Chromosome reproduction: units of DNA for segregation.

Evidence is summarized which indicates that the DNA loop anchoring proteins in chromosomes are effectively heterodimers that stack and are fastened into a bilaterally symmetrical array along the chromonemal axis. The evidence consists primarily of the observations made twenty five to thirty years ago on the pattern of sister chromatid exchanges and the way the DNA chains are sorted in the formation of diplochromosomes in cells that have undergone endoreduplication. The evidence indicates that each chain of DNA in the single duplex, which is assumed to run the length of a chromosome, is anchored to a bilaterally symmetrical axis of heterodimers that sort the two original chains among the four derived chromatids of each diplochromosome in a very precise way. These observations are considered in the context of investigations on the nature of scaffold proteins and the loop anchorage sequences, as well as the advances being made on the nature of DNA binding proteins and the roles of topoisomerase II.