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Science Frontiers ONLINE No. 41: Sep-Oct 1985 |
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Genetic Garrulousness
It is tempting to predict that those cells with the most genetic material will belong to the most advanced organisms. One would, for example, expect to find more DNA or nucleotide pairs in human cells than the cells of bacteria or plants. In the case of the bacteria, this expectation is realized. Some plants, however, have one hundred times more DNA per cell than humans. Some fish and salamanders do, too.
One reason why there is no simple relationship between a cell's genetic complement and the organism's complexity is that a lot of genetic material is apparently useless, with no known functions. Human genes, by way of illustration, possess about 300,000 copies of a short sequence called Alu. The Alu sequences seem to be simply dead weight -- functionless -- yet continuously reproduced along with useful sequences. One purposeless mouse gene sequence is repeated a million times in each cell.
(Stebbins, G. Ledyard, and Ayala, Francisco J.; "The Evolution of Darwinism," Scientific American, 253:72, July 1985.)
Comment. Why so much redundance? Or is there some purpose for this excess genetic material that we haven't yet descried? The "useless" sequences may merely be left over from ancient gene shufflings; or they may be awaiting future calls to action. The above tidbits come from a long review article that is generally supportive of the modern theory of evolution. (Would it have been printed in Scientific American if it weren't?) The article also treats the Neutral Theory of Evolution, noting that this theory still depends upon natural selection operating at the phenotype (organism) level.