So, What is an X chromosome?

It is impossible to avoid genes nowadays. The media are crammed
with news about genetic engineering, genetic modification, or gene
therapy. Yet very rarely does anyone try to explain what these genes
are and what they do, or try to reconcile the concept of the gene with
those other two bêtes noirs of the jargon-averse: DNA and chromosomes.
So that is what I will now try to do.

Probably the least impenetrable way to explain genes, chromosomes,
and DNA is to approach them in historical order. An understanding
of genes came first, in Moravia, with the publication of
Gregor Mendel’s work in 1866 on inheritance in peas (although the
significance of his work was not widely appreciated for many years).
His experiments appeared to show that characteristics are inherited
by physical means, by the bequeathing of sets of instructions by parents
to their offspring. No one at the time knew what these instructions
actually were, but they were soon given the name “genes” anyway.

Next came chromosomes, around 1880 in Kiel, Germany.Walther
Flemming first described how the dark-stained material in the cell’s
nucleus condenses into tiny threads each time a cell divides. He
counted the same number of threads in every cell in an individual,
and in fact, any member of the same species, and also gave them the
name “coloured bodies,” or “chromosomes.” It had long been claimed
that hereditary information is carried in the cell’s nucleus, so it was
not long before scientists started to wonder whether genes were
carried on chromosomes.

The story now pauses for several decades and leaps from Mitteleuropa
to Cambridge, England, and DNA, just after the Second
World War. For some years before James Watson met Francis Crick,
it had been known that the cell nucleus contains surprisingly large
amounts of an acid, deoxyribonucleic acid, or DNA. Watson and
Crick were both fired by the belief that DNA was the stuff that contained
genes, and passed them on to the next generation. However,
they had no good evidence for this belief, until they attempted to
work out the structure of DNA from extremely pure samples made
by Rosalind Franklin and Maurice Wilkins.

To cut a very long story short, DNA turned out to have a rather delightful design. It is a very very long molecule made up of two strands connected along their
length like a ladder, and as if to make it more attractive, this ladder is
wound up into a helix—a sort of ladder-cum-spiral staircase—the
“double helix” that has made its way into scientific folklore.

Watson and Crick were quick to notice that the rungs of the ladder could be
varied to create a code which might carry the instructions that make
up genes. Also, to use their own charmingly understated phrase from
their 1953 paper, “it did not escape their notice” that the two sides
of the ladder could be pulled apart, but each would retain the code,
so that each could be used to reconstruct a perfect copy of the original
ladder—providing a possible means by which genetic material
could be replicated.

So there, in a couple of paragraphs, you have it. Our individual characteristics
are created largely by the action of genes, which are held on
long linear molecules of DNA. These molecules of DNA are, in turn,
packaged into chromosomes, visible through a microscope in the
nuclei of dividing cells. That DNA is peeled into two halves every
time a cell divides, and so every chromosome must be teased into
two daughter chromosomes at the same time. This peeling also takes
place when eggs and sperm are made, and this is how genes are
passed to offspring. And since 1953, biologists have been sorting out
the details of how all this works.

Most important, we now know what those genes actually do. Most
of them provide codes for making proteins, and proteins do just
about everything that a cell does. Some proteins make the structural
components that support and move cells, and indeed our bodies.
Other proteins are called enzymes, and they spend their time chopping
and changing other molecules, including nutrients, waste, other
proteins, and DNA itself. Proteins are so important that now that the
Human Genome Project has deciphered the entire code of all the
DNA in all the human chromosomes, many scientists believe that the
next obvious step is to hunt down all the proteins that they make.

So this little history lesson tells us a great deal more about the X
chromosome. Rather than being an ill-defined splodge of stuff that
somehow came to control our lives, it is now revealed as a very
long ladder-shaped molecule and its rungs carry the code for lots of
genes.

A common misconception about the X and Y chromosomes is that
they were named because of their shapes. However, we have already
seen that Hermann Henking named the X because it was mysterious,
or exceptional in some way and that Nettie Stevens then simply continued
the alphabetical sequence when she named the Y. To be honest,
most of the time the X and Y chromosomes are very fuzzy in
appearance and do not look like any letter of the alphabet. But when
cells divide, all the chromosomes condense down, and transiently
look like chunky little cruciform things, with a blob in the middle.

images (2)

The X chromosome takes on an X shape, but this is hardly a defining
feature because at this point all the non-sex chromosomes look
X-shaped as well. However, the Y, although it has the same basic
structure, is so small that two of its four arms are often indistinguishable,
giving it an apparent Y shape under the microscope. Whatever
their appearance, very soon all these X-shaped chromosomes are
torn apart, and each daughter cell receives only half of each of them:

images (3)

Recently, we have found out how many rungs the X chromosome
ladder has—approximately 160 million. Because of this, we can set
about working out how long an X chromosome is, as we know that
one rung of DNA is 332 trillionths of a meter long. So, by simple multiplication,
we can calculate that the DNA in an X chromosome is 53
millimetres long, or just over two inches.

Two inches may not seem very much for something that carries
160 million pieces of information, but it is very long indeed for something
that has to be crammed into every cell in the human body. Fortunately,
DNA is not only very long, but it is also very thin, and the
reason that the X chromosome is not two inches long is that DNA
can be folded. The DNA is first wrapped around special packing proteins,
and these are then repeatedly packaged and repackaged until
all that long spindly DNA can fit into a single cell. This supreme feat
of folding results in a stubby little strand of material called an X chromosome,
about eight-millionths of a meter long.

But is length really that important? Perhaps instead we should
work out how much an X chromosome weighs. Luckily, once again,
the world of genetic trivia comes to our rescue. We know that a
single rung of DNA weighs 1,054 septillionths of a gram. That means
that an X chromosome weighs 169 quadrillionths of a gram, which is
considerably more, but still not very heavy. In fact, it gives you an idea
of how very thin DNA is when you think that the DNA in an X chromosome
is two inches long, but only weighs 169 quadrillionths of
a gram.

Anyway, this leads us on to our next feat of biologico-mathematical
trivia. If we know how much an X chromosome weighs, can we work
out how much of the human body is made of X chromosomes? This,
unfortunately, is the first time that we will have to make a guess. No
one can agree on how many cells there are in the human body, as no
one can be bothered to sit down and count them. I would say that a
good estimate is 20 trillion, but please do not ask me why. This would
mean that the average male human would contain 3.3 grams of X
chromosomes, or just over one-tenth of an old-fashioned ounce, and
of course, an XX woman would have twice as much X chromosome
in her. This means that about .005 per cent of a man is made up of X
chromosomes, and about .01 per cent of a woman is X chromosomes.
So, in brief, the X chromosome that controls our lives is a two-inch-
long string of code that is wrapped up tight, is not usually
X-shaped, and is unlikely to form a significant part of any weight-loss
program.

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