Why Women Are Stripey

Why Women Are Stripey


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.

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  1. So, if a female has XX chromosomes and a female has XY chromosomes, would a hermaphrodite then have XXY chromosomes? Or XXXY chromosomes? Something like that?

  2. If only one of a woman's X chromosomes are active, then why do people with Turner's Syndrome (a genetic condition where a person only has one sex chromosome, an X) look different from ordinary women?

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