Introduction
Translation, or protein synthesis, is a fundamental process in cellular biology where messenger RNA (mRNA) is converted into a sequence of amino acids, forming proteins essential for life. This process is crucial in understanding how genetic information is expressed within organisms, and it involves several types of RNA, particularly mRNA, transfer RNA (tRNA), and ribosomal RNA (rRNA). In this article, we will delve into the mechanics of translation, discussing the various stages and critical components involved in this biological phenomenon.
What is Translation?
Translation is the process by which ribosomes synthesize proteins using the information encoded in mRNA. It occurs in the cytoplasm and involves the following key steps:
- Initiation: Assembly of the translation machinery at the start codon.
- Elongation: Amino acids are sequentially added to the growing polypeptide chain.
- Termination: The finished protein is released once a stop codon is reached.
Overview of RNA Types in Translation
Before diving into the translation phases, it is essential to understand the key RNA molecules involved:
- mRNA (messenger RNA): Carries genetic information from DNA and contains codons that specify the amino acid sequence. For a deeper understanding of how this information is processed, check out our article on Understanding DNA Transcription: A Comprehensive Guide.
- tRNA (transfer RNA): Brings amino acids to ribosomes during protein synthesis, with each tRNA having an anticodon that pairs with the corresponding mRNA codon.
- rRNA (ribosomal RNA): Essential component of ribosomes, facilitating the translation process by providing a site for mRNA and tRNA interactions.
The Genetic Code
The genetic code consists of triplet codons (three nucleotide sequences) that specify which amino acids will be inserted into a protein. The codons are recognized by tRNA, decoded during translation:
- There are 64 possible codons formed from four nucleotides: adenine (A), guanine (G), cytosine (C), and uracil (U).
- 61 codons specify amino acids, while 3 codons (UAA, UAG, UGA) are stop signals, indicating the end of protein synthesis.
Codons and Anticodons
Codons in mRNA are recognized by complementary sequences in tRNA called anticodons. For example, if the mRNA codon is AUG (which codes for methionine), the corresponding tRNA anticodon would be UAC.
Phases of Translation
1. Initiation
The initiation phase sets the stage for protein synthesis.
- In prokaryotes, the small ribosomal subunit binds to the Shine-Dalgarno sequence upstream of the start codon (AUG) in mRNA. This binding is assisted by initiation factors.
- In eukaryotes, the process begins when the small ribosomal subunit binds to the 5' cap of mRNA, with the help of several initiation factors. This machinery scans the mRNA for the start codon (AUG).
Once the ribosome is assembled and the initiator tRNA is bound to the mRNA's start codon, the large ribosomal subunit joins to form a complete ribosome. Energy from GTP hydrolysis assists this process.
2. Elongation
During elongation, tRNA molecules sequentially bring the appropriate amino acids to the ribosome, facilitated by elongation factors:
- The ribosome moves along the mRNA in the 5' to 3' direction, facilitating the addition of amino acids to the growing polypeptide chain.
- Within the ribosome, the A site (arrival), P site (peptidyl), and E site (exit) play crucial roles in positioning tRNA molecules and peptide bonds formation.
- The peptide bond formation is catalyzed by the enzyme peptidyl transferase, facilitating the transfer of the growing peptide from the tRNA in the P site to the tRNA in the A site.
3. Termination
Translation concludes when a stop codon is encountered:
- A release factor recognizes the stop codon, binding to the A site, and catalyzes the release of the newly synthesized polypeptide from the tRNA in the P site.
- Following termination, the ribosome subunits, mRNA, and release factor dissociate, allowing the protein to fold and perform its biological functions.
Translation on Ribosomes: Free vs. Membrane-bound
Translation can occur on free ribosomes in the cytoplasm or on ribosomes attached to the rough endoplasmic reticulum (rough ER).
- Free Ribosomes: Synthesize proteins primarily involved in cytoplasmic functions, such as glycolysis enzymes and nuclear proteins.
- Rough ER-bound Ribosomes: Synthesize proteins destined for secretion, incorporation into the cell membrane, or delivery to lysosomes. This is initiated by a signal recognition particle (SRP) that recognizes a signal sequence on the growing polypeptide chain, guiding it to the rough ER. For more on the role of the rough ER in protein synthesis, see our article on Understanding the Endoplasmic Reticulum: Structure and Functions Explained.
Post-Translational Modifications
Once proteins are synthesized, they often undergo post-translational modifications (PTMs) such as:
- Glycosylation: Addition of carbohydrate chains, important for cell recognition.
- Phosphorylation: Adding phosphate groups to activate or deactivate proteins.
- Acetylation: Modifying proteins to regulate transcription.
- Methylation: Another regulatory modification affecting gene expression.
- Trimming: Cleaving particular amino acids to activate certain proteins, like digestive enzymes.
Conclusion
Translation is a vital process in cellular function, translating genetic code into functional proteins. Understanding the stages of translation and how proteins are synthesized provides insight into the fundamental biochemical processes sustaining all life forms. By exploring translation, we enhance our knowledge of genetics, molecular biology, and the underlying mechanisms of life itself. For a broader context on these processes, consider reading about Understanding DNA Replication: The Science Behind Cell Division.
what's up ninja nerds in this video today we're going to be talking about translation or protein synthesis but
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academic journey all right ninja so let's start translation when we talk about
translation we have to have a basic definition of what the heck it is and that is you're taking rna in this
case what type of rna we really is our primary one that we're going to focus on mrna we're taking that mrna
that we made from dna what was that process called transcription so we're taking the mrna
that we got from dna and now we're going to make proteins that is the process of translation
taking rna and making proteins there's so many different types of rna the three main ones that i need you guys
to remember that are crucial for translation is mrna trna and rrna and we'll go through these
as well as a couple other things before we get into the phases of translation the first thing that we need to talk
about is this concept of the genetic code okay it's really simple it's not as like
scary as it seems a little boring but we're gonna make it fun the first thing you need to know is here
we have a molecule of mrna right so this is our mrna now mrna is very
very important for this translation process right messenger rna messenger rna it has a very specific
sequence of nucleotides if you will that are in these triplet forms you see how this is a
there's three little lines there that line is a nucleotide that's a nucleotide that's a nucleotide so
there's three nucleotides there and we have a couple of these spanned along the length
of this mrna molecule right and the other thing you need to know is the orientation
right so a little bit about the topology of the mrna on this end i'm going to have a five
prime cap do you guys remember that whole process we talked about in post
transcriptional modification this is the five prime end on this end you have the three primate and we
had on this side do you guys remember what happened here in the transcription process we've added
the poly a tail on that end right so on the mrna you have a five prime end a three prime end
and sequences of nucleotides within it these sequences of nucleotides that are in these
triplet forms along the mrna are given a very special name that you need to remember
and these are called codons so let's write that down so these things these triplets let's put down
triplets triplets of what triplets of nucleotides okay and you guys need to remember the
nucleotides and rna are different than the nucleotides in dna
let's write down primarily the nitrogenous bases that are associated with rna
what are they we'll represent that by kind of just the single letter abbreviation one is
you have adenine right that's one of the nitrogenous bases in rna then you have guanine
cytosine and uh what else if you guys said thymine i'm gonna be really upset with you it's not thymine
it's uracil in rna it's uracil that's the big difference between dna it has thymine in rna it has
uracil okay so we have codons are triplets of nucleotides what
type of nucleotides nucleotides that contain these four nitrogenous bases okay now here's the next thing i need
you guys to know we know what they are how many are there let's take this let's do a little bit of
math i know it's a little boring let's take and do a little bit of math here we have the triplets we described that what
those triplets are made up of and let's talk about how many of these triplets
of these four nucleotides you can have well there's four total nucleotides right so let's put a four here four
different types of combinations of nucleotides each one of there's three of these
nucleotides in a codon so if i take four raise it to the third power
what does that give me 64. 64 possible codons based upon the four nucleotides i have
and that there's three nucleotides in that codon so that means that there are 64
different possibilities of codons okay so what do we need to know here that there's 64 different types of
codons now we've got to talk a teensy little bit about the different types we're not
going to go through every single one of them that's unnecessary you'll have these if you guys don't go
into the back of your textbook like an appendix you'll have the entire genetic code that
you guys can look at there's no need to memorize them but need to know a couple things about
these 64 different types of codons and what should you know the next thing you should know is that when you take
these 64 codons okay that are triplets and we'll kind of talk about them
61 of those codons read you read them right and we'll give you an example here for an example
let's give you an example now let's say i take a codon right which is a triplet and it contains
one of these three of those nucleotides let's use the example a u g that's a codon
if i look in the back of the textbook at that genetic code kind of thing and i see aug it's going to code for an
amino acid and that amino acid is very specific to that codon
and so what type would it be this one's an easy one to remember and this is probably one of the few that
you should remember and memorize but this is methionine and methionine is an amino acid
so out of the 64 codons 61 of them right in this kind of form three nucleotides which there's so many
different types of possibilities will code for an amino acid i'm just giving you an example
so out of these 61 codons they code for an amino acid so very important to remember that
okay out of the remaining how many are remaining 61 there's three codons left that we
have to talk about these other three codons do not code for an amino acid they
code for terminating the translation process and these are called stop codons and
we'll get into these a little bit more in detail when we go through the phases of translation but
you can remember these by the mnemonic or kind of like the phrase if you will a little memory trick
which is you go away you are away you are gone these do not code for an amino acid
so in other words if i were to look in the genetic code in the back of the textbook these would
not give you a particular amino acid they would stop the translation process so that's important and we'll go over
what that kind of looks like a little bit later so basic concept i want you guys to get
out of the genetic code particularly is mrna contains codons codons are made up of
nucleotides how many three what are the particular types of nucleotides they have to contain adenine guanine
cytosine and uracil there's how many different types of codons so many 64. do you need to know
all of them no out of those 64 61 of them code for amino acids you can look at all
those up in the textbook three of them do not code for amino acids they stop the translation process
that's all i want you to know about that the other aspect of the genetic code is that we need something that's going
to carry so we said that these codons code for amino acids how the heck do they do that
you guys should be asking that question there's another molecule called trna right what is it called
t rna transfer rna transfer rna if you guys kind of look at the structure of it
it contains what's called anticodons let's write that down so it contains anti
codons okay that's the first thing what the heck are anticodons it's really simple anticodons
are a triplet all right so three nucleotides that are complementary to the codons in
mrna that's it so what are they they are a triplet of nucleotides
that are complementary to codons in the mrna that is it
the other aspect of this is there's some enzymes we'll talk about a little bit later
so there's a funky little enzyme that'll come in it's kind of like a little bit it's a little gropy
grabs the anticodon portion reads it says okay i got the anticodon there all right so i
know what it is i need to find an amino acid that is very specific right to the codons that this anticodon is
complementary to so let's give the example here let's say the codon that you have an anticodon
complementary to is this one aug okay so let's give the example here that you have a codon
we're going to use this example here so let's say here we have a codon and the codon is aug
what would the anticodon be has to be complementary to this so the anticodon
for this example would be u a c right because that are copper complementary to each other right
you guys remember that that if it's a that's complementary with u that g is complementary with c and
technically these should be triple bonds right so this would be a is complementary with
u u is complementary with a and g is complementary with c so we'll go u
a c what will happen is the trna enzyme that'll come in we'll talk about later it'll say oh uac
that's complementary to aug if i go into my genetic code because it's got all of it in its head this enzyme
it says aug is specific for what amino acid methionine so then this enzyme takes and we use a
particular amino acid domain we'll call it there's a specific amino acid domain
on this trna that we'll talk about and that is going to carry the amino acid specific to this
codon what was the amino acid methionine so it'll carry the methionine in this example
now let's talk a little bit about this kind of like anatomy or structure of the trna that kind of
coincides with what we just talked about if we take the trna the first thing is where are the anticodons
the anticodons if you look at a trna kind of has the shape of a t in a way right
on this portion this bottom loopy portion this right here is where you'll have your anti-codons
on this portion so this is the part of the trna that'll interact with the codons in the mrna the
next portion here is you have another loop i'm not really too worried about you knowing about
these loops this portion here though okay this is called the
five prime end of the trna so what would be on that end what group hydroxyl group or phosphate group
engineers phosphate group this end over here which has like the little copper loopy thing
is the three prime end three priming contains what oh phosphate group contains the hydroxyl
group or the oh group but there's also a very specific sequence of
nucleotides that are in this area of the three prime end which hold on to the amino acid this is the amino acid
holding domain and this is containing c c a so we'll put the three prime c c a
domain or region on the trna what is this portion the little cup that holds on to what
we'll bind in here so here we have c c a it'll bind on to the amino acid in this case it was what
the finding if the anticodon is uac which is complementary to aug holy crap we went through all of
that all right so the next thing we're going to talk about is the characteristics of the genetic code i
don't want to go too long into this let's just breeze over really quick but it's things that can be asked in
your exam so you should know it when we talk about the genetic code all the stuff we talked about with the
codons the anticodons the mrna tr and all that good stuff when we take the mrna and we read it
right from five prime end to three prime end for the most part there's a couple
exceptions you start at the five prime end and you go to the three prime end continuously
you don't like have any stops or anything like that so in that way when we talk about the
genetic code this translation process understanding the genetic code
is what's referred to as kamalas so what does that mean here's a codon right
i'm going to read this codon utilizing the trna and the ribosomes and i'm going to give an amino acid then
i'm going to go to the next codon read that make amino acids and i'll keep going down this way going
through each sequence of three nucleotides are a triplet now
what does this mean that it's homolos let's say that i read this codon read this codon and there's a couple
nucleotides in between to between this next codon that i want to read i don't
skip these nucleotides and go to the next codon okay so this this thing does not happen
you don't go three nucleotides read through nucleotides next and then
skip a couple nucleotides and go to the three next nucleotides it's consistent this does not happen in
the genetic code or the translation process the only exception to this is viruses they're the only
exception so we'll put exception to this where they can have some type of translation
process that does have commas in it or you kind of skip a couple nucleotides okay
the next thing that we need to know about the genetic code is that not only is it
kamalis but it's non-overlapping what does that mean that means that when i read these
again five prime to three prime i'm gonna read it all the way down continuously
i'm gonna read this codon give an amino acid read this codon give an amino acid what i'm not going to do is read this
codon give an amino acid but then let's do a different color start here at the second nucleotide read
these three and give an amino acid now let's do one more color start at this third nucleotide after and
read here and give an amino acid that does not happen
in the translation process according to the genetic code there is one exception and again that exception
is that this overlapping process can occur in viruses that is the only
exception okay so the when someone says can you give me characteristics of the genetic code you
will say it is comless it occurs continuously from five to three and non-overlapping continuous
from five to three the only exceptions are viruses that's it the next thing that you need to know
is a little bit more important than this gibberish up here
and that is that the genetic code is what's called redundant so it's redundant
and it's degenerate okay so it has degeneracy or it's degenerate so let me explain what that means let's
say i have a couple codons and i'm going to actually give specific nucleotide sequences to i'm going to
give this a nucleotide sequence of a u a a u c and a u
u okay now here's what's really interesting about these i have three different codons these
three codons you would probably say each one of them codes for you know a different amino acid but you'd be wrong
and that's where redundancy or degeneracy comes in if you guys look in the back of your
textbooks or the appendix where the genetic code is if you were to look there is a amino
acid called isoleucine and if you take isoleucine and you try to track back to
its codon you'll find that it has three different types of codons that can actually code for it
and that is a u a a u c and a u u that's really interesting so that
tells me that i could know the amino acid that i'm making but i won't be able to track
it back to one to the specific codon there's two exceptions to that and that is um
if you truly want to know it the only exceptions to this concept of redundancy or degeneracy
the exceptions are methionine so we'll put here methionine and what's called tryptophan
and here and let's i want you guys to think about why these would be exceptions because it actually does help
methionine there was only one codon what was it aug tryptophan only has one codon
you know you don't need to know this but it's ugg there's no other codons that code for
these amino acids they're just one so they're the only exceptions all the other amino acids
have multiple codons that code for it so that's the concept of redundancy now here's the thing
you guys are like how the heck does that happen i asked this question when i was learning it
so let's take an example here let's say here i have my mrna right and again this is my five prime
end this is my three prime end of the mrna let's say i start here and i'm going to
go in sequence here so the sequence is which one let's kind of keep these colors
we'll do red here for the mrna so this is going to be which ones a u a a
u c and the next one is a u u how the heck does the trna do that in a particular way does each
anticodon have to be different because i thought they have to know the trna the enzyme
that we talked about kind of like a little bit said it has to read the anticodon and it
has to be complementary to the codon to give me this amino acid how does it do that
here's the way it does it there's something called the wobble effect wobble baby wobble baby wobble right and
it's called the wobble effect or the wobble phenomena let's write that down and let's talk about what the heck that
is so let's take and this is particularly for the trna that kind of allows this
process to occur it's pretty cool so where's the anticodon on the trna here's our trna
where would it be on this bottom loop what would it have to be specifically
you would say oh complementary to a is u complementary to u is a complementary to a is u but here's where
it's different on this position what was this point here on the trna at this point here
this was the five prime end right what's this portion here the three prime end so going from five prime of the trna to
the three prime of the trna right because if you kind of follow this down like that
so you start five prime this would be the first position this would be the second this would be
the third position and then you continue to work your way back up on this first position on the five prime
end it's actually containing a something called inocine
you're like what the heck believe me i thought that too so the same thing we'll talk about what
that does in a second but let's go to the next one same thing you'll read this here you have your five
prime three prime and it'll have come down the first one will be ionosine
and then what will you have what's complementary to you a what's complementary to a u do the same
thing over here start at 5 three prime for the trna work your way down first one has to be
i next one will be what a and the next one will be u let's explain what happens with all this do you notice
a difference here remember i told you that the enzyme has to read that anticodon it
has to be complementary to the codons in the mrna to pick the correct amino acid
well do you notice how all these are dif they all differ in that third position on their codon and they differ on this
kind of like first position in the anticodon here's how this happens ionosine okay that i
i'm representing with is actually called ionosine ionosine is not talked about too often in the watson and crick model
you know in your dna stuff the interactions complementary stuff ionosine is complementary to
adenine ionosine is complementary to uracil ionosine is complementary to cytosine so whenever you have something
like this where the third position is a c and u on the trna they can have an ionosine
that can be complementary to a complementary to you and complementary to c
and still give you the same amino acid which would be what i already told you this will be
isoleucine isoleucine isoleucine okay so that's called the wobble effect
you're probably like why the heck do we do this why don't we just make it specific to each
of these types of um you know codons why don't we just make it u a u u a g
uaa why don't we do that the reason why is the wobble effect reduces the risk
it decreases the risk of mutations so what do i mean by it can decrease the
risk of mutation it can and specifically it can decrease the risk of mutations
how does it do that it's all based upon this fact that if i have any mutations in the dna
that'll lead to mutations in the mrna and if there's mutations in the mrna i'm going to have changes like substitutions
or things like that in the codons and if i kind of substitute or switch up
some of the nucleotides i'll code for a different amino acid particularly
so if i have a little bit of that wobble effect i have a little bit of you know wiggle room in that that first position
on the trna i may reduce the risk of giving a wrong amino acid
leading to a abnormally structured protein so that's kind of the big effect here
is when we talk about redundancy or degeneracy it's that one amino acid can have multiple codons
with just these two exceptions and how does that work via the wobble effect in trna where on that first
position on the five prime end of the anticodon it is an ionoscene which has multiple
complementarities with a uc whole purpose of this is to decrease the risk of
mutations so when we're talking about when i'm mentioning all this stuff about the genetic code and you can look in
your textbooks i use marieb kind of a human anatomy physiology book and again you can find
the in the appendix all the information about that genetic code but you can find this in various
textbooks campbell's biology as well but again i'm just referring to in any book you'll have that appendix to talk
about the genetic code so that you guys know what i'm talking about here all right so we got a
pretty decent idea about the genetic code right i'm talking about codons anticodons and some of the features of
it our characteristics the next thing i want to talk about is trna a little bit
and i want to go through something called trna charging we'll review the structure of the trna
really briefly as a nice like little review but we're going to talk about this process called trna charging which
is very important when we talk about the translation process
so here we have our trna molecule right so this is our trna transfer rna tiny little guy right
again what is this end here it doesn't have the little kind of like little socket or pocket there where the
amino acids bind what is this end this is your five prime end what's this end
this is your three prime end which contains the oh group but particularly what nucleotide
sequence cca what binds here this is the amino acid kind of like binding domain if you
will so this is where an amino acid will bind correct now this arm was the one i really wanted you
to focus with we had the three loops we'll briefly talk about these other two loops not
super worried if you guys know it but here in this bottom loop what do we have down here
on this bottom loop you contain the anticodons and the anticodons will be in a triplet
form and these triplets can be in the form of again containing nitrogenous bases like
what adenine you know adenine guanine uracil and the cytosine okay so again this
portion here will be what this will be the anti-codon portion okay the last thing here is
these two little arms or loops and this little thing that's kind of like sticking out the side
this portion here near that three prime end this is called the t arm so what is this
portion here called nor the three prime end this is called the t arm
the t arm what you really need to know about this is that it tethers the trna to the
ribosome that's all i want you guys to know is that it tethers
the trna to the ribosome so it kind of is one of the big things that allows for interaction between the trna
to the ribosome okay that's it this other arm over here this loop near the five prime end
this is called the d-arm and the d-arm is what allows for the identification of the trna by
the enzyme called trna the aminoacyl trna synthetase so it allows for
identification identification of the trna by what's called the uh we'll just a br
we'll kind of a basic thing trna synthetase enzyme okay so basic concept here you have the five
prime end then you have the first arm which is the d arm it allows for the identification of
the trna by the trna synthetase anticodons on the bottom loop t arm is near the three prime end that
allows for the trna to interact with the ribosome three prime end has the
cca domain which allows for it to interact with amino acids this last thing here i'm not really
concerned if you guys truly know it it's the invariable domain it's uh it can i mean it can
actually it's the variable domain it can change from trna to trna nothing too big to know about that
portion okay so if the basic structure of the trna the next thing i need you guys to know
about is called charging so this is really simple it's basically talking about
how do we get the amino acid to bind on to that three prime end that's all it is and it's really simple here i have an
amino acid okay so here's my amino acid and let's let's use this example that we've
continuously been using a lot let's say that we're con we're going to start the translation process
and let's just pretend for example here's my mrna okay let's use this example that this is
a u g what would the anticodons be if we were to kind of write them in here
if it was aug it would be u a c what is that aug code for methionine we've already said that multiple times
right so let's say here's our here's our example this is our methionine we'll
just abbreviate it as met okay what we're going to do is the first step we're going to do in this process
is we're going to add an atp molecule onto it we're going to add an atp molecule onto the methionine
so let's say that here i use an atp and i add it into this process here okay then what i'll have here is i'll have my
amino acid and what happens is when atp gets added in it actually we break two of the
phosphate groups off of the atp okay so if we break two phosphate groups off that gives you
what's called a pyrophosphate and so the only thing that's kind of hanging onto this amino acid
is an amp they want you to know these kinds of names of it right so when i take this
amino acid and add on an amp it's called i know it's annoying it's called the
amino acyl amp molecule okay then here's the next thing
we have this aminoacyl amp and we have that three prime kind of amino acid domain with the cca portion
imagine we draw a big old enzyme here so here's this enzyme okay here's this enzyme
this enzyme has in one end is holding the trna right so it's holding that trna molecule
so we're just gonna we're gonna draw a very generic structure of it here's gonna just be this process here okay so
here's the generic structure and we'll just kind of show that this is our three prime end right there
okay just generic it's holding in one pocket this trna molecule in the other pocket
it's holding the amino acid with what bound to it the amp then what it does is it basically just
says hey let me make sure that this anticodon is appropriate
is it appropriate to the mrna codon that we need oh it is good clicks them together
and so it takes and adds that amino acid with the amp onto the three prime cca region
so let's draw the little cup what was that little cup thing the cca portion it'll add on this reaction will occur so
we're going to just fuse these two things together and when we fuse these two things together what
do you get you'll get this structure where all you'll have the trna
with the little cup and what will be kind of sitting in that little pocket there
the amino acid and what amino acid was this in this example methionine in the process though do you
see amp still bound to it no so what are we going to do
we're going to release the amp during that process okay what you need to know is what the
heck is this enzyme this enzyme is called the amino acyl trna
synthetase i kind of quickly abbreviated it for you like a shorthand version of it when we
talked about with the d arm that's the enzyme i'm really referring to is the amino acyl trna synthetase
and if you really wanted to remember what part of the tna is keeping it kind of like identified
the d arm of the trna will allow for it to be identified okay so to recap really quickly
i want to take the amino acid put on the three prime end what do i have to do first thing take the amino acid add an
atp onto it i'll pop off a pyrophosphate so i'm truly only adding a amp
that's called an aminoacyl amp a amino acyl trna synthetase will come in have two pockets in one pocket it will
hold the amino acyl amp in the other pocket it will bind the trna with no amino acid
it will read make sure that it's the proper anticodon that is complementary to the codon of mrna
click them together when it clicks them together it puts the amino acid on the three prime in and
spits out the amp now what do i have a charged trna so what is this thing here called
this is called a charged t rna okay
that's the process that's all i really want you to know out of this okay so let's now move on to the next thing
which is saying okay we've already talked about mr now we talked about codons anticodons some features we
talked about trna charging now we need to get into these things called ribosomes a little bit all right
so now let's talk a little bit about ribosomes and what are their kind of significance because we're going to go
into all these phases of translation it's all going to make sense it might seem a little bit scattered right now
but i promise we're really building our foundation so we truly understand the translation
process so the next thing we need to talk about is these ribosomes ribosomes are
definitely very very crucial for translation as well as the mrna and the trna
but some of the things that you guys need to know particularly is the difference in ribosomes between
eukaryotic and prokaryotic cells and there is a very brief clinical significance that we'll
talk about with that so let's say here i have ribosomes and they're interacting with the mrna
they will interact with the trna but we're going to talk about these specific differences between
eukaryotes and prokaryotes because this is something that you guys will be asked
eukaryotic cells when we talk about ribosomes they have two subunits okay we're going to say this subunit up
here is bigger than this one down here right so it's pretty straightforward this
is the large subunit or ribosomal subunit and then this one down here is the small
ribosomal subunit okay now these have different ways that we can kind of like describe
their size okay large and small according to a zved zvedberg unit
and eukaryotic cells that zvedberg unit for large rebels almost sub units are called 60s large ribosomal subunits
and the small and eukaryotic cells are called 40s ribosomal subunits but we sometimes
generally in textbooks refer to them as ads ribosomes and eukaryotic cells you're probably like zach
that those numbers do not make any sense 60 plus 40 is a hundred zach what are you losing your brain
i promise you the there the way that they do this via this vedburg unit gives you an ads ribosomal subunit for
eukaryotic cells and prokaryotic cells it's the same concept again we're not going to write
these down but this is your large here we'll put large ribosomal sabine small ribosomal
subunit and prokaryotic cells the large one is a 50s ribosome
and then in prokaryotics the small is a 30s ribosomal subunit and you're probably like oh that's going
to give you 80. nope according to this vedburg units it gives you a 70s
ribosomal sub ribosomes in prokaryotic cells you're probably like okay is that cool
i'm glad that i know that now why do i need to know that before we talk about why you need to know that the
next thing i need you guys to remember is what are ribosomes made up of you guys need to remember this
ribosomes contain a very specific kind of molecule if you will that's kind of a sitting and
a part of it very integral to its structure what is this it's got little like nucleotides on it
it's rrna so ribosomes contain two different types of things that make them up
it's equal to r rna and what else proteins so proteins so when we're talking about remember
when i said in the beginning translation requires three types of rna mrna trna and rrna we usually just say
ribosomes but ribosomes contain rrna and proteins now why did i spend the time talking about all this
stuff a common clinical relevance here is that they
love to say when you're talking about prokaryotic cells prokaryotic
cells okay we can use different types of antibiotics to target these ribosomal subunits and prokaryotic
cells for example if i give someone an antibiotic like an aminoglycoside and there's so many
different types of these but the commonly one that you need to know is like gentamicin
and another one called tetracyclines and there's a bunch of different types of these doxycycline
tetracycline minocycline all those these love to target and inhibit the translation process by affecting the
30s ribosomal subunits so they inhibit the activity of the 30s ribosomal subunit in prokaryotic cells
the other antibiotics is going to be particularly not the aminoglycosides and the tetracyclines
but let's say that we're talking particularly about something called macrolides
and these are things like azithromycin clarithromycin erythromycin these love to target and inhibit the
activity of the 50s ribosomal subunit and prokaryotes which inhibits protein synthesis think
about this prokaryotic cells like bacteria let's use this example like bacteria need
proteins in order for them to function if you give an antibiotic if a bacteria is infecting a particular tissue you
give them an antibiotic something like an aminoglycoside a tetracycline or a macrolide
it's going to inhibit these ribosomal subunits you can't now use them to make proteins if you can't
make proteins the bacteria will die so you see how there's a clinical relevance to something at the molecular
level okay we've gone through all the players that we
really need to understand and know for translation we went through the mrna we went through the trna we went through
the ribosomes and the rrna now let's head home and talk about the phases of
translation all right so we're going to talk about the phases of translation we've really built up our foundation to
understand translation now so there's three phases of translation the first phase that
we're going to go through is called initiation so what's the first that we're going to
talk about here called the first phase we're going to discuss is called initiation of translation and
it's probably like it's really it's it's not that hard it's a really simple step
we have to kind of discuss though the differences between prokaryotic initiation and translation
and eukaryotic initiation and translation so let's first talk about prokaryotes
because they're easier so here's our mrna right and on the mrna again what do you have
you'll have a five prime end and you'll have a three prime and let's just kind of uh write here now
that this is specific for prokaryotes okay we're talking about this for prokaryotes right now
let's say here on the prokaryote is my start codon and what are your start codons
we didn't talk about that yet did we but there is a particular star codon we kind of talked about a little bit
what i want you to remember is that your start codons we talked about there were 64 different
types of codons 61 code for amino acids and three don't they're stop a star codon
we did kind of talk about it is aug do you guys remember what aug coded for methionine right so methionine but
here's the difference this is an important thing to talk about and they'll probably throw this on an
exam for prokaryotic cells it's technically not methionine
it's called informal methionine so what is it called in formal
methionine okay we'll put met so again the start codon is aug in prokaryotic cells same as it is for
in eukaryotic cells but what it codes for is not methionine like it is in eukaryotic cells it's
called informal methionine sometimes it's even abbreviated
as f met okay either way that's my start codon so we're going to put here
a u g on this mrna there is a sequence of nucleotides
particularly like purines that are a couple nucleotide bases upstream towards the five prime end from
that start codon and for whatever reason they love to give this a particular name
because this is where your ribosomes a lot of initiation factors things like that
bind and recognize the mrna and the prokaryotic cells and bind it helps to start the translation process
and this sequence that's like eight nucleotides upstream from the aug is called the shine
delgarno sequence okay and if you really want to know it contains a lot of a's
adenines and guanines okay so it contains a lot of adenine and guanines or your purine
nucleotides in that region okay so there's a shine delgarno sequence it's kind of like an identifier on the mrna
and what happens is a couple things first thing is you have your small ribosomal subunit
okay your small ribosomal subunit will come and bind to this area
right and what happens is when it binds to the area here on the mrna it uses
a very special type of protein let's represent these in brown actually no let's do it in pink so
it's kind of different here there's these things called initiation factors
and there's these initiation factors that recognize the shine delgarno sequence that are in the small ribosomal
subunit are bound to the small ribosomal subunit and so what happens is the initiation
factors in the small ribosomal subunit will bind the shine delgarno sequence then
once it does that it starts kind of moving towards the start codon so two things happen
these pink things called initiation factors that are associated with the small ribosomal subunit will identify
the shine delgarno sequence when they bind they then move down about eight nucleotides until they hit the
start codon which is aug that's the first thing okay so if we wanted to kind of show that
that's the first event to happen let's put one here first to event event to happen is
initiation factors and small ribosomal subunits bind shine delgarno move down until they hit the aug the
second thing to happen here is that there is a molecule called trna right and trna is going to have to have
anticodon specific to this aug which is u a c and it'll be carrying with it an amino
acid what is that amino acid specific we already kind of talked about it we're going to abbreviate it called
f met now when the trna comes what is this called this is your trna
containing the fmet when it comes in as its anticodons interact with the codons here
there's something that help to bring it or drag it into this area what do you think that is this
represents another little pink color there's a pink protein that kind of helps to yank that
trna the initiator trna right which contains the fmet and bring it into
where the start codon is what is that pink protein called it's called an initiation factor that's
it so first step initiation factor small ribosomal subunit bind shine delgarno move down until they hit the start codon
second step initiator trna in the prokaryotes which contains
trna and n-formal methionine with a initiation factor come to the area
where the start codon is and bind that's the second step third step there is
a molecule bound to this initiation factor and that molecule is called
let's bring it over here a gtp this gtp is a high energy molecule what's going to happen is this
initiation factor will break down the gtp into gdp and an inorganic phosphate and
that'll create a lot of energy and what happens is at the same time the gtp gets broken down
into gdp and inorganic phosphate the large ribosomal subunit will represent it like this
the large ribosomal subunit will come over and bind to this area and so what would it look like if we had
kind of like showing all of this happen here this process and the large ribosomal
subunit coming in here this would be in your third step
so third step here is gtp gets broken down to gdp and inorganic phosphate and the large robot
is almost up and it comes and gets added in what would be the final thing that it
would look like if we drew it down here if we drew it all down here at the end product here
you would have what large ribosomal subunit small ribosomal subunit bound here then what else would
we have we would have the trna kind of sitting in here
with the f informal methionine bound with the codon in this case it would be
aug and then what would we have released during this process we would have released gdp in an
inorganic phosphate and what else would we release we don't need this thing anymore we don't need this
pink protein anymore the initiation factors we can just spit those out as well
so we can spit out the initiation factors as well what are these things called we're just
going to abbreviate them initiation factors so to recap really quick because i know
it's a lot of crap and one thing shine dog arnold sequence identifier of the mrna small ribosomal sub being it's
initiation factors bind to it identify it move down till they hit the start
second thing trna which contains the fmet right which is particularly based upon
the anticodons complementary to the codons and mrna it gets brought to this area by the
initiation factors they bring it to the area and bind the trna then
third step there's a gtp associated with the initiation factors it gets broken down
into gdp and inorganic phosphate at the same time a large ribosomal subunit will bind
and what will you get at that process you'll get the large and small bound to the mrna with the
trna sitting in the ribosome in what site we didn't talk about this yet but
there's three sites in a ribosome one of them if we start them here this first one is called the a site
that's the kind of the arrival site this one is called the p site and this one is called the
e site and we'll go through these all in detail but that trna is going to be sitting right smack dab in the middle
which is going to be the p site okay so that covers the initiation and prokaryotic cells thank
goodness in eukaryotic cells it's pretty much the same we just give different names for
stuff so this step here in initiation this is for particularly what
eukaryotic cells they still have a five prime end and a three prime end but guess what
they don't have a shine delgarno sequence they just have this start codon what
happens is first thing that happens is you have a molecule called a eukaryotic initiation factor
so a eukaryotic initiation factor will come and bind to this five prime end and we call this eukaryotic initiation
factor type four it'll bind to this five prime end okay that's the first thing that will happen
the second thing that will happen is that you'll have your small ribosomal subunit and other
you know initiation factors that we're not too concerned with just yet that'll come in interact with this
mrna so let's draw here your small ribosomal subunit that'll come in bind that's the second
thing that will happen and then what else is happening you're having some initiation factors some
small little initiation factors that'll help that small ribosomal subunit to bind
to the mrna this the third thing that happens okay so so far we've had two things
happen eukaryotic initiation factor type 4 identifies the mrna second thing is the small ribosomal
subunit with the initiation factors bind to the mrna the third thing to happen
is that you have a eukaryotic initiation factor type 2 eukaryotic initiation factor type
2 that will bind your trna right it'll bind the trna that contains anticodons that are complementary to the
codons and mrna which is uac it'll have an amino acid that'll be based off of that start codon
what is it in eukaryotic cells what is the start codon in eukaryotic cells it's the same one we talked about
in prokaryotes right aug what's the difference aug and eukaryotes codes for methionine
not in formal methionine that's all that's different so this is just methionine eukaryotic
initiation factor type 2 will bring with it the trna with the methionine
and bind it to this portion on the start codon the fourth thing to happen here
is that you have a gtp molecule that is going to be bound to the eukaryotic initiation factor type two
this is the fourth thing it's going to get broken down into gdp and an inorganic phosphate and the other
event to happen here is that the large ribosomal subunit which contains the
e site p site a site will come and bind to the mrna and what will it look like if all of
this stuff kind of happens accordingly you'll have here your large ribosomal subunit with the e site
p site a site small ribosomal subunit you'll have the trna which will have its anticodons complementary to the codons
of the mrna and you'll have your methionine sitting there and what would be of release
because we don't need them anymore in this process we would release the gdp
and the inorganic phosphate and we would also release the eukaryotic initiation factors
right like type 2 and type 4. do you see how it's pretty much the same in prokaryotic cells
the only difference is is that in order to start this you have a shine delgarno sequence that's identified
by initiation factors and eukaryotes it's a eukaryotic initiation factor that binds the five prime end okay
the other thing is you still have a trna that's coming in and binding with initiation factors
to where that star codon is the only difference is is that's informal methionine and eukaryotic cells
it's called methionine and these are just called initiation factors this one's called
eukaryotic initiation factor type 2. they just wanted to be annoying but the same thing happens in the
remaining steps which is the large ribosomal subunit has to bind and you have to break down gtp into gdp
and inorganic phosphate and you have to release the initiation factors all of it's the same with just
some minor changes in it that's it we finished initiation
thank the lord now let's move on to the next step which is called elongation so what's the next step that
we're going to talk about here the next step is probably one of the more difficult ones to kind of visualize
but this is called elongation this is the second phase in translation so let's pick up where we
left off we initiated the translation process let's pretend this is the same thing
thank goodness this is the same and eukaryotic cells and prokaryotic cells but we're going to
use a lot of the examples here in eukaryotic cells so this is primarily going to be used
in eukaryotic cells that we're going to be using this as an example and it's because we're going to be using
particular types of factors okay so in this example just so you know it's the same and prokaryotes and
eukaryotes just in this example i'm going over it in eukaryotes because i'm going to use specific
factors and you'll see what i mean so to see if you guys remember everything we just talked about up here
you had to initiate it right small ribosomal large ribosomal have to bind initiation factors help that process
break down gtp into inorganic phosphate and bring a trna which contains a amino acid the initiator trna which is
going to be informal with ionine and prokaryotes and methionine and eukaryotes
in this example what was our start codon aug what would be the anticodons that are complementary to that on the
trna uac okay that's where we are we just finished the initiation
now we're gonna do is okay we have to quickly review what is this site here the a site now if you really want to
know the a site is called the acyl site p site is called the peptidyl site and e is called the
exit site you can remember ape in that order okay because that's the order we're
gonna have things coming in and leaving so a site is i like to remember the arrival site
piece i like to think about as the synthesis site and e i like to think about is the exit
site that's how i remember them okay so the first thing we have to do with this elongation process is we have to
bring something into the a site let's just make up we use isoleucine as an example over there
let's bring them back let's put here a u a as the next codon that i'm going to read if that's the case then what do i
need to bring into this area a trna in order for me to bring a trna that is has anticodon specific to
that let's draw that in bringing him in here so we're going to have him come into
this step here so we're going to bring in what are the
anticodons to this u a u right if you really wanted to be specific according to the wobble effect
what would it be the ionosine but just in this example we're going to put uau
okay this is going to be containing what an amino acid and that amino acid in this example doesn't really matter but
it's called isoleucine since we talked about that one before now in order to bring this trna into
this a site we need something to help bring it to that area
and that is going to be called an elongation factor so it's called a elongation factor it's
called eukaryotic elongation factor type 1. eukaryotic elongation factor type 1
will bind this trna which is going to have anticodons complementary to these
codons on the mrna and the a site now once that happens let's show what that would look like so here
we're still going to have that same initiator trna right here right which contains the methionine and
if you really wanted to know here this would be uac and then what would these codons be
a u g this is the p site in the a site what does it look like a uua is my codons
and with the help of the eukaryotic elongation factor type 1 he brings in the trna that's
complementary to this one so that's going to have trna which is uau
and again if you really wanted to be specific according to that wobble effect it would technically be
ua i if you really wanted to but it's going to contain the isoleucine in the a site
who helped to bring him into this area the eukaryotic elongation factor type one but guess what else
this eukaryotic elongation factor on its back it's got a gtp molecule and really in order for this guy to get
in there and to bind what do i need to have enabling this process
energy so on the back of this molecule we have gtp when we add him in here
and he finally gets added in what do i spit out i spit out gdp in an inorganic phosphate
and what else do i spit out my eukaryotic elongation factor type one okay and now
i have my trna in this spot here's where it gets a little interesting because now what do i need
to do i need to take this amino acid that is bound to the trna in the p
site and transfer it onto the amino acid of the trna and the a site and then i need to shift this one that's
in the a site into the p site and shift the one that's in the p site into the e site you're probably
holy crabs act that's too much we're going to go through it so how does this work it's really cool
i'm going to show you in a very generic way and then we're going to show it in a zoomed in way because it is important
that you understand this what happens is there is a a little kind of like uh nitrogen
on this amino acid here and what was this one if you really wanted to remember isoleucine
that nitrogen comes over and attacks the carbon end on this amino acid that's in the p
site and you know those like little things when you were a kid there were like the little sticky things
with the hands on the end of it and you can throw it it could stick to something and kind of like suck it back in
that's kind of what this guy is doing it's going and it's grabbing the amino acid and the p
site and sucking it back onto it in the a site and then what it would look like if we
kind of did that process so let's say that we did this process here what would that look like if this were
to be if this were to occur that amino acid would be gone
because i transferred it over to this guy in the egg site isn't that cool so now
in the a site i'm going to have the amino acids two amino acids the one that was originally coming from
the uh the isoleucine right which was brought in in this step
and the amino acid methionine that came in from the initiation step in the a site now what does that look
like kind of in a zoomed in view we were to really take these and zoom in on them in a really kind of like zoomed
in view here is my isoleucine and on this end it has a
interminus the same thing over here from methionine it has a interminus and then on this end if you
really wanted to know it has a carboxy terminus same thing here it has a carboxy terminus the interminus of the
isoleucine nucleophilically attacks the carboxy group on the
methionine and then again sucks it back into where that area is like the little kind of like hands the sticky hands that
yank it back in in order for this process to occur the ribosome has an
enzyme kind of intrinsically associated with it and this enzyme is called uh a
peptidyl transferase pretty ironic right so the peptide transferase which is kind
of like imagine here that the that's kind of associated in this kind of uh
ribosome it's the one that's going to be helping to perform this process taking and catalyzing it so this step
that we just talked about is catalyzed by an enzyme intrinsic to the ribosome which is
called the peptidal transferase okay so we brought in a new trna into the a
site we used the peptide transferase to catalyze this step where this amino acid and the p
site gets added onto the amino acid and the a site all right so now we've already kind of done this little
peptidal reaction where we transfer to this amino acid from the trna and the p site onto the amino acid
of the trna and the a site what would that look like over here then after this process
occurred which was catalyzed by the peptidyl transferase in the
ribosome it would look like this so here we'd have our trna and would it have a here let's just
represent by an x does it have an amino acid no it's gone because we transferred it
then over here and that's in the p site here in the a site what would it look like
well now we would have that trna and it would have the amino acid isoleucine first
and then it would have the next amino acid that was added onto it which is the methionine right that's it
now what did i say that we had to do that was the first thing i said we had to do in this kind of elongation process
the second thing that we have to do is something called so we did kind of this like
peptidal reaction now we have to do something called translocation so the next step here is
called translocation and that's basically just kind of like moving things along
moving whatever was in the p site into the e site moving what was in the a site into the p
site that's all it is but in order for this to happen i need energy to generate this process
so what happens is i have this in the not an enzyme but a kind of a factor here
called a eukaryotic elongation factor type 2. and this eukaryotic elongation factor
type 2 contains a molecule called gtp we need that energy baby so it brings in this gtp and puts the
gtp into this reaction which breaks it into gdp and inorganic phosphate so this guy
brings them the eukaryotic elongation factor type 2 brings the gtp to this area where the ribosome and mrna
are interacting creates energy and then shifts what was in the a site into the p
site what was in the p site into the e site what would that look like then come over
here this should be in the e site which is my trna
with no amino acid bound to it and the p site what i have i'd have my trna which contains the
isoleucine and the methionine what would i have an a site nothing all right so now that we've kind
of moved and shifted or translocated the trna that was in that site into the e site
eventually because of that energy i generated i'm also just going to spit it out right i'm going to spit it
out of the e site and so now this is no longer going to be associated with the
mrna and the ribosomes it's going to be spit out and it'll go back up remember in the trna charging
it'll go back up and it'll get charged get a new amino acid added on to it and then it'll come back into the a site
eventually but after we spit that trna out that we have finished
what does it look like we'll come up here right if so what do we do we spit out the trna out of the e
site come back to this point here we now have if we were to take from this point what
was the difference from when we started we just added on an amino acid so now the only difference here is that i have
a amino acid added on to a trna in the p site then what would i do i'd have
another eukaryotic elongation factor bring another amino acid into the a site i'd have that then do
what have that amino acid and the a site attack the amino acids in the p
site pull them over when they pull them over that's catalyzed by the peptide transferase
then i'll use gtp to shift the amino acids at this point which would be now what three
in the a site into the p site then after i do that i'd spit the trna that i already
used out of the e site and i'd come back and i'd have three amino acids and then i would just
keep doing this process and going and going and going as i continue to elongate my peptide
eventually though you hit a certain point so let's pretend this trna has been going ham and you've
just been bringing in tons and tons and tons of uh amino acids and by this time it's it'll start to
look like this because you've gone through that elongation step like you know a thousand times at this point
and you got a nice long peptide at this point okay because you've gone through this
step multiple times eventually again we're in the p site here e site a site eventually you come to the third
phase of translation which is called termination termination eventually
you hit a stop codon okay and let's say that we used any of the three stop guns do you guys remember the
thing that the memory trick you go away you are away you are gone if at any point in time
you get a u r away you all go away you are gone in that a site am i going to have a trna
come in and interact no no trna will be coming into this step sir so no trna
with an amino acid will be brought into this step instead what am i going to bring in i'm going to
bring in something called a release factor so i'm going to bring in something called
a release factor a release factor has like a little pocket if you will
that'll come in and interact with that uag that stop codon it'll then prevent the ribosome from
continuing to move along the mrna continuing to translate it so it'll
bind to the stop codon stop the translation process and then what xing cleaved shiatsu that peptide
away from the trna that's in the p site so what else will it do it does three things what i want you to remember
binds the stop codon second thing is it stops translation third thing is it cuts peptide
in p site so then from here that release factor would then use its little shiatsu and
cut that bond right there separating the trna from the peptide and then what will happen
this peptide will then get released and then from there once we've released this peptide it can go and do whatever
it needs to do maybe it's going to get incorporated into the cell membrane maybe it's going
to be in the cytosol maybe it's going to be secreted we don't really care at this point we just know
that we terminated the translation process utilizing
a release factor to identify the stop codon stop the ribosome from moving along the mrna
and then cleaving the peptide from the trna and stopping the translation process but
now what i want to talk about is that this translation process can occur on what's called free ribosomes
or it can occur on the rough endoplasmic reticulum so we have to understand the differences between those two processes
so let's go talk about that now all right engineer so we've gone through we've built up the foundation talking
about mrna tr and arrna ribosomes we talked about the genetic code we went through the phases of
translation and we talked about particularly how translation is occurring on
ribosomes right with the mrna the trna we talked about all that stuff but here's the thing translation or
protein synthesis can occur on ribosomes that are just kind of like freely circulating in
our cytosol our cytoplasm or it can occur on membrane-bound ribosomes which are bound to what's
called the rough endoplasmic reticulum and you guys should be asking when do i do it on the rough er when do
i do it on the cytoplasm and we'll answer that because it's a good question
for the most part the simple answer is that when it occurs on the rough endoplasmic reticulum
that is for proteins that are either going to be secreted from the cell incorporated into the cell membrane or
proteins that are going to become incorporated into lysosomes so three reasons why it would occur on the
rough er and not in the free ribosomes is secreting the protein embedding it into
the membrane and becoming a part of lysosomes so now let's talk about the difference
between the translation process that occurring on a free ribosome and when it has to bind or translocate
from that cytosol where it's a free ribosome to a membrane-bound ribosome
there's a very important process that we have to talk about so let's pretend here that we're covering this it's the same
thing that we've already gone over you've taken dna and you transcribed it when you transcribed it
you made it into mrna right so we took and you made mrna the mrna was then
gone through its modification got sped out of the nucleus and came into the cytosol
and bound with a ribosome starts getting translated we've already gone through it goes through the initiation elongation
process and it's making these peptides that are coming out of what site
the p site right as it's synthesizing these peptides there's about a sequence of amino acids about maybe
nine to ten amino acids that become an identifier on this peptide and this is represented by
the orange portion so we can we're translating it just like we did over here
we're just continuing to go through the elongation steps and making a long peptide
there's a sequence of amino acids on that peptide that is recognizable by a very specific protein that is kind
of floating around in our cytosol this sequence here it's not hard is called the
signal sequence okay but it's important to remember the signal sequence is what
amino acids so let's make sure that we understand this is amino acids it's not any type of
nucleotides or anything like that it's amino acids we're making proteins peptides amino acids make up peptides or
proteins and you make a very specific sequence of them that is recognizable by a protein
what is that protein that's going to be kind of floating around out here let's do it
here in purple there's a protein that's kind of just floating around out here
and it is going to come and recognize that signal sequence what is this called this is called a
signal recognition particle or protein that's all it is
so this is the signal sequence the signal recognition protein or particle will bind to the signal
sequence that's it once it binds it then has a high affinity
for these receptors that are located on the rough endoplasmic reticulum a very very high affinity for these
receptors that are located on the rough endoplasmic reticulum so what is this here called signal recognition
particle will identify the signal sequence on the growing peptide from the translation
process that occurring on the ribosomes once it identifies it it binds it and then starts
dragging it towards what this membrane here what is this membrane here this membrane is the rough
endoplasmic reticulum membrane right so if i were to kind of show that here like a general way let's say here
if i took a cell i took a cell for example here's my nucleus here's my dna
i make my mrna comes out here's your ribosome the mrna will interact with the
ribosomes and then the translation process that we talked about over here was just basically occurring on that
free ribosome but if we wanted it to occur on the rough endoplasmic reticulum that
would be kind of like over here and we'll just kind of represent this by these like lines over here we're not
going to get too fancy what would happen is we're going to move this
ribosome mrna and the growing peptide towards the rough endoplasmic reticulum membrane
which we're just zooming in on right here okay so if we zoom in on it this is what we're going to get
on that membrane are two proteins that i need you guys to know two proteins that's it
this one right here is the pink protein and this is called the signal recognition particle receptor not hard
that's it so what do you think the signal recognition particle receptor is going to bind onto
the signal recognition particle or peptide so now let's draw that purple protein here kind of
binding here with the signal recognition particle which is then bound to what bound to the
signal sequence and the signal sequence is from the growing peptide so here we're going to show kind of like our
ribosome here here's the large here's the small and then what's going to be kind of
in between here sandwich between it that's getting red right now the mrna and if we were to just kind of
show this here here's our protein that's being kind of synthesized out of here and there's one
particular thing that's on the end of it which is what the signal sequence and the signal sequence is bound to the
signal recognition particle which is bound to the signal recognition protein receptor
that's it okay after that process occurs this molecule right here this protein that we haven't talked about yet this
black protein here is called the translocon this protein is called the trans
locon now in this state right the translocon is closed nothing has kind of triggered it to open
yet it is closed so signal recognition particle binds the signal sequence brings it towards the
rough er binds it with the receptor and the translocon is still closed
how do i get that translocon to open let me explain how here's my signal recognition protein or
particle receptor i know this is a lot and we're going to just keep it's going to be a good review
here bound to it is going to be the signal recognition particle bound to that is going to be the
what the signal sequence from the growing peptide chain so here we will kind of just represent
the growing peptide chain and then what's going to be over here my ribosome right
and my ribosome is going to have my large my small and then what's sandwiched in between it
the mrna good now the signal recognition particle and the signal recognition
protein receptor particle receptor contain gtp molecules bound to them
okay they contain gtp molecules that are bound to them when this is bound nice and snug with
each other the gtp molecules get broken down into gdp and inorganic phosphate so how
many gtps are we actually going to break down in this process two that's important because they're
each one are associated with the particle and the receptor i'm gonna break these
down into gdp and inorganic phosphate bake this one down to gdp
inorganic phosphate that's breaking down two total gtps in order for this process to occur
when i break that down what happens to the translocon you guys see the translocon was closed
it was still closed here so the translocon was still closed in these two states but once i broke
down the gtp into gdp and inorganic phosphate i created energy and what happens to the
translocon once this happens the translocon opens because it was dependent upon
breaking down the gtp into gdp and what in inorganic phosphate okay now the translocon is opened
what do you think i'm going to do i'm not going to draw all this stuff here again because we don't really need to
know that we opened the whole thing that we talked about is the same i'm just going to
continue to keep taking that ribosome here we'll just draw the ribosome the ribosome is going to kind of really
line up perfectly like this it's going to line up perfectly with the translocon and now that peptide that was
growing is just going to kind of get pushed right through the translocon
into what this whole thing this whole thing right here is the lumen
of the rough endoplasmic reticulum all that it's just going to start getting pushed into the lumen of the
rough endoplasmic reticulum so i'm going to have this peptide getting pushed in here and what was at
the end of that peptide what was there the signal
recognition i'm sorry the signal sequence so here i'm going to have that signal
sequence now since the signal sequence is kind of inside the lumen at this point
do i need my signal recognition particle anymore no so what can i do spit him off go back
and bind another ribosome and bring him here to you know to another site so while i spit off right here i'm going
to spit off also in this step my signal recognition particle i don't need him
once i start have this growing peptide line up with the translocon the peptide gets pushed into the lumen
there's another little freaky little enzyme in here that loves to identify the signal sequence and cut him off
so that we don't have him in there anymore because we don't need him he was primarily needed just to bring the
peptide the ribosome to the rough er we don't need him for anything else
so this enzyme beautiful cute little enzyme that's inverted is called a signal
peptidase thank goodness that's an easy name right and he comes over here and he cuts off
the signal sequence and when he cuts off the signal sequence that signal sequence
will just kind of get spit off over here and there's going to be some enzymes that'll come and degrade
that signal sequence into amino acids because we don't need them but what happens is that peptide is just
going to continue to keep going and being translated and translated and translated through the elongation
process until what happens till we hit a stop codon you go away you are away you are gone right remember
that little trick once you hit that stop code on translation ceases
and what do you do this was once lined up continuing to push the peptide in
continuing to push the peptide into the cell we already broke off the signal sequence so we don't have that anymore
more once you hit the translation process that stop codon the translation will
stop occurring the translocom will close and the ribosomal subunits and the mrna will disassociate away from
this site so what happens the translocon closes the peptide
is released into what into the rough endoplasmic reticulum's lumen into rough er
lumen and then the ribosomes and the mrna will disassociate okay and they'll actually
go and get degraded as well that is because why does this happen because at this point you hit a
stop codon and once you hit that stop code on it terminates the translation process
closes the translocon releases the peptide with no signal sequence on it into the rough er lumen
and then the ribosomes and mrna will disassociate and that's covered the ribosomal translocation process now
really quickly when you have this process ribosomal translocation coming and binding with the rough er
i want you to know why we already talked about the three reasons why this would occur it's proteins that are
going to be secreted proteins and that's why because they need to go to the rough er
then to the golgi make a vesicle and then go and get excreted or get incorporated into the membrane
second thing is they're going to be a membrane protein and the last reason is that they'll become lysosomal
proteins okay these are the three reasons why it would be
rough er ribosomes okay i need you guys to know that and it's a really simple process because
whenever if you guys know come back to this diagram over here if i take a protein it gets synthesized
in the rough er then where does it have to go to the golgi
then from the golgi it has to get packaged into vesicles and those vesicles can either go to the cell
membrane get incorporated go to the cell membrane get excreted or they can become
a lysosome okay so that's the whole purpose of why we go through that process with the rough er
what about the other ones i know you guys are probably like well zack what about all that
free ribosomes that don't bind to the rough er what what how do where do they go what do they do
if we talked about let's say that we kind of use a line here and say that these structures are where the proteins
are going to be incorporated into these are going to come from
free ribosomes and the ones that we already talked about these proteins that will be either
be incorporated cell membranes secreted or become lysosomes are going to be rough er
ribosomes we already know the ones for the rough er secretive proteins membrane proteins lysosomal proteins what about
the free ribosomes where are those proteins going to go if it's just in the
free ribosomes these proteins will be for cytosolic proteins what are the reasons
that we have cytosolic proteins just use a very simple example a lot of the metabolic processes that
occur in the cell glycolysis that occurs in the cytoplasm some other steps that occur in the
cytoplasm we need those proteins to catalyze things that are in the cytosol the second one proteins that are
incorporated into the nucleus different types of enzymes that are involved in things that
are involved in dna transcription things that are involved in replication and modification of things
so we also need them for nuclear proteins proteins that are actually going to be
involved in the mitochondrial processes certain metabolic processes that are involved there
so mitochondrial enzymes and the last one is enzymes that are very very important
catalases and a bunch of other enzymes that are involved within peroxisomes so peroxisomal enzymes
okay very very important to remember those things okay so free ribosomes gives ways to
cytosol nuclear mitochondrial peroxisomal proteins and rough vr gives way to membrane-bound
proteins lysosomes and excreted proteins simple as that we've now made the protein we've either got it
we've either made the protein via the free ribosomes or we've made the proteins from the
rough endoplasmic reticulum now what do we got to do we got to modify the protein a couple different ways let's
talk about that very briefly all right guys so at this point in time we have gone from
dna we transcribed it we made mrna then we translated and made proteins in this case we made a protein
we went through all of these stages in sequence of videos transcription and then translation in this video now
what we're going to do is we've got to take this protein that we've synthesized whether it was via
the free ribosomes or whether it was via the rough endoplasmic reticulum ribosomes
and we have to modify them a little bit in other words we add things on or cut things off that's it add on cut
off let's give some examples we're not going to go too ham let's say on one of these i add a
sugar residue i'm just going to represent that with a g what does this call when you add a sugar
residue onto a protein like oscillation so that could be a reaction called glycosylation
and we'll talk about a couple examples of these very briefly a little bit later but that's one thing i add a sugar
residue onto these proteins the next thing i could do is i could add a lipid
onto these proteins what do you think that's called lipidation here we'll just kind of represent like this
little thing called lipidation and we'll talk about reasons that this is important
the next thing we could do is we could add on a phosphate groups so we could add on
phosphate groups so we'll just kind of show here phosphate groups what is this called phosphorylation
we could add on hydroxyl groups what is this called hydroxylation okay
what else could i do i could add on like a methyl group here let's put down a couple of methyl group i could add
acetyl group or i could cut some some of the amino acids off so let's put
cut or trim some of the amino acids off so what would this be called if i add a methyl group on this is called
methylation what would it be called if i added an acetyl group on not hard right an acetyl group you would
call that acetylation and the last thing is i could cut so here i would just
represent maybe i'm going to cut some of these amino acids out of the reaction if i cut some of these
amino acids off okay what is that called that's called
trimming we actually specifically we call that trimming now these are the basic kind of most
important types of modifications that you truly need to know when you're taking a protein and doing
things to it but glycosylation lipidation phosphorylation hydroxylation
methylation acetylation and trimming what are examples of those that's kind of the big thing that you
really should know not going to go ham on it but just think about examples if i took a protein and i added a sugar
residue onto it what would be a reason that i would want to do that the best example that i can
think of is antigens okay so you know like your red blood cells
your red blood cells you have different antigens like a antigens b antigens rh antigens
those have sugar residues on them they're proteins with sugar residues on them
and they help to identify what's what type of protein that this has on it which can determine your blood type
right so that's an example so it can be good for identifying particular proteins or antigens specific to a cell
also good for transporters you know transporters different types of channels like glut channels that we
talked about in this membrane transport or other different types of voltage-gated ion channels those can
sometimes have some sugar residues on them lipidation these are good for proteins that are
going to be incorporated into the cell membrane so these are going to be lipid proteins are
good for cell membrane as well as organelle membranes for example the rough endoplasmic
reticulum that's a that's a phospholipid bilayer which we could use some proteins with
sugar lipid residues on them the golgi the smooth endoplasmic reticulum things like that
or the cell membrane itself phosphorylation this is a really big one i
really need you guys to remember these use the example that we've talked about like a million times like protein kinase
a or cyclin-dependent kinases things like that we've talked about a lot in other
videos these guys add phosphate groups right so if you had a protein here and we added a
phosphate group that could either activate the protein or it could inhibit the protein
and that's important in a lot of cells like you know your cell cycle when you go from your g1 to your s phase
to your g2 phase through mitosis we phosphorylate particular proteins that modulate that activity or modulate
cellular signaling pathways so this is very very important hydroxylation
is very very key for making collagen collagen synthesis collagen is extremely important because
it's incorporated into our bones our cartilage our connective tissue our basement membranes
and hydroxylation is one of the biggest ways that we make collagen okay methylation acetylation
this is best talked about and i know you ninja nerds know this we've literally just talked about it in dna structure
and organization if i methylate a histone protein what do you do if you add a methyl group onto it
does it decrease transcription or increase it keeps the interaction tight so is it can an rna polymerase fit
between that no so that would decrease transcription if i put an acetyl group onto it it
relaxes the dna increases the space the rna polymerase can come in read it and does what increases the
transcription so something as simple as modifying our protein in that way can make a huge
difference and my favorite example is trimming i like to think about this as let's say
that you just worked worked out you got yourself some gains you're going to go home and eat chicken breasts you know it
tastes like a bike tire you know because you know sometimes chicken isn't that good but anyway you're getting
trying to get your gains you're getting your protein and when you do that the protein gets
into your small intestine and you have a particular enzyme called trypsinogen you know enzyme called
trypsinogen trypsinogen it's kind of like the precursor
it's not active but if i take and i use an enzyme that cuts the trypsinogen and turns him into
trypsin i'm going to cut a piece of it this is the inactive protease this is the active protease
if i activate him by cutting some pieces off of him now he can go and shiatsu the proteins that i ate from the chicken
so that i can absorb it that's kind of the simple examples of how we can modify proteins
that they can either become activated deactivated be incorporated into a membrane
be particularly an antigen all these different things so that's taking proteins and modifying
it away for particular cellular examples and that concludes our video on
translation of protein synthesis all right ninjas in this video today we talk about translation or protein synthesis i
hope it made sense and i hope that you guys enjoyed it alright engineers as always until next
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