The asymmetric segregation of parental nucleosomes during chrosome replication.

SV40 DNA replicated in the presence of cycloheximide was more sensitive to staphylococcal nuclease digestion and had a lower superhelical density than viral DNA replicated in the absence of this drug. These data indicate that fewer nucleosomes are associated with progeny SV40 DNA molecules after DNA replication in the absence of protein synthesis and that these nucleosomes are derived from the parental histones. We designed an experiment to determine whether these parental SV40 nucleosomes segregate to the leading side of the replication form where DNA synthesis is continuous, the lagging side of the fork where DNA synthesis is discontinuous or randomly to both sides of the fork. The results indicate that the parental histones distributed themselves asymmetrically, preferentially (80-90%) segregating with the leading side of both SV40 DNA replication forks during bidirectional replication in the absence of protein synthesis. In the case of SV40, the same parental DNA strands are the templates for the leading side of DNA replication at both forks as well as the templates for the informational or coding strand of early and late viral mRNA synthesis. Based on this correspondence, we designed an experiment to test whether chicken cells growing in culture and replicating their DNA in the absence of protein synthesis segregated their parental histones asymmetrically to the progeny DNA strand that also coded for stable nuclear RNA transcripts. The results of these experiments indicate that, like SV40, parental cellular histones segregate asymmetrically and are preferentially associated with those DNA template strands that code for stable nuclear RNA species detected by hybridization to single-copy DNA.