Deoxyribonucleic Acid Side-Chain Model of the Chromosomes

Abstract

AT the 1954 Oak Ridge Symposium on Genetic Recombination, I proposed a model of the chromosome in which the molecules of deoxyribonucleic acid projected as side-chains from the main protein axis of the chromosome¹. The advantages of such a model for explaining the results of cross-over studies involving closely linked markers were briefly discussed at one of the sessions of the 1956 Cold Spring Harbor Symposium on Genetic Mechanism. In this model, the genes (deoxyribonucleic acid) are linearly arranged along the chromosome; however, the linearity within the gene is not continuous with, or a part of, the linearity between genes. Intergenic cross-overs would be of the classical type involving exchanges of whole segments of the chromosome, whereas intragenic cross-overs would not exchange other markers on the chromosome. According to this conception, the chromosome is composed of alternate sections of deoxyribonucleic acid and protein. To satisfy the requirement for chromosomal continuity it was suggested that the two chains of the Watson–Crick deoxyribonucleic acid double helix are tied together at the free end. Taylor² has published a chromosome model that differs from mine in that the chromosomal elements are tied together at the other end of the deoxyribonucleic acid instead by making the backbone continuous. In this communication I propose to consider three lines of evidence that may serve to distinguish between these two models.