"Without mutations, we would still be in the primaeval slime... along with the chemists, says Professor Steve Jones" (2005)

View from the lab: mutations
> <span class="filed">(Filed: 0405/2005)

Without mutations, we would still be in the primaeval slime... along with the chemists, says Professor Steve Jones

To a chemist, we ought to be dead. That gloomy prognosis (leaving aside the ignoble thought that if you do chemistry you might as well be dead) turns on thermodynamics, on the random noise that interferes with all chemical reactions and causes them to go wrong. The chemistry of DNA is simple (before it became famous, it was known as "the stupid molecule") but the double helix - cheap and cheerful as it might be - is very, very, long (long enough in your own body to go to the moon and back 8,000 times) and very, very, busy (and if you make it to the end of this article you will be rewarded with several miles of new and accurately reproduced genetic material).

A simple sum proves that this is impossible. The chances of physical error as each DNA molecule is copied are such that mistakes - mutations - should build up with great speed and stop most of the dividing helices in their tracks. Even those that make it would be so damaged that their carriers would not survive.

Fortunately, biological sums are never simple. The genetic material is indeed much damaged by the laws of chemistry, which work as inexorably in our cells as in a test tube. In cells, though, most of the mistakes are put right. The thousand natural shocks that life is heir to are fixed in many ingenious ways. Special enzymes clean up the mess as they snip out a mutated segment, join together broken bits of the molecule, or replace a faulty piece with the correct version. The process is like the spell-check in a word-processing program: make a mistake and the machine puts it right. More than 100 genes are now known to be involved in the DNA repair business. Without them we would not survive.

To check the spelling in a document, one needs a back-up based on the correct versions of words (a dictionary) - but where is the dictionary for DNA? In fact, the genome is built on back-ups, with repeat copies of genes that can be used to check when one has gone wrong. Even the double helix is a sort of spell-checker, for editorial enzymes can compare one DNA base with its opposite and undamaged number to check that it fits.

Now comes news of a spectacular new talent in the world of biological proofreading. The plant Arabidopsis has lots of mutations, one of which causes various bits of the flower to fuse together. At first sight, the gene is simple, for it follows the ordinary laws of inheritance of pea colour or of blood groups. As expected, two parents who each carry a double dose of the damaged gene produce only mutated plants - except that, quite often, they produce perfectly normal offspring. The damaged gene has been fixed - but a careful search finds no back-up copy in the genome. A closer look at the DNA shows that, in every case, the version restored is present not in the parents, but in an unmutated ancestor, as far back as a great-grandparent. Somehow, the altered gene has checked back for several generations to see what the correct answer should be (which is rather like turning to the ghost of Dr Johnson for advice on how to spell "lexicographer").

Those who made the discovery suggest that RNA, the molecule that carries information from DNA and helps to make proteins, is involved. Perhaps the substance is preserved over the generations. As a result, it bears a hidden copy of the genetic information to be referred to when things go wrong. The idea is startling indeed, for it proposes a completely new means of inheritance, not based on a solid and permanent genetic dictionary, but instead on a scribbled and carefully concealed piece of paper which can be thrown away once the biological exam is over. All this is alarming news for geneticists - but they face many other baffling questions about the stupid molecule. Damaged DNA is fixed with great efficiency, in many unexpected ways. Why, then - given that nearly all errors are put right - is it not repaired absolutely, with no mistakes at all? Why is the mutation rate not even lower than it is?

We have no idea, but the errors are essential. Without mutations we would all certainly be dead (or, at least, unborn), for a system that copies itself precisely cannot move forwards. Evolution is a series of successful mistakes. Mutations are the fuel of the Darwinian machine and without them it could not keep running. We would find ourselves down in the primaeval slime, among the chemists; and for a biologist no fate could be worse than that.

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