You have to wait until the end for the fun facts, but they're better if you read the article first
The problem is a complex one, but it is in simple terms:
Chromosomes carry genes, which carry instructions to make proteins
Humans have 23 pairs of chromosomes
In humans, females have twice as many X chromosomes as men, and so twice as many X chromosome genes, and so twice as many instructions to make proteins
If females made twice as many X chromosome related proteins as men do, they would quickly die.
Obviously, by the number of females walking around living lone functional lives, this doesn’t happen. So how is this prevented?
In each cell in a female human (for the sake of this article, any human with two X chromosomes), one of the X chromosomes is inactivated, or silenced. That is, it’s wrapped up tightly and put to the side of the cell so it doesn’t do something silly like make way too many proteins and kill the cell. The wrapped up cell is known as a Barr body (named after its discoverer, Murray Barr, who, as could be inferred, had no Barr bodies). So which X chromosome becomes the Barr body? The paternal or the maternal? It’s a little random. I say a little, because in some kinds of cells it is always the paternal X chromosome, and in marsupials, such as koalas and kangaroos, it is always the paternal X chromosome. Having it be random has obvious evolutionary advantages. This way, if one of the genes on one of the chromosomes is faulty, you still have half the body using the gene on the other X chromosome, which hopefully is not faulty.
This is all well and good, but how does this happen? Our understanding of how X chromosomes are inactivated is still growing, and has been bolstered by a recent study at Massachusetts General Hospital. What was already known was that the inactivated X chromosome produced a product called Xist. In a new cell, both X chromosomes produce Xist and the chromosome that produces the most Xist the fastest gets to become a Barr Body and hang out at the edge of the cell and chill while the other X chromosome does all the work. What wasn’t well known was how Xist works. The new study has suggested Xist ‘evicting’ a group of proteins known as cohesins from its associated X chromosome. What does this do? Well, cohesins help genes activate by connecting the gene with its associated ‘regulator’ (a part of the chromosome which activates its associated gene). If the cohesins are evicted, the genes and regulators don’t connect, and the genes can’t be activated.
Of course, there’s still a lot we don’t know about this process, and there’s no reason that Xist doesn’t perform other functions. But this is a huge progression in our understanding. This will help in our treatment of diseases which have large problems with X chromosome inactivation, most notably Rett Syndrome.
Oh, and as promised, here are some fun facts on sex in animals (fun to me):
- Flies do the opposite of humans. Instead of females inactivating one of their X chromosomes, males have specific mechanisms of making twice as much X chromosome product per chromosome.
- Female humans (and flies) are homogametic: they have two X chromosomes, whereas male humans (and flies) are heterogametic: they have an X and a Y chromosome (actually, flies don't have a Y chromosome. They have a whole one less chromosome less than their female counterparts). In birds, some fish, some reptiles, and some invertebrates, the roles are reversed: males are homogametic and females are heterogametic.
- The Y chromosome may be going extinct in humans. Scientists believe it may be getting shorter, and if the more essential genes it carries migrate to other chromosomes, there seems to be no reason it couldn't disappear entirely.
- Some organisms' sex don't even depend on their genes. For instance, female sea turtles lay eggs all over the beach. Those lower on the beach have the tide wash over them, and develop at a cooler temperature, which differentiates them into males, whereas eggs developing at warmer temperatures hatch as females. This is known as environmental sex determination, and may be even more common than genetically based sex determination.
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