God, DNA, and the Sheep
MEDICINE TODAY
By JOHN ROWAN WILSON
Ah, sweet mystery of life, at last I've found thee! Ah, I know at last the secret of it all!
Cto sing, these days, the molecular biologists. aEvery area of scientific research has its hey- day, its golden age, and this is theirs. The atomic physicists have had their brief moment of glory, but having shown us how to blow the entire world to pieces they have found themselves with little left to say. It is now the turn of the molecu- lar men to tell us how the pieces came to be put together in the first place. And this they have done in the most startling and spectacular manner. Nobel prizes have been showered on them like confetti. Like all successful scientists they have turned to prediction and philosophy. They have traced the progress of evolution and suggested ways by which man may himself con- trol its future course. They have hit God for six, right out of the Universe.
Well, perhaps not quite that. But they have certainly suggested an alternative explanation for one of the great mysteries of existence—the origin of living matter. And this explanation has sprung largely from one historic piece of research —the discovery of the genetic code.
It isn't easy to explain the genetic code in a few simple words, but it is so fundamental to our existence that surely every educated person ought to have some idea of what it means. It is certainly a better candidate for an eisential piece of scientific knowledge for laymen than Lord Snow's second law of thermodynamics. The principle of the thing is roughly this. It has been discovered that the reproductive cell contains within it all the information required to make the final living organism that it eventually pro- duces; and that this information is carried in code form in the molecule of a chemical sub- stance known as deoxyribose nucleic acid, usually known for short among scientists as DNA.
DNA, as one might have guessed, is a pretty complicated molecule, consisting of a very long backbone made up of alternate groups of phos- phate and sugar. To this chain are attached side groups known as bases. These bases come in only four types, but they can be repeated a vast number of times along the great chain, in a variety of different orders. They thus form a four-letter code. Their arrangement and sequence contain the genetic information.
A gene is a piece of DNA which causes the synthesis of a particular protein. The protein molecule also is very long, with a backbone to which side-chains are attached. It consists of a string of smaller molecules known is amino- acids, of which there are twenty varieties. A protein may consist of as many as 400 amino- acids, and its nature is determined by the sequence in •which its amino-acids are joined together. The DNA of the gene contains the information which determines this. The four- letter code of the DNA is thus translated into the twenty-letter alphabet of the protein. Once made, the proteins act both as structural com- ponents of the cell and, more important, as enzymes or biological catalysts which control the chemical reactions the cell needs for growth and division.
This is a highly simplified account of the pro- cess. However, it is perhaps enough to illustrate what an important discovery the molecular biologists have made. It reduces the miracle of creation from the construction of a complex organism down to the construction of a mole- cule of nucleic acid, and holds out the possibility that one of these days man may be able to make molecular structures which possess that capacity for growth and reproduction which we generally define as life. It opens up the possibility of a completely mechanistic creation, with a piece of nucleic acid formed by accident and the whole process going on from there. It doesn't actually dispose of God, of course, since it doesn't ex- plain where the necessary chemicals come from in the first place. And if God is there at all, he is presumably just as capable of talking to Dr Francis Crick in a four-letter nucleic code as to Moses in ancient Hebrew.
This work has been carried out on very simple organisms, particularly viruses. And it has great medical significance in that it has managed to explain for us some very baffling aspects of virus behaviour, particularly the fact that they cannot reproduce outside living cells. It is now clear that viruses, while they contain nucleic acid, do not contain the necessary chemical machinery to translate the nucleic acid code into the langu- age of the proteins. So they enter the cell of some plant or animal and take over its chemical machinery. They supply it with the information contained in their own nucleic acid and force it to manufacture the proteins needed to form more virus particles. These then erupt from the host cell and move off to infect others.
A very remarkable situation, if not without its gruesome aspect. It makes it fairly clear why we have had so little success in developing chemi- cals which would kill viruses without damaging the cells of the infected individual. It explains the smallness and simplicity of virus structure, and the fact that they can pass through the smallest filters and revive after being crystallised. So long as they have DNA, anything is possible.
It is a beautiful explanation—but there is one small cloud on the horizon. Recently, animal pathologists have been investigating a certain disease of Sheep. This has the elegant name of `scrapie,' arising from the fact that it causes the infected sheep to scratch themselves against fences. At first it was thought to be hereditary, but it has now been found to be an infectious disease caused by a very tiny organism which is alive in the sense that it is capable of reproduc- tion. Investigations to date on the scrapie agent, whatever it may be, have not definitely revealed its constitution, but recent work has led to the disconcerting suggestion that it may not contain any nucleic acid at all. If this turns out to be the case, it is not going to be just the sheep who are scratching their heads..
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