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

Long stretches of protein-free, nonbeaded DNA were observed electron microscopically in nuclear spreads prepared from 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. In addition, 26 presumptive replication forks were observed with beaded chromatin on two arms and free DNA on the third. These results suggest that new histones are usually deposited onto new DNA, that the cellular histone pool is very small, that histone migration is minimal in vivo for at least 18 hr, that for most fibers nuclesome assembly and segregation is conservative for stretches of DNA as long as 100 kbases, and that some part of the octameric histone core may remain bound to DNA during the replication process. The regularity we have observed for the assembly and segregation of nucleosomes is likely to be important for our understanding of how chromosomal information is segregated during development.