In science there's this age old question, are you the way you are because of nature

or nurture? In other words, am I the way I am because of the genes I inherited from my

parents or from the experiences I have during my life. And a great place to study this is

using identical twins. And so if I had an identical twin, we would have exactly the

same DNA. But we wouldn't look exactly the same. And that's because during our life we're

going to have different experiences. Get different nutrients at different times and so we would

be different. And so nurture is important but so is nature. But what we're finding is

that the delineation between nature and nurture is blurred. And a great study that relates

to that and epigenetics came out in 2003 when they were looking at mice. And so this is

going to be a typical mouse. It's grey in color. And it's going to be relatively thin.

It's a normal mouse. But there's a mutation found in mice called the agouti mutation.

You have the agouti mutation, you're not going to be dark in color. You're going to be this

kind of yellowish color and you're going to be fat. You're going to be obese. And associated

with that, you're going to have a higher incidence of diabetes. You're going to live a less amount

of time. And what they found is that the scientists could actually take cloned mice, mice that

were exactly the same and by feeding the mother different amounts of nutrients, they could

produce agouti mice. In other words they could produce mice that are genetically identical.

So they have the same exact DNA. But they're going to express different genes. And so that's

what epigenetics is. It's taking the genes that we have and manipulating those. And we've

known about this for a long time. And so if we look at some stem cells. These are stem

cells here. Those are going to become the cells that are eventually an adult. They're

going to have all the same DNA in all the stem cells. But we know that as those cells

eventually start to become different cells and different cells and different cells, they're

going to differentiate. They're going to turn into different cells. And so the DNA is going

to be the same between all of those cells. But the genes that they express are going

to be different. And so what they have is they have all the messages to make all the

different types of cells inside the DNA but they're not expressing all of those. And so

what is epigenetics? Epigenetics is controlling which genes we're going to express at which

time. And so if we express just the lip genes, then we're going to make a lip cell. And if

we express just the eye genes, we're going to make an eye cell. And if we express the

ear genes were going to make an ear. But if we express all of them at the same time we're

going to make a cell that clearly doesn't function. And so this is something interesting

that you should know. That all of our cells have the same exact DNA. But they're not expressing

all of the genes at the same time. We call that differentiation. How do we control what

genes are actually being expressed? It is called epigenetics. And so we finally come

up with a definition for it. And if I were to read it out its "Stably heritable phenotype

resulting from changes in a chromosome without alterations to the DNA sequence." What does

that mean? Well remember phenotype is going to be the physical appearances that you have.

And so what epigenetics does is allow us to change the phenotypes without changing the

underlying DNA sequence. And this is heritable. In other words once we change that you can

actually pass that on to the next generation. And so before we talk about the specifics

of how epigenetics works, we should really talk about what DNA is. So DNA remember is

going to be a code and it's code to make all the proteins inside the cell. It's found in

all life. But DNA just doesn't sit loose within the nucleus. It's made up of something called

chromatin. And chromatin is basically two things. You have the DNA, which is going to

be the genetic code. And then you're going to have these proteins. They're called histone

proteins. And the DNA is wrapped around the histone proteins. The histone proteins are

wrapped around themselves. You eventually get threads and fibers. And you eventually

get what we think of as a chromosome. And so what is a chromosome? It's a bunch of proteins

with DNA kind of wrapped around it. And so in epigenetics what we want to be able to

do is to express specific genes. And so how do we do that? There's basically three mechanisms

of epigenetics. And the first one is called DNA methylation. So what does methylation

mean? It means we're adding a methyl group. We're adding a functional group. In this case

we're adding it to cytosine. So remember DNA is going to be made up of four different nucleotides.

We have adenine, cytosine, guanine and thymine. And the one I'm talking about right here is

called cytosine. So this is going to be a nitrogenous base. It's going to be those rungs

on the inside of a ladder. And if we methylate cytosine what that really means is we're adding

a methyl group. You can seen the methyl group right here. We're adding a methyl group to

the cytosine nucleotide which is going to be found on the inside of the DNA. When we

do this, when we methylate cytosine, it's almost like turning a switch off. So we're

turning that gene off. And so basically RNA polymerase now can't grab on to the DNA. It

can't make RNA and it can't make those proteins. And so once we methylate our DNA, we are turning

it off for good. Now where is it a good example of this? Well this is going to be a fertilized

egg or a zygote. That eventually makes stem cells. And those stem cells eventually are

going to differentiate to make all of the cells in our body. But how does it do that?

Well again it does that by methylating the genes. And so inside the circulatory system,

let's say a heart cell, we're going to methylate all the genes that don't make that heart cell.

And so the same thing is going to happen in all of the cells in our body. Now an interesting

thing, well how do we make those stem cells in our children again? Well when we're forming

those cells, those gametes cells, we're going to demethylate the DNA. So we're going to

remove the methyl groups and now it can become a stem cell again. So that's one mechanism.

Histone acetylation is going to be another one. So remember we said the DNA is wrapped

around these histone proteins. And so how tightly is that DNA is wrapped is going to

determine if we can express the genes on the DNA or not. And so if the DNA is wrapped really

tightly then RNA polymerase can't get on. We can't transcribe those genes. And so that's

controlled by a couple of different enzymes. And so before we get to the enzymes, we should

talk about what a histone is. A histone is going to be a protein. So it's made up of

a number of different amino acids, but the important ones are going to be lysine. So

lysine is going to be a specific amino acid. You could see here's the R group hanging off

the end. And what we could do is we can add an acetyl group to that. As we add an acetyl

group to that, right down here, what that's really going to do is it's going to change

the structure of these histone proteins. And that's going to loosen up the DNA that's attached

to it. Once we loosen up that DNA then we can start to transcribe the genes that are

found wrapped around the histone. It's almost like having thread wrapped around a spool.

And if we loosen up that thread, then we can start to code for those genes. What if we

don't want to express those genes? Well we're going to go in the other direction. We're

going to remove that acetyl group. And so the functions, or excuse me, the enzymes are

going to be histone, acetyl transferase. And that enzyme is going to transfer an acetyl

group on to the histones. And then we're going to have histone deacetylase. And so that's

going to remove the histone group. And so again, what does that mean? If we add the

acetyl group to it, then we can code for the genes here. If remove the acetyl group, then

RNA polymerase can't get on to the DNA and we're not going to code for it. And so this

is occurring all of the time. It's not like methyl groups when we're just turning a gene

off permanently. We're constantly acetylating and deacetylating those histones. And so we're

coding for the genes. And then we're not. And then one other important thing that we're

starting to discover is something called microRNA. MicroRNA is little bits of RNA. And so let's

kind of figure out where we are. This is the nucleus. So this would be a eukaryotic cell.

This is going to be the RNA. And then this is going to be a ribosome. Ribosome remember

is going to translate that protein. And so what we also produce inside our DNA is we're

producing a bunch of microRNA. MicroRNA is little bits of RNA that aren't going to code

for specific proteins. What they're going to do is they're going to bond to the regular

messenger RNA. When they do that they block the ribosome. And so we can't code for those

specific enzymes. Can't code for those specific proteins. So it's another way that we can

say, okay we've got the DNA. We've got the gene, but we're not going to make the protein

because we're going to control that post-transcriptionally after the RNA has been made. So why is this

important? Well it's super important that you take care of your genome. Because that's

what you hand on to your kids. But what we're finding is it's your epigenome thats incredibly

important. And so we can mess up our genome using all of these things over here. Changes

in diet, drugs, getting older. All those things are going to change what genes we're actually

expressing. And the neat thing about that and the scary thing about that is that we

pass that on to the next generation. And so diabetes, we found forever that diabetes is

going to be much more common if you have a parent who has diabetes. Well, what's going

on? They're actually changing their epigenome. They're changing what genes they're expressing

and then they're handing that off to their kids. It's a little bit scary, but that's

epigenetics. It's really cool. It's revolutionizing a lot of the ways we look at health problems.

And I hope that was helpful.