Chromosome structure--a model based on DNA replication.

Abstract

A model of chromosome structure is proposed which assumes: (i) that DNA replication is accomplished via right-hand (RH) rotation; and (ii), that where replicating DNA segments arc very long RH-rotation will not proceed with absolute freedom. It is expected that inhibition of rotation in daughter molecules will lead to the formation of left-hand-individual (LH-I) coiling systems in the two daughter molecules. It is also expected that inhibition of rotation in the parental molecule will cause the LH-I coiled daughters to be held together in a right-hand-relational (RH-R) association. The interaction between LH-I and RH-R coiling is expected to cause separation of the daughter molecules. Multistranded DNA-containing structures are expected to show, in addition to the LH-I coiling heirarchies formed within individual strands, an RH-R coiling heirarchy formed by the complex as a whole. An LH-I coiling heirarchy was looked for, and found, in Cleveland's drawings of flagellate chromosomes. Evidence for the existence of RH-R coiling was also found. Results of electron-microscope studies on chromosome structure were briefly examined, as were the structures of lampbrush chromosomes, salivary chromosomes and "normal" chromosomes. These studies provided additional, though less direct, evidence in favor of the replication hypothesis and the predictions developed from it.