Understanding the Cell Cycle: Stages and Importance Explained

all right ninja nerds in this video we are gonna talk about the cell cycle the cell cycle is so important why because

the cell cycle which we're gonna talk about interphase and mitosis is the series of phases and steps that a cell

goes through to replicate itself so we're gonna turn one cell into two cells and this is an important important

process not only is the cell cycle it just important for being able to replicate cells but it's also important

to be able to control cell growth we'll talk in another video about the regulation of the cell cycle will talk

about proto-oncogenes we'll talk about tumor suppressor genes and we'll talk about DNA repair enzymes and genes okay

but in this video we're gonna discuss the cell cycle so we're gonna go through the various stages of interphase then

we're gonna go through mitosis and then another thing for you guys is during the mitosis part I'm gonna show you what's

going on in the board but just to get a different view we're gonna take models that are gonna show

you guys a little bit more of what it would look like in the cell during prophase metaphase anaphase telophase

okay so let's go ahead and get started on the cell cycle before we do that how would you describe a cell what is a cell

a cell is basically it's the basic unit of all living things and the cell is classified by technically having three

different things so this is important to remember a cell is classified by having three different things what are these

three things generally since we're talking about eukaryotic cells because there's eukaryotic and prokaryotic cells

right we're gonna talk about eukaryotic and specifically human cells they have to have what's called a cell membrane so

they have to have a cell membrane and remember that the cell membrane is a phospholipid bilayer right that is

actually surrounding the entire structure it also has to have a nucleus where it houses its genetic material

okay in the form of chromatin which is the DNA wrapped around different types of histone proteins and the last thing

is you wanted to have cytoplasm this is the three basic units that are needed for an actual cell so a cell is

made up of three different things a cell membrane a nucleus and a cytoplasm what we were going to do is we want to take

this and make another one an identical cell in the nucleus we have a structure though we just we briefly described it

here and we said its DNA all right so we're gonna take the DNA during this process of the cell cycle we want to

duplicate the DNA we want to replicate it we want to synthesize a new double-stranded DNA and we're gonna talk

about that in this video so let's go ahead and get started here so the first part of the cell cycle let's say we take

a normal cell alright a normal cell that cell is gonna get ready to go into the cell cycle what's the first point that

it'll go into in the cell cycle the first phase is called the g1 phase so it's called g1 phase sometimes you might

even hear it referred to as gap one it's the gap one phase so it's either g1 or gap one phase now

in this phase what is the cell going to be doing so now let's pretend we take a cell right so here we're gonna have a

cell in the cells entering into this phase here alright it's entered into the g1 phase now cell we said has a cell

membrane right it has a nucleus which houses genetic material and around that has the cytoplasm well the first thing

we're going to want to do is is we have to be able to get this cell ready so it can replicate

right we want to take one cell and turn this one cell into two cells that's the whole goal and we want them to be

identical not only just identical but how is the same amount of genetic material

so in general we know that this is a diploid you know in all of our cells we have our chromosomes right and there's a

total of 46 chromosomes 23 of them are maternal at 23 over the paternal we want to be able to pass the chromosomes down

so we have to duplicate it in order to duplicate it we have to have both of these cells also be diploid so we have

refer to to in as diploid meaning that has a total 46 chromosomes so two men is

representing 46 chromosomes the end is n is basically representing the number of chromosomes and again we have 23

maternal and 23 paternal so if we take that 23 times 2 is going to give us 46 total chromosomes and then what we want

to do is we want to replicate this into two identical cells with the same number of genetic material same number of

chromosomes that is mitosis all right but in order for us to go into mitosis we have to have this first part here

called interphase and we'll talk about this all right so now first thing for the gap one phase if we need to be able

to replicate these cells what should I do well you know another thing that these

cells have our eukaryotic cells have is they have different organelles like ribosomes they might have mitochondria

you know they can have different types of organelles so the first thing we should do is we should increase the

number of organelles let's make more organelles so the first thing here that we're going

to want to do here is make more organelles okay cool what else they're gonna want to do well

you know inside we said inside of this actual nucleus what do you have you have your genetic material your DNA well you

know there's a process we'll talk about it it's called DNA replication in order for DNA replication to occur we need to

have certain types of enzymes certain types of proteins right and an order for an even transcription factor so if

that's the case then what do we need to start doing we need to start preparing the cell by making tons and tons of

different types of enzymes so we need to start synthesizing proteins and enzymes now because we're gonna start making a

lot of protein and enzymes to help to aid in this actual DNA replication process we have to say one more thing

sometimes ourselves most of our cells hey this is another important point you know most of our cells usually exist in

the g1 phase most of the cells stay in the g1 phase so out of the cell cycle if you if you were to ask if you were asked

which out of the whole cell cycle which phase is this cell most likely in most of the time it's in the g1 phase because

it's variable for certain types of cells what do I mean for certain types of cells they might only be eight hours

that existed in this phase other cells it might be years you know there's different types of cells we should

actually talk about that let's come over here for a second will deviate for a second but we'll come back there's three

different types of cells that I want to talk about one are called labile cells or another LOI I like to think of them

as proliferative cells I'd like to think about them as proliferative cells so what are lay bio cells are proliferating

cells think about it simply out of your whole body where your cells constantly proliferating they're constantly going

through the cell cycle all of the time right here we're constantly shedding skin cells so all the stratified

squamous epithelial tissue on your epidermis and where else in the GI tract in the urethra the vagina many different

places that's constantly undergoing replication so for these lab ourselves what can I say we could say the

epithelium of skin where else the GI tract and maybe even the urinary tract so even the urinary

tract okay in other places this is the coolest one I like this one if you think about it we have to constantly be making

red blood cells and white blood cells and platelets all the time so because of that you have to have some type of stem

cell that's constantly replicating and producing more of these blood cells what is that cell called it's called a

hematopoietic stem cell so you know our hematopoietic stem cells that are located within your red bone marrow

they're also lab off cells so what are they called they're called your Hamato poetic stem cells that are in the red

bone marrow the red bone marrow these two types these basic types of cells these labile proliferative cells they're

constantly going through the cell cycle now there's some cells that they don't want to go through the cell cycle all

the time they're kind of stable of just resting staying in a kind of like I just not really doing anything a kind of like

a resting area those types of cells are called stable cells so what are they called they're called stable cells now

stable cells stable cells if we think about these guys they're okay with not having to replicate that often they

replicate when the stimulus is strong enough when there's a strong enough stimulus so these guys don't necessarily

replicate a lot but they can if the stimulus is strong enough like different types of growth factors to push them

into the cell cycle so what are these different types of cells the liver oh my goodness the liver is such an amazing

organ you want to know why because if you can now you can take a good portion liver almost 40% of the liver and what

happens is let's say I cut 40% of my liver off my liver can regrow itself that's one of the beautiful things about

the liver there's different types of growth factors that the actual liver cells will release to make more liver

cells so your liver is real good I think the hepatocyte s-- within the liver are really stable self so

let's put here patio sites alright so your have pata sites within the liver what else other ones is like your kidney

tubules you know what the epithelial cells within the kidney tubules those are also stable cells but if we have a

stimulus necessary to push them into going into the cell cycle they can so the epithelium of the kidney tubules

okay like your proximal convoluted to be a loop of Henle all those different types of things right and then if you

want even the alveolar cells of the lungs right now there's the last one and these are the ones that pretty much

everybody usually knows you have a last one and these are called permanent cells so these cells once they go through the

cell cycle they don't ever go into it again what are these again we said these are your permanent permanent cells and

these ones are the usually the ones that people usually remember and in other words we call these they're a mitotic in

other words they don't undergo through might they don't undergo mitosis these are your neurons so your nervous tissues

alright so your neurons what else you know your skeletal muscle that's another one your skeletal muscle cells so your

skeletal muscle and another one your cardiac muscle the one that's responsible for the heart so the

myocardium right so the cardiac muscle so it's really important to understand the three different types of cells

because some cells are going through the actual cell cycle very often labile some will go through the cell cycle if they

have a proper and strong enough stimulus stable and then some of them will not go into the cell cycle and that is the

permanent cells okay now that we understand that one thing we need to talk about for the g1 we make more

organelles we synthesize proteins and enzymes but we got to do one more thing sometimes these cells can have certain

types of damage they might have certain types of problems sometimes they call them five

minima Dean dimers so in the g1 phase you want to be able to prevent or repair these things called thymidine dimers so

you have different types of enzymes that can actually scan the DNA because you want to make sure that before you start

replicating the DNA there's no stakes within the DNA so sometimes people can get Diamond Dean dimers and what you

want to do is you want to repair those time adding diamonds before you get ready to replicate the DNA so in the gap

one phase or g1 phase we make more organelles in the cell we make more proteins and enzymes to help to

replicate the DNA and we repair any thymine dimers that when we go into DNA replication there's no mistakes

previously okay and the reason why you're making more organelles why because you only have right now

organelles for one cell you need to make organelles for two cells that's the whole purpose there okay from the g1

phase where does it go into it's gonna go into this next phase the S phase the S phase stands for synthesis so this is

the synthetic phase the synthetic phase or the ass phase now what happens in the S phase we've kind of already talked

about it right what we're doing here is we're taking a cell all right let's say I take this cell and I take the genetic

material you know there's the genetic material right here let's say I'm taking this genetic material here's the DNA

what am I trying to do with this DNA I'm trying to we'll talk about this in a separate video but what I want to do is

I want to take this DNA I want to open it up so I want to open the DNA up and form what's called a replication bubble

all right you get this thing called a replication bubble and then what happens is I want to be able to synthesize new

DNA based upon whatever nucleotides I have here so I'll make a whole new strand and this is going to be what's

bio it's called the semi conservative model so what I want to do in this phase is I want to take and replicate that DNA

so in this phase the primary thing that is occurring is going to be DNA replication what's really cool about

this DNA replication is it's maintained by specific types of enzymes there's what's called DNA polymerase a--'s and

there's two types type 1 and type 3 now these enzymes are so good at their job so so good that generally they're

replicating the DNA so fast but very faithfully they don't make that many mistakes you know sometimes they can

make a mistake every million or billion base pairs that's insane so they don't make very

many mistakes that often but we still want during this this synthesis phase we want to make sure that there was no

errors in replication so sometimes there certain genes we'll talk about that and they're called tumor suppressor genes

and and also DNA repair genes and we have other genes that can read the DNA we'll talk about all these things but we

want to make sure that whenever we replicated the DNA that there's no errors so we're gonna want to fix that

we'll talk about different checkpoints alright so synthetic phase RS phase we know what

it's doing it's replicating the DNA all right so we're replicating the DNA and technically if you want to remember for

replicating the DNA or going from 2 in in to 4 in right because we're taking it from a total of 46 chromosomes in one

cell and doubling it and if we're going from 46 and doubling it you'll have 92 chromosomes right and 46 will go to one

cell 46 will go to the other cell that's the whole purpose here another thing is how long does this phase take we said

that that one can vary from eight hours to years it depends on the type of cells but this one is usually constant in

duration usually it's about six hours this phase usually is approximately about six hours okay so now we know the

gap one phase and we know the S phase remember I told you though that before we go into the S phase we want to make

sure that the DNA is okay because we don't want to waste energy and time on replicating DNA if it's not even good so

what we'll talk about in another video and the regulation is there's a little checkpoint right here right here there's

like a little checkpoint where we're gonna stop this cell and just check it to make sure everything is okay that

checkpoint is called the g1 s-phase checkpoint and again we'll talk about the regulation through tumor suppressor

genes and proto oncogenes and stuff like that but I just want you to get an idea of what's happening within the cell

cycle okay so g1 in order for the go to S phase it has to have this checkpoint

where we kind of check the DNA make sure that there's no issues make sure that there's enough proteins and enzymes or

organelles for it to go and replicate after it replicates though now we have to we have a cell here right we have a

cell at this point time now who not only he's actually going to be what he's no longer gonna be - in this cell is going

to be four in total of 96 99 other thing we need to do here this next face there's g2 phase what color should we do

let's do this one this is the g2 phase or gap - phase okay now in the g2 phase this one's kind of a simpler phase we've

already done what to this cell we've already replicated the DNA we've already made more organelles alright so let's

just assume that those are ribosomes we had one we went to two we had a mitochondria right here and what do we

do we went to two so we already made more enzymes we made more organelles we replicated the DNA we checked for any

types of damage now what do we got to do is this enough cytoplasm is this cell big enough to split into two equal cells

has to be perfect right our cells are very particular right so because of that we want the cell to grow in size so in

this phase the main function of this phase is primarily focused on cell growth that is its primary function the

primary function of this phase is to regulate cell growth by doing what increasing the cytoplasm

and the different types of components within the cell to make it big enough that whenever we pinch these two this

one cell into two cells it's equal we want it to be perfect okay so what are we up to now we did gap one we did S

phase we've done gap to face our G to face these three make up a whole phase if you will and that whole phase is

called interphase so again I want you to remember interphase is made up of to come up sorry three components what are

those three components one is g1 the next one is s and the last one is g2 and in order it goes g1 - ass ass - G - okay

now and remember remember that one point right here before we go - from g1 to s you have to have a g1/s checkpoint okay

and we'll talk about that in the regulation of cell cycle now we finished the interface we have to talk about

something else now now we have to go into what's called mitosis the M phase so again let's come up here and write up

here mitosis mitosis are sometimes they refer to it as the M phase in mitosis you have to remember that there's

specifically four parts and there's technically a fifth part in there we'll discuss it but you're gonna have P

mapped okay P matte and there's another one here which is going to be cytokinesis that's kind of a part of

telophase we'll talk about it but P is for prophase M is for metaphase a is for anaphase and t is for telophase and

there's a part here which we'll discuss which is like the end of telophase which is called cytokinesis where we'll

separate the cytoplasm equally so let's go through the first part here prophase okay so here's what you have to remember

when we were going through this wrap location this whole interface the genetic material inside of the cell so

what is this inside of the cell what should you have inside of it you should have a nucleus right but inside of the

nucleus it was has a bunch of different it has a lot of DNA the thing is though the DNA originally

was really loose it was loose DNA we also call this loose DNA call it you chromatin but here's the thing in order

for us to be able to separate the DNA properly the chromosomes we don't want it to be loose we want to condense that

chromatin so what is this first phase here this first phase here is called prophase

okay so prophase and again what did we say we said that the actual chromatin what is chromatin how would you define

chromatin chromatin is actually two basic things one is it's your DNA and the other one is your histone proteins

will talk about these when we talk about how DNA is organized into what's called nucleus ohms but there's many different

types of histone proteins but all chromatin is is we're taking DNA and wrapping it around these histone

proteins like octamer x' of them so what I want to do is I want to condense that chromatin so let's condense that

chromatin now and when I condense it you're gonna get something which is going to look kind of like this it's the

easiest way to represent it you're gonna see what's called these chromosomes so you're gonna see these chromosomes and

they're gonna be nice and condensed so there's my chromosomes now what did I tell you a cell has to have it has to

have a nucleus but here's the thing if I want to I've already duplicated the DNA right because before it would look like

this pretend here was the cell before it was going in it would look like this it would have before it would only had one

chromosome right before I went to the S phase then after the S phase it would actually replicate and make two

chromosomes now from here we want to be able to separate these chromosomes into opposite ends into two cells so should

we have a nucleus blocking it now because if I have the nucleus blocking this there's no way I'm gonna be able to

separate these into two ends of the cell so guess what the nuclear envelope is actually going to get

dissolved there are special types of cyclin dependent kinases and things like that that will phosphorylate different

proteins of the nuclear envelope like for example they'll phosphorylate like lamins

they'll phosphorylate some of the histone proteins like h3 a there's even other proteins here too that they can

phosphorylate that are a part of the nuclear envelope right so different parts of the nuclear envelope it's going

to phosphorylate these guys and when you phosphorylate them it sets up specific enzymes to break

them down it activates certain proteases so there will be some specific enzymes that will phosphorylate different

proteins of the nuclear envelope like lamins and histone proteins and other different types of proteins and cause

them to get degraded by proteases so the nuclear envelope is gonna start dissolving what else is gonna happen you

know you have these other things here right you start seeing these these structures that are part of the

cytoskeleton and these aren't here you're gonna start forming these things called your microtubule organization

center you know have these things called centrioles so you have these things called centrioles and these centrioles

are gonna be important for forming what's called the microtubule organization Center so what is these

things right here called these are my microtubule organization Center M TOC microtube the organization Center so

three things have happened one thing i condensed the chromatin second thing I start dissolving the nuclear envelope

the third thing I start seeing the appearance of these things called centrioles or centrosomes and it's going

to be we're gonna call them the microtubule organization Center because from these the actually gonna have these

things called polar and astral microtubules guess what they do they connect to the chromosomes to help to

separate them okay so we got prophase that's the first part now we go to the second part the

second part is going to be metaphase now in metaphase what happens here you're gonna have the nuclear envelope

should now be dissolved right but what's gonna happen is remember that microtubule organization Center it's

gonna start going towards during this process of where we get to metaphase the microtubule organization centers start

taking up residence in the opposite ends of the cell the different poles of the cell so one will see right here and the

other one will see it the opposite pole of the cell so here's the one pole to cell here's the other one what did I say

comes from these organization centers these microtubule organization centers these different microtubules you know

there's microtubules that go to where the actual chromosomes are and there's ones that actually come off like this

those are called your astral microtubules and these are your polar microtubules now what do we say should

be in here we should have the chromosomes so let's actually show here here is our chromosome we're here we'll

have another one here right so here's our chromosomes now since we have the chromosomes what should be connecting

the chromosomes to these actual micro - of organization center we should have these microtubules connecting here now

we need to come up with a little definition here because sometimes people get confused alright so a chromosome

when we talk about a chromosome it's actually right here here's a chromosome right so chromosome how would you define

a chromosome a chromosome again is actually made up of chromatin DNA and histone proteins so like in this I'm

gonna have DNA moving in throughout it right but a chromosome has a short arm and a long arm right so it usually the

short arm is up on top long arm on the bottom right but more important part the ends of it the ends of the chromosome is

called your telomeres this is a telomere and this is a telomere and the center of it is what's called your centromere the

centromere determines the number of chromosomes you have so for example let's pretend I'm get I'm just going out

there with this how many chromosomes do I have one even though this thing is a freaking freak of nature it's still one

chromosome because we determine the number of chromosomes by how many centromeres we have but a better way of

describing this is we take that replicated part here right so pretend here and here was the old DNA well

generally it's actually actually that's wrong because if it's if we actually replicated it it should be by the semi

conservative model right so we should have old and new mixed in so here I have one strand that's the old strand

here's another old strand and then what should you have here you should have a new strand and a new strand this is one

chromosome but the two individual components of that chromosome what do you call these two little things here

what is this guy and what is this guy these are called sister chromatids okay sister chromatids but this whole thing

is a chromosome all right the whole thing is a chromosome but these two individual entities is the sister

chromatids but this whole thing is a chromo so all right so just so we understand it I wanted to make sure that

we really get an idea of that okay so now we're going back to metaphase so from here these polar microtubules what

are these guys right here these are called your polar micro tubules here's your chromosomes and here's your

microtubule organization Center the microtubules are now connected to the chromosome we got actually be specific

at what part of the chromosome what we said we had the centromere right so if we said here we had chromosome

chromosome like this there's a protein a protein structure that's right on the outsides of it right here you know what

that structure is called that that purple structure they call that the kinetochore canítö

or it's a protein structure and guess what connects to the kinetochore the microtubules those polar microtubules

they connect to the kinetochore imagine them like a hook right because what they're gonna do is they're gonna hook

one sister chromatid hook the other sister chromatid and separate the suckers right so what we need to do is

is we have to have these polar microtubules connecting to what structure again what's this purple

structure the kinetochore okay now once they're connected at the kinetochore you're gonna notice

something I've only drawn to here but imagine there was tons of these bad boys all of them lined up in a row and

they're lined up kind of like along this mid line if you will they're kind of lined up or along this mid line or

another way of saying it is on the metaphase plate so they're aligned very very perfectly all of them are aligned

perfectly what are we gonna do now okay now we've set up the stage to start separating them okay so a metaphase we

aligned them up on the metaphase plate we have the polar microtubules are connecting to the kinetochore of the

chromosomes and we're gonna separate those sister chromatids okay so now it's going to the next step

the next stage is anaphase you can remember away so sometimes how they remember this is metaphase in the middle

or metaphase plate anaphase is their going away from one another right so what should I have over here again I

should have my microtubule organization center all right microtubule organization Center and then what should

I have coming over here and connecting I should have will draw three this time since we only did two last time what

should I have it connecting to let's say right here I'm going to have my chromatids because what am I going to do

remember that centromere there I'm going to split the two I'm going to split the two there's a protein that's connecting

them together called cohesin I'm gonna split the cohesin and there's a special regulation point of that I'm gonna split

the cohesin so I can take this sister chromatid go to this pole this sister chromatid go to that pole so now look

here chromatid is gonna be coming over here this chromatid is gonna be coming over

here but really this is a chromosome the sister chromatids were separated but now how many central means do I have one so

that's a chromosome then what do I have over here another chromosome what do I have over here another chromosome

another chromosome so now what am i doing I'm separating the chromosomes for one another because eventually I want

all these chromosomes to go to this end I want all these chromosomes to go to this end because originally what was

this whole thing for in there's a total of 92 chromosomes I need 46 of them to go to one end 46 of

them to go to the other end so that's what we're doing here it's just so darn cool all right so we're separating these

two opposite into the pole so where will these guys be going they'll be going this way now an important concept here

we're not gonna go into super depth on them but how the heck do they get there that's how important thing with science

is you have to ask yourself the question sometimes why are these things happening so you know there's different types of

proteins here I call them motor proteins so special types of motor proteins we're not like I said we're not gonna go into

super depa that's once you get the idea there's motor proteins and these motor proteins can literally walk along the

microtubules carrying whatever structure they have with them towards a specific direction isn't that cool

so there's different motor proteins that can move these microtubules towards the actual microtubule organization Center

to the opposite ends of the poles what are these things called again they're called motor proteins there's

particularly too in this situation one is called dining and the other one is called kinesin technically this is a

minus in directed motor protein and this is a plus and directed motor protein I'm just throwing out there you don't

necessarily have to know this I just want you to get the idea that there is two motor proteins dynein and chi Nissen

and what are they doing they're helping to move these actual chromatids towards the microtubule that's important so now

we've done anaphase we've separated the actual chromosomes now once we've done that what do I need

to do I need to equally distribute this into two cells so what this cell starts doing you have different types of actin

and myosin proteins here let's put here you have these different types of actin I'm going to represent this with like

red here's some myosin proteins or contractile proteins here's some myosin proteins which are contractile proteins

and then let's say near it we have some actin molecules so here's some actin molecules which are contractile proteins

these guys start contracting the cell and they produce this little constriction ring so we're gonna try to

take this cell and just squeeze it when I try to squeeze it to push this stuff into the amount of stuff and equally

into both cells I produce this little constriction ring but they don't like that name they call it a cleavage furrow

they call this right here a cleavage furrow okay and it produces this thing called the constriction ring now it

looks like I'm getting ready to have two cells all right so now what am I gonna do remember what we had before we didn't

have a nuclear envelope guess what we start forming again guys I don't know why I get so excited about this stuff I

just think it's so cool but you start actually beginning to reform your nuclear envelope so now you want to get

ready for this cell to be complete so you start reforming your nuclear envelope you start pinching and forming

this constriction ring called the cleavage furrow through myosin and actin proteins then what what should you have

over here you should have your chromosomes how many should be over here there should be a total of 46 here right

or we say 2n how many should be over here a total of 46 we say 2 in and that cool what else should you have over here

you should have an equal amount of ribosomes you should have an equal amount of ribosomes I'm only gonna do a

couple things but you I want you guys to just get the idea and then what else you should have equal amount of mitochondria

we're separating these cells just perfectly our body's amazing now before we end this off what else do you

have in this cell what's pretty much the fluid in the cell we already talked about rember there's

three parts of the cell cell membrane nucleus cytoplasm the cytoplasm is all the fluid all the fluid of this cell so

now we want to be able to distribute the cytoplasm evenly between the two cells so whenever we do and we finish this

process we're gonna squeeze that constriction ring completely together cause these actual membranes to fuse and

equally distribute the actual cytoplasm here's one more thing right so we said how we've squeezed the cytoplasm equally

into both cells which is the cytokinesis process right we produced that constriction ring and we said that the

nuclear envelope starts reforming well you see how we said that we have these chromosomes here right we equally

distribute the chromosomes something else happens before they were condensed but guess what they need to become loose

again so the chromatin starts actually becoming a little bit more loose again so now we can see it like this in the

telophase right so now we're gonna have this loose chromatin all right now after we've pinched these actual cells off

right we've equally distributed the cytoplasm what does that call it again whenever we pinch the cells and we

actually form that constriction ring eventually separate the cytoplasm equally it's called cytokinesis right

that's an important part now we've pinched this cell so really we should have two cells here we should have two

cells and these two cells should have an equal amount let's assume that their actual nuclear envelope completely

reformed so here's a nuclear envelope here's the nuclear envelope on this one and what should you have in there you

should have the chromatin right you should have the chromatin and this should be a total of how many

chromosomes 46 chromosomes which means it's 2n 46 chromosomes here's which should be two in now even though these

cells aren't perfectly identical in size they should have the exact same amount of cytoplasm and the same amount of

organelles all right guys so we said that we're going to take a look at the phases of the cell cycle just a models

right kid getting a different look at it so if you look here the first one we said his interface and interface was

consisting of the three parts right g1 s g2 easiest way to identify it again is if you remember what was happening here

you see how the chromatin is really loose within the nucleus right it's really really loose and again what

should have happened by now within at the end of interphase at least you should have actually replicated the DNA

so now it's no longer to in but it should be for in in this cell now another thing is actually after we get

done with this interface we're gonna go into the next phase which is prophase now in prophase what's gonna be really

different with this one look here you see how the crewmates chromatin is still really kind of loose here well another

thing that should happen is that the nuclear envelope should actually start breaking down the lamins and condense

and proteins all the things that are making the nuclear envelope up remember we're gonna phosphorylate those proteins

other proteins will phosphorylate is like the histone proteins and then what did we say again what are these guys

right here these are the microtubule organization Center remember we have the centrosome and then we have the

microtubules that are beginning to form here then from the prophase we can distinguish it different from metaphase

how remember what we said as we go from prophase to metaphase the mitotic spindles right those microtubule are an

organization centers start taking residence up in the opposite poles of the cells and then those microtubules

the polar microtubules start connecting to the chromosomes along this midline of the cell which is called the metaphase

plate right then after that if everything is successful at that checkpoint the EM checkpoint there's a

protein we'll talk about them in the regulation video it's called APC and he'll help to initiate this

segregation or the separation of these chromatids from one another when they start separating from one another let's

go over here because now we're in the next phase anaphase anaphase remember here's those mitotic star the

microtubule organization centers and the microtubule are connected to those chromatids and they're pulling the

chromatids to opposite poles of the cell this one's pulling it up this one's pulling it down this is how you can

distinguish anaphase for the last and final phase we're assuming that the kids and all the organelles and all the

cytoplasm is getting equally distributed into the two different cells right but then you produce this little contractile

ring or this constriction ring which produces this thing called a cleavage furrow right but we want to equally

distribute all the different cytoplasmic contents into both cells which is the cytokinesis process right so what do we

have here again you can notice the two cells that we're trying to form t-to telophase right we're trying to form two

cells another thing is what do you notice here what's happening with the chromosomes

right the chromatin is a little bit more loose again where here was condensed now it's a

little bit loose also the nuclear envelope should be reforming and again look for that cleavage furrow and that's

how you can identify telophase all right so again real super quick recap what are these phases of the cell cycle again

it's interphase prophase metaphase anaphase and telophase these cells that we just replicated what can they do well

some of them guess what they can go right back into the cell cycle right back into g1 some of these cells which

type of cells is the proliferative cells the lab aisle cells this the epithelium of the skin the GI tract the urinary

tract amout of we text em cells they can go right back into the cell cycle but some of the cells they don't really go

back into the cell cycle they go into another area so they kind of go into this are their area where they wane a

little bit what is this area called this area is called the quiescent so they call this g0 right just called g0 phase

or we also called the quiescent phase and this is where the cells go to rest so they can rest in this phase they

don't have to go into any type of replication they can remain dormant if you will but then let's say that there's

a stimulus some type of stimulus whatever it might be there's a stimulus to this cell and the stimulus is strong

enough to put it back into the cell cycle to go back into G you want to start undergoing the cell cycle those

could be some of those stable cells but there's other cells that no matter what one

they're done they're a mitotic those are your neurons your schedule your cardiac muscle

they're not gonna proliferate anymore another thing that can happen with this cell cycle is you know as you get older

as we get older remember we had that chromosome right here right here's our chromosome and as we get older remember

these were the telomeres these ends up here as there's consistent DNA replication after DNA replication either

DNA replication the telomeres start getting shorter over time so as you age as we get older so with age that's a

terrible marker as we get older with age during the aging process what happens to the telomeres this causes the telomeres

to shorten and sometimes because of that these cells can go into what's called cell citizens where they are

irreversibly out of the cell cycle they can't enter into the cell cycle no matter what so sometimes in situations

as people get older their telomeres shorten and shorten and shorten as a result some of these cells with their

telomeres are shorter and shorter and shorter we put those cells into an irreversible state to where they can't

enter into the cell cycle that's called cell citizen's okay so we've covered these cycles and we said that there's a

g1/s checkpoint I should also say that there's one other check point two other checkpoints so we said that we had the

g2 phase and we said the times this phase is approximately about two hours about two hours just to throw that out

there and in Phase is probably about the time that you guys have almost watched this video about an hour so by the time

of this video isn't over you guys have almost undergo mitosis that's kind of cool but anyway there's an actual

another checkpoint this next checkpoint is right here as you're going from the g2 phase into the M phase so about right

here there's another checkpoint this is called the g2 M checkpoint we need to make sure that there was no

mistakes in the DNA replication process because again even though these DNA polymerases are very very faithful and

they're very good and they only make mistakes by very one two out of a hundred thousand million base pairs we

still need to make sure that there was no damage and there's special genes that do that called ATM genes and we'll talk

that they produce proteins that read the DNA but we have to regulate it at that checkpoint where's another one you know

right here at metaphase right here before we get ready to go into anaphase there's another checkpoint before we get

ready to separate these chromosomes we have to make sure that these guys are aligned at the metaphase plate perfectly

we need to make sure that there's no mistakes here and this checkpoint is called the EM checkpoint and we'll talk

about the proteins like the APC proteins secure in all those different proteins that help to ensure that from that point

on everything has occurred successfully and properly measure so if you guys have watched this video I really hope that

you guys now understand the cell cycle I truly do it's our goal here in engineering science to help this stuff

make sense for you guys so if you guys did please hit that like button comment down the comments section please

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