The culprit here is known as the X chromosome, and it’s prevented from causing harm by a process that relies utterly on junk DNA. But before we move to exploring how this protection happens, we need to explore the nature of the X chromosome itself.
Most of the time the chromosomes in a cell are very long and stringy, and difficult to distinguish from each other. They appear like a great bundle of tangled wool when viewed under a normal light microscope. But when a cell is getting ready to divide, the chromosomes become very structured and compact, and are really discrete entities. If you know the right techniques, you can isolate all the compacted chromosomes from a nucleus, stain them with specific chemicals and examine the individual ones through a microscope. At this stage they look more like separate skeins of embroidery wool, with the centromere as the little tube of paper that holds the skeins in place.
By analysing photos of the whole complement of chromosomes in a human cell, scientists were able to identify each individual chromosome. They literally used to cut and paste the individual chromosome pictures to arrange them in order. This is how researchers discovered the causes of Down’s, Edward’s and Patau’s Syndromes, by analysing the chromosomes in cells taken from affected children.
But before identifying the underlying problems in these serious conditions, the early researchers discovered the fundamental organisation of our genetic material. They showed that the normal number of chromosomes in a human cell is 46. The exceptions are the eggs and the sperm, which each have 23. Our chromosomes are arranged in pairs, inherited equally from our mother and father. In other words, one copy of chromosome 1 from mum and one from dad. The same for chromosome 2, and for the others.
This is true for chromosome 1 up to chromosome 22. These are known as the autosomes. If we only looked at the autosomes in a cell, we would not be able to tell if the cell was from a female or a male. But this information becomes immediately apparent if we look at the last remaining pair of chromosomes, known as the sex chromosomes. Females have two identical large sex chromosomes, known as X. Males have one X chromosome and a very small chromosome, called Y. These two situations are shown in Figure 7.1.
The Y chromosome may be small, but it has an amazing impact. It’s the presence of the Y chromosome that determines the sex of the developing embryo. It only contains a small number of genes, but these are vitally important in governing gender.
Figure 7.1 Standard female and male karyotypes, showing all the chromosomes present in a cell. The upper panel shows a female karyotype, the lower a male one. The only difference is in the last pair of chromosomes. Females have two large X chromosomes, males have one large X and a small Y. (Wessex Regional Genetics Centre, Wellcome Images)
In fact, this is predominantly controlled by just one gene[13],{116} which drives creation of the testes. This in turn leads to production of the hormone testosterone, which results in masculinisation of the embryo. Remarkably, a recent study has shown that just this and one other gene are sufficient not just to create male mice, but also for these mice to generate functional sperm and to father pups.{117}
The X chromosome, on the other hand, is very large, containing over 1,000 genes.{118} This creates a potential problem. Males only have one copy of the X chromosome and hence one copy of each of these genes. But females have double that number, so in theory could produce twice as much of the products encoded by the X chromosome as males. The trisomic conditions described in Chapter 6 demonstrated that even a 50 per cent increase in expression of the genes from a small chromosome has a hugely detrimental effect on development. How then can females tolerate a 100 per cent increase in expression of over 1,000 genes, compared with males?
The answer is that they don’t. Females produce the same amount of X chromosome-encoded protein expression in their cells as males. They achieve this by a remarkably ingenious arrangement whereby one X chromosome is switched off in every cell. This is known as X-inactivation. Not only is it essential for human life, the process by which it occurs opened up new and totally unanticipated areas of biology that are still the subject of intense scrutiny.
One of the oddest things we have come to realise is that our cells can count the number of X chromosomes. Male cells contain an X and a Y chromosome and they never inactivate the single X. But sometimes males are born who have two X chromosomes and one Y. They are still males, because it’s the Y chromosome that drives masculinisation. But their cells inactivate the extra X, just as female cells do.
A similar thing happens in females. Sometimes females are born who have three X chromosomes in each cell. When this happens, the cells shut down two X chromosomes instead of one. The flip side of this is when females are born who only have one X chromosome. In this case, the cell doesn’t shut it off at all.
In addition to being able to count, our cells are also able to remember. When a female produces eggs, she usually only gives each egg one of each pair of chromosomes, including the X chromosome. A male produces sperm that contain either an X or a Y chromosome. When a sperm that contains an X chromosome fuses with an egg, the resulting single-cell zygote contains two X chromosomes and both are active. But very early in development, after just a few rounds of cell division, one X chromosome is inactivated in each cell of the embryo. Sometimes it’s the X that came from father, sometimes the X that came from mother. Every daughter cell that subsequently develops switches off the same X chromosome as its parental cell. This means that of the 50 trillion or so cells in the adult female body, on average about half will express the X chromosome that was provided by the egg, and the other half will express the X chromosome that was provided by the sperm.
When an X chromosome is inactivated, it adopts a very unusual physical conformation. The DNA becomes incredibly compacted. Imagine you and a friend each take hold of opposite ends of a towel. You start turning your end of the towel clockwise, and your friend does the same at the other end. Pretty quickly, the towel will start twisting in the middle, and the two of you will be pulled closer together. Now imagine that the towel is about five metres in length, but you manage to keep twisting it until it’s a dense clump of towel only a millimetre in linear length. By this stage, the towel is extraordinarily tightly wound up. Essentially, the X chromosome becomes as tightly compacted as that towel. One of the consequences is that it forms a dense structure that can be seen when looking at the nucleus of a female cell down a microscope, when all the other chromosomes are long and stringy and can’t be visualised. The condensed X chromosome is called the Barr body.
In order to try to understand how X chromosome inactivation happens, scientists studied unusual cell lines and mouse strains. These focused on examples where parts of the X chromosome had been lost, or where bits of it had been transferred to other chromosomes. Some cells that had lost part of the X chromosome were still able to inactivate one of their X chromosomes, as shown by the presence of the Barr body. But cells that had lost a different part of the X weren’t able to form Barr bodies, showing that they hadn’t inactivated a chromosome.
Where parts of the X chromosome had been transferred to other chromosomes, sometimes these abnormal chromosomes were inactivated, and other times they weren’t. It all depended on which bit of the X chromosome had been transferred.