A method for studying DNA chain growth and chromosomal organization of replicons in HeLa cells has been developed. DNA replication is initiated with bromodeoxyuridine followed by pulse labeling of active replicorts With [3H]thymidine and growth of the chains for finite intervals in unlabeled thymidine. Photolysis of the bromodeoxyuridine-DNA leader with 313-nm light releases the newly replicated chains that are then analyzed by sedimentation in alkaline sucrose gradients. This method of analysis provides data on the rate of chain growth, the bidirectionality of replication, and the distribution of the active replicons at specific intervals in the S period. Applying this method to cells caused to synthesize DNA at a lowered temperature (27 °C) or with protein synthesis restricted by cycloheximide revealed that the irrimediate reduction in the rate of DNA replication in both instances was due to a decreased rate of chain growth without derangement of the overall process.
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