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Replicative activity of isolated chromatin from proliferating and quiescent early passage and aging cultured mouse cells

Replicative activity of isolated chromatin from late passage cultured mouse cells has been compared to the activities of chromatin preparations from dividing and quiescent early passage cells. Rates of endogenous DNA synthesis are similar for chromatin from growing or resting cells but this activity is stimulated 2.5‐fold in senescent cell chromatin. Chromatin from growing young cells copies exogenously added single stranded DNA at the highest efficiency. Chromatin of senescent cells copies this template at a lower rate and resting young cell chromatin replicates single stranded DNA at the lowest efficiency. Similar relative rates are obtained when activated DNA is copied by the various chromatin preparations. Total activity of DNA polymerase extracted by salt from chromatin is similar for dividing and quiescent young cells but the proportion of DNA polymerase β is higher in the latter. Elevated activities of DNA polymerases are extracted from chromatin of old cells. It is concluded, therefore, that chromatin‐directed replication is differently arrested in nondividing senescent cells and in quiescent early passage cells. The possible regulatory mechanisms of DNA replication in quiescence and aging are discussed.