Changes in the Physical State of DNA during Replication in Regenerating Liver of the Rat

Evidence is presented which demonstrates that DNA synthesis in regenerating liver is discontinuous. In pulse-labeling experiments from 30 seconds to 10 minutes, the earliest detectable intermediate in DNA replication appears to be double-stranded DNA which has a significant degree of single-stranded character probably due to the existence of gaps in the newly synthesized strand. The label begins to move into completely double-stranded DNA 10-30 minutes after the commencement of labeling. The average molecular weight of the single-stranded "unit" of DNA replication is determined to be approximately 7.5 million. Smaller units may be produced but not observed as a result of imperfect synchronization of DNA synthesis.