Tissue culture cells of the African green monkey, Cercopithecus aethiops, were synchronized by release from contact inhibition and pulse-labeled with [3H]thymidine at intervals throughout the ensuing DNA synthetic (S) period. Rapidly reassociating component α DNA, amounting to about 20 % of the total nuclear DNA, was separated from the bulk, non-reassociating DNA by centrifugation in Hg2+-Cs2SO4 density gradients. The specific radioactivity of component α DNA was lower than that of bulk DNA early in S, and higher than bulk DNA at the end of S. Diploid African green monkey kidney cells showed the same pattern of DNA synthesis as heteroploid CV-1 cells. CV-1 cells were also pulse-labeled with [3H]thymidine and the mitotic cells collected for DNA analysis at selected time intervals thereafter. Component α DNA, which has a G+C content equal to, or slightly greater than the bulk DNA, again showed its highest specific radioactivity compared to bulk DNA at the end of S. Earlier work has shown that the S period in several types of mammalian cells is characterized by a progressive shift from the early synthesis of DNA having a high average G+C content to the synthesis of DNA having a low average G+C content late in S. The synthesis of C. aethiops bulk non-reassociating DNA conformed to this pattern.
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