Science: The Secret of Life

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The charm of this structure for geneticists comes from its variability. Each step between the helices can be made of either pair of bases pointing in either direction. If the spirals should be pulled apart (the chemical bonds between bases are weak), each spiral would be left with the four bases arranged in any sequence. If arranged meaningfully along the spiral, the bases could carry information in a four-symbol code, much like digits on the magnetic tape of an electronic computer.

Here, the geneticists now believe, lies the high command of growth and reproduction. Double-helix DNA molecules, thousands of turns long and arranged by thousands in each chromosome, can carry a vast amount of coded information. They may very likely carry enough to determine whether a fertilized egg grows into a clam or an elephant. When chromosomes replicate during cell division, the DNA molecules that they contain presumably replicate too.

Stealthy Viruses. This concept of the DNA molecule has started a vast amount of excited work. Mathematicians are trying to break its four-symbol code. Chemists are trying to dig deeper into its structure. All sorts of biologists are looking for effects of DNA on the behavior of living organisms, and they are finding a wealth of strange things. Loose DNA can penetrate certain bacteria, changing them permanently into a new strain. Many viruses are packets of DNA wrapped in a coat of protein. When a virus infects a living cell, it leaves its coat outside. The DNA enters the cell and takes charge of its activities, issuing chemical orders as if it owned the place. Its orders are simple: "Stop everything and make more virus particles packed with DNA." The cell obeys helplessly, turns its contents into virus particles, bursts and dies.

Sometimes a virus enters a cell and makes it multiply over and over, even if its unruly growth kills the animal of which the cell is a part. Several kinds of animal cancer are caused by such viruses, whose DNA presumably takes command and makes the cells multiply wildly.

Anticancer Orders. Some geneticists think that many if not all kinds of cancer are caused by invading viruses. Others think not. But all agree that the genetics of cancer-causing viruses and cells that are their victims is a promising road toward the cure or prevention of cancer. If cancer cells multiply wildly because the DNA of a virus is giving them orders, it may be possible to countermand those orders with another kind of DNA. Knowledge about DNA may also help prevent some kinds of radiation damage.

If DNA can change bacteria from one true-breeding strain to another, it may have some similar effect on higher animals, including humans. If such a process is discovered, not much DNA will be needed. The entire supply of DNA that could control the heredity of the next generation of the human species (several billion individuals) could be put in a cube one twenty-fifth of an inch on a side.