Inside each one of your cells there is six feet of DNA, made up of 6 billion letters
of genetic code. Now your DNA is split into 46 pieces, each 3-4cm 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 2nm wide but a chromosome is centimetres 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 for proper development
to occur.
This happens when a female embryo is just four 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. 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 signalling 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's 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 migration of all these first a hundred cells, when the embryo was just four days
old.
Now of course you can't actually see that in humans, but you can see this with calico
cats and that's because the gene for coat colour 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 colour 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 behaviours 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 behaviour and the behaviour of your ancestors.