What is a Gene?-Twenty Years Later

I shall conclude with a brief summary of my present views about the gene. I regard a gene as a segment of a larger filamentous structure (chromosome) which is a center of specific biological activity and which is sufficiently different, chemically, from adjacent segments (other genes) as to possess different activity. I think of a gene as having three important properties: the capacity to reproduce itself; the capacity to produce a specific substance, which is responsible for its physiological activity; and the ability, during reproduction, to synapse with a homologous gene. It seems reasonable to assume that a gene reproduces by attracting from its cellular environment the materials of which it is constructed, and making it possible for them to join together. When two synapsed genes are reproducing, it may be supposed that each of them attracts its own set of compounds, and that components accumulated by either of them may be used in the formation of two new genes-a process similar to that proposed by Belling (1933) for the reproduction of chromosomes. In the course of such gene reproduction one may visualize various possible "failures," such as the substitution of one component for another, or a shift in the order of components, which would give rise to a slightly modified gene-that is, an allele of the original one. In the reproduction of two synapsed alleles, one having a failure in one region and the other a failure in another region, a newly formed gene might acquire one, both, or neither of these regions, and consequently there might occur recombinations of the characteristics of the original alleles (pseudoallelism). According to this concept, gene changes (mutations) occur during gene reproduction. It is postulated that agents which increase mutability-such as mutagenic radiations and chemicals, mutator genes, specific chromosomal rearrangements-do so by creating conditions in treated cells that increase the frequency with which the genes fail to reproduce as exact replicas of themselves.