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History of Genetics

The Molecular Gene

It has been remarked that molecularizing the gene, far from establishing it as a discrete entity, had the opposite effect, fragmenting and destroying it. True, ever since the 1930s there had been discussion of what was euphemistically called "position effect"—the idea that a gene defined by its function can suffer two distinct mutations and that even if one normal example of the two mutated sites is present in the cell, the gene will only function when both lie on the same chromosome. The neat coincidence between the gene defined by function, by location, and by mutation was broken in these cases. But unraveling the mystery of protein synthesis, in which the function of the genes is to specify which amino acids are to be incorporated into the protein and in what order, revealed further complexity. It turned out that the genetic message (messenger RNA) from many genes is cut into parts, some fragments being rejected (known as introns) and the remainder (exons) incorporated into the chain that is transcribed into a sequence of amino acids. Different parts of the same message may thus be incorporated into different gene products. To those working in the field, these complexities are par for the course. The word gene has not died and been buried, but the context in which it is used tells the informed listener in what sense to understand the term. The simple conception of the Mendelian factor or gene of classical genetics has surely suffered a major series of revisions. The simple picture of genes like beads on a string, though essential to get a hand-hold on the problem, has not survived. But the early claim of Richard Goldschmidt that any discreteness of the gene be dropped, and that the chromo-some be considered one continuous developmental unit, has never been accepted. There are codons in the DNA for starting and terminating the genetic message. Genes are interspersed with non-coding regions in the DNA. But the relation between the DNA and the proteins around it is a subtle and dynamic one.

Since Johannsen introduced the term "gene" first in 1909 it has served as a flexible concept, suffering many revisions and acquiring many meanings. In the process the extreme hereditarian view of DNA as dictating the life of the cell has been undermined by the discovery of a hierarchy of interactions between DNA and proteins. Genetics has in truth found its place at the core of biology, and in doing so it has revealed a machinery of the cell more intricate and subtle than could ever have been imagined.

BIBLIOGRAPHY

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Robert Olby

Additional topics

Science EncyclopediaScience & Philosophy: Heterodyne to Hydrazoic acidHistory of Genetics - Gregor Johann Mendel, Rediscovery, The Chromosome Theory Of Heredity, Culture Of Heredity, Fine Structure Of The Gene