Understanding Life: A Journey Through Biology and Genetics

Hi. You’re on a rock, floating  in space. Have did we get here?  Well, about 4.5 billion years ago, the earth  was big ball of flaming rocks, constantly   bombarded by even more rocks from space. Fun  fact! Those rocks probably had some water  

inside them, which has now turned into steam. Breaking news! The earth is cooling down. Oh yeah,   did I mention tha- [it’s raining.] Whoops, everything’s flooded, but hey,   at least there’s some cool stuff at the bottom,  like hydrothermal vents, which are piping hot  

and filled with a bunch of chemicals, that can  make some very interesting stuff. Wait a minute,   what the heck is going on here? [Biology]  Biology is the study of life, but really,  it’s just chemistry in disguise. I mean  

you and I are basically just a big ball  of molecules that can make funny sounds.  Carbohydrates give you quick energy, lipids store  long term energy and make membranes, proteins make   up tissues and nucleic acids make DNA. Also, to  make all the chemical reactions possible, living  

beings, have inside of them a bunch of enzymes. They’re special proteins that act as catalysts,   which just means they help chemical reactions  speed up by either breaking down or combining   one specific thing. For example, lactase  breaks down lactose, the sugar found in milk. 

Ok, so enzymes make life possible  by speeding up chemical reactions,   but what even is…life? Scientists don’t really  seem to agree, but obviously a cat is different   from a rock. The cat can produce energy by  metabolizing food, it can grow and develop,  

reproduce, and it responds to the  environment, whereas the rock does not.  Also, unlike rocks, every living thing on  earth is made of cells, of which there’s   two main categories: Eukaryotes and prokaryotes. Eukaryotes have fancy organelles which are bound  

by membranes, like the nucleus, inside of which is  DNA. Prokaryotes, have none of those organelles,   and the DNA is just kind of chilling  there, like freely floating around.  This is why Prokaryotes are just  single cell organisms like bacteria  

and archea whereas eukaryotes can form  complex organisms like protists, fungi,   plants and animals. These are what’s known  as “kingdoms”, which is a taxonomic rank,   so basically, how we classify different living  things and how they’re related to one another. 

Because there are quite a few species of  life on this planet, and naming them cat,   dangerous cat and water cat wouldn’t really be  all that helpful, we also give every species   a unique and unambiguous scientific name  consisting of the genus and the species. 

One thing every species has  in common is homeostasis, aka,   keeping certain conditions in check, so ya don’t  die. If you feel warm, your body will sweat,   if you’re cold, your body will shiver. A cell does kind of the same thing just  

that it balances out concentrations of certain  chemicals. You see, enzymes for example, only   work in a very specific environment, let’s say at  some specific pH value. If this changes too much,   the enzymes will denature and won’t work anymore.  To counter this, the cell needs to constantly keep  

up this specific pH value, which is controlled  by the concentration of acid and base molecules.  Ok. But like, how does the cell do that? The secret lies in the cell membrane. You see,   it’s a semipermeable phospholipid bilayer,  okay that’s way too many words, all it is,  

is two layers of these funky looking molecules  with a polar head and a nonpolar tail.  This allows small molecules like water  and oxygen to slip right through,   whereas larger particles like ions need special  channels that can be opened or closed, which  

gives the cell control of what goes in and out. Naturally, particles move with the gradient,   so from a place of high concentration  to a place of low concentration. Or,   in the case of water, it can also move to a place  of high solute concentration, so for example salt. 

Welcome to Biology Pro Tips Season 1, tip  of the day: do not drink too much saltwater.   There’s a bunch of salt in saltwater, in  fact, more salt than inside of a cell,   which means it will draw water from your cells and  dehydrate you. Yeah that’s it have a great day. 

The process of balancing out gradients is known  as “diffusion” and happens automatically, but,   by using a little bit of energy, particles  can actively be moved against the gradient.  The energy comes from Adenosine  Triphosphate or ATP. To be exact,  

the highly energetic chemical bonds between the  phosphate groups can be broken to obtain energy.  This is kind of important, as  in, every organism and every cell   needs to make ATP for example, through cellular  respiration which happens in the mitochondria: 

Together with oxygen, glucose, so sugar, is  turned into water, carbon dioxide and ATP.  This is nice, but it only works if you already  have glucose. Humans are “heterotrophs”. They   eat food, inside of which is sugar,  which is then broken down into glucose. 

Plants on the other hand are “autotrophs”.  Simply put, they said “screw food, I’ll just   make my own glucose by staring at the sun”. You  see, plant cells have small organelles called   “chloroplasts” inside of which is chlorophyll,  which absorbs red and blue light but reflects  

green light, which is why most plants look green. The absorbed energy from light is used to split   water and make a special form of carbon dioxide  which can then be turned into glucose and oxygen.   Okay quick recap, once you have glucose, either  from food or photosynthesis, you can do cellular  

respiration, to get energy in the form of ATP. Chemically, ATP is what’s known as a nucleotide.   It has a phosphate group, a five carbon sugar and  a nitrogenous base. You know what else is made of   nucleotides? Deoxyribonucleic acid, or DNA. It consists of two strands of nucleotides,  

with the sugar and phosphate groups, but the  actually important part is the nitrogenous base,   which comes in four flavours: Adenine,  Thymine, Cytosine and Guanine.  These bases can form base pairs through  hydrogen bonds, where Adenine goes with Thymine,  

and Cytosine goes with Guanine. These bonds  are what holds the two strands of DNA together.  Okay, but, how the heck does that store  genetic information? I’m glad you ask!  A “gene” is a section of this DNA  that codes for a special trait,  

by carrying a certain sequence of base pairs,  which is like a recipe for making a protein.  Why proteins? Because they’re like really  important, they transport molecules,   act as enzymes and determine the way you look.  For example, the difference between brown and  

blue eyes is the amount of a pigment called  “melanin” in the cells of the iris. The OCA2   Gene codes for “P-Protein” which we believe  controls the amount of melanin in cells,   meaning that the proteins made from this gene,  could be what determines your eye colour. 

Cool! There’s just one issue: Your DNA  and its information is in the nucleus,   but proteins are made in organelles  called the ribosomes. How do we get the   information from A to B? The answer is RNA. It’s kind of like DNA, just that it’s most  

often a single strand, it uses a ribose instead of  deoxyribose and instead of Thymine it uses Uracil,   which makes it less stable, but that’s besides  the point, here’s what RNA actually does:  Let’s say you want to make the protein  coded for by this gene. An enzyme called  

“RNA polymerase” will split the DNA and make  a strand of RNA with the complementary bases,   essentially copying the information from the  DNA to the RNA. This is called “transcription”.  The new strand is called messenger  RNA or mRNA, because it carries this  

message out of the nucleus to a ribosome. Remember how I said that a gene is like a   recipe for a protein? Well, on the mRNA, which  carries the same base sequence as that gene,   every group of three bases, which is called  a “codon”, codes for a specific amino acid,  

which are the building blocks for proteins. Welcome to Biology Pro Tips Season 1, if you want   to decode a sequence of RNA, there is actually a  chart for that! Yeah that’s all have a great day.  These amino acids are carried by special  molecules called transfer RNA or tRNA,  

which have a unique anticodon that can only  attach to its matching codon on the mRNA.  The job of the ribosome is to read over codons on  the mRNA and attach the matching tRNA molecules,   which then leave behind their amino acid. As the  ribosome moves along the mRNA and attaches more  

tRNA, which happens a couple thousand times, the  amino acids combine into a “polypeptide chain”,   which is just a really long chain of  amino acids, that can be bunched up,   creased, smacked and folded into a protein. Okay, let’s recap: A gene is copied onto mRNA,  

which is then used to build proteins  by assembling a chain of amino acids.   Aka transcription and translation. Hey, this genetics stuff is pretty   cool, can we learn more? Absolutely. Oh yeah did I mention that you have, like,  

a bunch of DNA? You have about 20000 protein  coding genes, each thousands to millions of   bases long, and that only makes up around 1% of  your entire DNA, the rest is just non-coding.  PLUS, almost every cell in your body contains your  entire genetic code, but genes can be turned on or  

off depending on the cell, which is good, because  otherwise your brain cells might just start   making stomach acid, which would not be good. FUN FACT! If you were to stretch out all the   DNA of just one single cell, it  would be about 2 meters long. 

Wait a minute, how does that fit into a  microscopic cell? Well, if you were to look inside   the nucleus, you wouldn’t find the DNA floating  around like this or even this, no, you would   actually find lots of these worm looking things. To be exact, DNA is coiled up around Proteins  

called “Histones”, which are then condensed into  strands of Chromatin, which are then coiled up   even more to make tightly packed units of DNA  called “Chromosomes”, which kinda look like   worms. Different sections on a chromosome carry  different genes, and the entire human genome is  

split amongst 23 different chromosomes, although  every body cell has 2 copies of every chromosome,   one from the mother and one from the father. For most chromosomes, the two copies are   said to be homologous, meaning that they carry  the same genes in the same location. However,  

the two versions of a gene can be different,  so the mother’s gene could code for brown eyes,   while the father’s gene codes for blue eyes. These  different versions of a gene are called “alleles”.  For most of your genes, you have 2 alleles, one on  each chromosome from either parent. These alleles  

can be dominant or recessive, which determines  which of them is expressed. For example,   brown eye color is a dominant trait, which  is shown by an uppercase B, whereas blue   is recessive, which is shown by a lowercase b. All this means, is that if you have the dominant  

brown allele, you will have brown eyes, no matter  what the second allele is. Only when there are   two recessive alleles will it be expressed. With this knowledge, we can predict the future!  Let’s look at how this trait is  inherited from parents to children: 

Both of these parents have brown eyes, but  also have a recessive blue allele in their   genotype. Every child receives one allele  from each parent randomly, so these are the   possible combinations for the children. Most combinations contain the dominant  

brown allele, so the child will have brown eyes.  But, there is a small chance that a child gets   two recessive alleles and has blue eyes, even  though both parents had brown eyes! You see,   it’s what’s on the inside that counts. Alright, that’s cool, but reality is not always  

so simple. Some genes are not fully dominant, but  not fully recessive either, which means that the   phenotype, so the appearance, appears to mix. Crossing a red and a white snapdragon, where   red is “dominant” and white is “recessive” gives  you a pink phenotype which is somewhere inbetween,  

aka intermediate inheritance. Or, crossing  a brown and a white cow where both colours   are dominant could give you spotted cow, so both  phenotypes are expressed equally, aka codominance.  Hey remember how I said almost all  chromosomes are homologous? Well,  

there’s one exception: the sex chromosomes. Females have two big X chromosomes, whereas   males have one X and one smaller Y chromosome. These are partially homologous at the top,   but since the Y chromosome is so small,  it’s missing genes that are present  

on the lower part of the X chromosome.  These genes are called “X-linked genes”.  If one of these genes is a recessive trait like  colour blindness, males are stuck with that trait,   whereas females probably have another  dominant allele, to override it. This  

is why most colourblind people are male. Now, for genes to even be passed on,   the body has to make new cells which can  inherit the genes. There’s two main mechanisms:  Mitosis, which is how the body makes identical  copies of body cells to grow and repair tissues,  

and Meiosis, which is how the body  makes gametes, so sperm and egg cells.  Mitosis starts with a diploid cell, so a cell with  two sets of chromosomes. These chromosomes consist   of one chromatid, which has to be replicated  for the new cell. After replication is when  

you see the familiar X shape consisting of  two identical sister chromatids. These are   split into two identical diploid cells, with two  sets of chromosomes consisting of one chromatid.  Meiosis also starts with a diploid cell, but  after replication, the chromosomes comingle  

and exchange genetic information in a process  called “crossing over”. The cell is then split   into two non-identical haploid cells. These  have one set of chromosomes, but they still   consist of 2 sister chromatids. These cells split  again into 4 genetically different haploid cells,  

where each chromosomes has one chromatid. Meiosis produces haploid cells, so that when two   gametes combine into a fertilized egg or “zygote”,  it again has the correct number of chromosomes.  This is cool, but, cell division is only a tiny  part of a cell’s entire life cycle. Most of its  

time is actually spent in interphase, aka just  chilling. All it does here, is grow and replicate   all of its DNA, so that it actually has enough  genetic material and size to divide in M-Phase.  There’s multiple checkpoints in the cell  cycle which are controlled by proteins  

like p53 or cyclin to check if the cell is  healthy and ready to reproduce. If a cell   is not quite right, it’s either fixed  or it destroys itself, which is called   “apoptosis”…or at least, that’s what it should do. Normal cells replicate until there’s no need to,  

but some cells just keep going. This is because  they don’t respond correctly to these checkpoints   and end up replicating out of control and  functioning wrong, which is also known as cancer.  This damaging behaviour is often a result of a  gene mutation, which is a change somewhere in the  

base sequence of a gene. This can happen during  DNA replication, when a single base is changed,   left out or inserted into the original sequence. This often changes the protein coded for by that   gene and let’s just say that  change is often not optimal. 

Another type of mutation happens in chromosomes,  where entire sections of DNA could be duplicated,   deleted, flipped around or transferred between  chromosomes. The most famous chromosomal mutation   is probably when the 21st pair of chromosomes  has an additional copy, so that there’s 3  

instead of 2. The result? Down syndrome. Mutations might seem like a bad thing,   but actually, they can also be neutral  or even beneficial. For example,   a species of yellow grasshoppers might  mutate and become green, which makes them  

blend in with the grass and get eaten less. Over time, you can expect to see more and   more green grasshoppers, as their fitness  has increased. Not that kind of fitness,   fitness as in, they can have more  offspring, because they get eaten less. 

This is natural selection and the driving  factor behind evolution, as the poorly adapted   species gets selected against and the fittest  species, which has adapted to the environment,   survives and and has the most offspring,  passing down the trait that made them survive. 

If you think adaptation is cool, yes,  but also it kind of sucks. You see,   humans can get sick from bacteria or viruses,  but nowadays, we have medicine that works. Good!  However, what if the bacteria mutates and  suddenly, the medicine doesn’t work anymore? Well,  

that’s kind of exactly what is happening,  aaand we have no clue how to fix it. So, yeah.  Oh yeah by the way, one thing many people confuse  is bacteria and viruses, and NO, they’re not the   same. Bacteria are prokaryotes, so they consist  of a single cell which can reproduce on its own,  

and we treat bacterial infections such as  strep throat and tetanus with antibiotics.  Viruses are not made of cells, in fact,  we’re not even sure they’re alive. They   share some signs of life, but they can only  reproduce inside a host, and they don’t grow,  

so it’s not really alive, but it’s not dead  either, it’s more of non-living kind of thing.   Also, you cannot treat viral infections with  antibiotics, most of the time you just have to   chill out and let your immune system do its thing. Now you might think bacteria are a bad thing, but  

actually, you have millions good bacteria inside  your gut. The live in symbiosis with you, so you   give them food, and they help you digest it. Speaking of digestion, your body is made of   many complex organ systems that work  together to make sure you don’t die,  

and I know what you’re thinking. Actually  I don’t, but I know how you’re thinking.  The nervous system, consisting of nerves,  which connect to the spinal cord and lead   to your brain, is made of cells called  “neurons” which can conduct electricity  

along this long tube called the “axon”. Anything you see, think and feel, it’s   all just electrical signals going to your brain,  and your brain telling your body how to respond.  To be exact, the signals are called “action  potentials” and happen at the same strength  

and the same speed every time, so  the only difference between “hey,   I’m a little cold” and “OMG I AM ON FIRE” is  where it happens and how frequent the signals are.  When a neuron is just chilling, the axon is  more negative on the inside than on the outside,  

because there’s an unbalanced amount ions. This  causes an electric potential of about -70mV.  When there is a stimulus, signalling molecules  called neurotransmitters dock onto ion channels on   the axon and open them, letting the ions flow and  changing the electric potential around that area. 

Now, action potentials are all or nothing.  A small stimulus won’t really do anything,   but, if the potential exceeds about  -55 mV, boom, action potential.  Ion channels around the stimulus  open and ions rush into the cell. 

This causes the charge distribution in that  section of the axon to reverse for a split second,   which is called “depolarisation”. The ion channels that are next to   this area are influenced by this and open as well,  

which causes a chain reaction and sends  the signal all the way down the axon.  Some neurons have a myelin sheath made  of Schwann cells, which insulate the   axon and only leave tiny gaps called nodes of  ranvier. If there’s a stimulus, the charges  

can “jump” across the nodes which transmits  the signal way faster than a normal neuron.  But either way, at the bottom, the electric signal  reaches a terminal button, which connects the   current neuron to the dendrites of the next. If  you zoom in, you’d notice that the two cells don’t  

even touch, there is actually a small gap. This  is once again where neurotransmitters come in:  Once the button is depolarized, tiny packages  of neurotransmitters get released, and bind   to receptors of following dendrite, either  blocking it from doing anything or causing  

another action potential, which repeats the cycle. Hmmm. Something in my brain’s telling me that you   should definitely subscribe, and also, if you  want to stimulate your neurons and find out   how math is used in Biology, a resource I can’t  recommend enough is Brilliant, which has thousands  

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