Conservative segregation of parental histones during replication in the presence of cycloheximide

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Abstract
Long stretches of protein-free, nonbeaded DNA were observed by EM in nuclear spreads prepared from [chicken leukemia cells] cells that had replicated their DNA in the absence of protein synthesis. The amount of this DNA increased with increasing time of replication in the presence of cycloheximide and was greatly decreased when replication was inhibited with 1-.beta.-D-arabinofuranosylcytosine (cytosine arabinoside). This DNA is considered to be free DNA because it has the same diameter as marker PM2 DNA and it is preferentially sensitive to DNase I digestion. Reversal of the cycloheximide block resulted in a burst of histone synthesis and repair of the depleted chromatin within 5 min. Presumptive replication forks [26] were observed with beaded chromatin on 2 arms and free DNA on the 3rd. New histones are probably usually deposited onto new DNA and the cellular histone pool is probably very small. Histone migration is probably minimal in vivo for at least 18 h for most fibers nucleosome assembly and segregation is possibly conservative for stretches of DNA as long as 100 kb, and some part of the octameric histone core may remain bound to DNA during the replication process. The regularity observed for the assembly and segregation of nucleosomes is likely to be important for an understanding of how chromosomal information is segregated during development.