The sedimentation coefficient and buoyant density of nucleosomes from replicating chromatin in heated cells

The sedimentation coefficient and buoyant density of mononucleosomes from the replicating chromatin of heated (45 degrees) and unheated HeLa cells were determined. It was observed that both physical parameters were the same (S = 10.8 and p = 1.165 g/cm3) for nucleosome particles from replicating and mature chromatin in heated and unbeated cells. The size of DNA in the nascent and mature nucleosome from heated or unheated cells was observed to be 145 base pairs of double-stranded DNA. A significant fraction of nucleosomal DNA from heated cells was observed at subnucleosomal sizes, principally at 125 base pairs of DNA. It is concluded that 45 degrees thermal shock does not alter appreciably the subunit structure of chromatin at the replication fork.