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DNA is as complex as the system it directs. Even after two decades of intensive study only about one-third of the genes have been mapped along the length of DNA in the chromosome of so elementary a creature as the digestive-tract bacterium Escherichia coli. The reason: just a teaspoon of E. coli DNA has information capacity approximately equal to that of a computer with a storage capacity of about 100 cu. mi.
MAN, FOR HIS PART, is even more generously endowed—with 1,000 times as much DNA as one E. coli in each of his reproductive cells. Even so, the cells of such relatively primitive animals as salamanders, lungfish and even certain one-celled algae contain far more DNA than man's. Does this mean that such lowly beasts have a richer genetic capacity than man? The Carnegie Institution's Roy Britten and David Kohne, after much painstaking investigation, may have found the answer to that embarrassing question. A few years ago they discovered that in the DNA of higher organisms many genes seem to be repeated. In calf cells, they calculated, up to 40% of the DNA consists of segments that are repeated as many as 100,000 times apiece. As a result of this work, some scientists are now convinced that in this seeming redundancy of genes, rather than in the total number, lies the secret of the genetic sophistication of higher organisms.
How would such genetic repetition help man? Some theorists suspect that the "spare" DNA plays a regulatory role, perhaps switching other genes on and off at just the right moment during the involved process of protein manufacturing. Harvard Biochemist Charles Thomas, however, supports a more radical idea. He thinks that the repeated segments are actually "slaves" of a "master" gene from which they have been copied. Working in tandem, explains Thomas, such "slaves" could produce proteins more quickly and efficiently—though, he admits, not necessarily in greater diversity.
Molecular biologists are also probing ever more deeply into the process of cell differentiation. It has long been known that the DNA in every body cell of an individual organism is identical; this DNA contains all the information necessary to construct the whole organism. Why then, in a human being, for example, is a liver cell so different from a hair cell, a heart cell so different from a skin cell? The answer, Jacob and Monod theorized in 1961, is that only a small percentage of the genes in any cell are giving instructions for the operation of that particular cell. The rest are "turned off" by protein repressers, which wrap themselves around long stretches of DNA and prevent them from transferring their coded information to messenger RNA.