Why Women Are Stripey

[Applause] Inside each one of your cells, there is six feet of DNA made up of six billion letters of genetic code. Now, your DNA is split into 46 pieces, each 3 to 4 cm long, called chromosomes. Now, normally we think of chromosomes as looking like this, but they only take that form when a cell is ready to divide. So, usually, DNA is just a wiggly thread within the nucleus.

Now, if you can imagine, DNA is only about 2 nanometers wide, but a chromosome is cm long. So, you would think that it would get tangled worse than the headphones in your bag. So, the DNA is actually wrapped around proteins called histones. Now, those histones have wiggly tails, which will come in handy, as we'll see in a moment.

Your unique set of DNA first formed when 23 chromosomes from your mom mixed with 23 from your dad. Now, 22 of those chromosomes from each parent form matching pairs, but the 23rd set is the sex chromosomes. So, two X chromosomes make you female, and an X and a Y make you male. Now, since the male sex chromosomes are different, both can remain active for the rest of your life.

But for females, one of the X chromosomes needs to be inactivated in order for proper development to occur. This happens when a female embryo is just 4 days old and consists of only 100 cells. Right now, in this cell, the X chromosome from Dad and the one from Mom are both active, but through a tiny molecular battle, one of the X chromosomes wins and remains active while the other is inactivated.

This is done by packing the DNA closer together and making modifications to those dangly histone tails that signal this inactivation. New structural proteins are also added to bind everything closer together, and finally, methyl groups—these tiny little molecular markers—are added to the DNA to basically signal to the cell that this DNA shouldn't be read. So, all of this together makes the DNA very difficult to access for the molecular machinery that would harness the code in this DNA. It is switched off.

This DNA is silenced. In contrast, the active X chromosome DNA is more spread out. This allows better access to the genes on the chromosome. Histones can be slid along the DNA or removed entirely, and the histone tails have a different modification signaling this DNA is active. Now, all of this makes it possible for RNA polymerase to access and transcribe this DNA into messenger RNA, which then goes out into the cell and is used to make a protein.

Now, what's surprising about X chromosome inactivation is that it seems kind of random which X chromosome wins. I mean, in some cells, Dad's X chromosome wins, and in others, Mom's X chromosome wins. So, this 100-cell embryo ends up with a mixture of active X chromosomes. But from this point forward, as these cells divide, they maintain the active X chromosome that they had inside.

So, all of the cells with Dad's active X chromosome give rise to further cells with Dad's active X chromosome, and this continues on into adulthood. So, if you could look at a woman's skin and see which X chromosome has been inactivated, you would see a stripy pattern which shows the growth and mixing of all of these first 100 cells when the embryo was just 4 days old.

[Music] Now, of course, you can actually see that in humans, but you can see this with calico cats, and that's because the gene for coat color is actually on the X chromosome. So, just by looking at the pattern of her spots here, her dark and light spots, you can see where her mom or dad's X chromosome has been inactivated. And this also shows us that only female cats can be calico cats, and that's because, well, only female cats can inherit two X chromosomes with two different color genes.

Now, this is just one really cool example of epigenetics, but epigenetics doesn't normally work on one whole chromosome. In fact, it's at play in all of your chromosomes, turning on and off your genes. For example, it's epigenetics which makes a pancreatic cell capable of producing insulin because that gene is switched on there, but switched off everywhere else.

What's more interesting is that it seems the behaviors you take can actually affect your epigenetics, and even weirder, perhaps the things that your parents or grandparents did can affect your epigenetics. Now, can affect your DNA. So, you are not just a product of your genetic code. You're not just a product of your DNA. You are also a product of your epigenetics, and that is influenced by your behavior and the behavior of your ancestors.

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