DNA replication in mammalian cells. I. The size of newly synthesized helices

The newly synthesized DNA of heteroploid human cells cultured in vitro has a single-stranded molecular weight of approximately 1 x 10(5) following 30- to 60-second pulses of labeling with tritiated thymidine. With increasing pulse times the amount of radioactive DNA having a low single-stranded molecular weight remains approximately constant, whereas increasing amounts of label are found associated with the bulk DNA. After removing the label from the medium, the radioactivity associated with the small molecular weight DNA is chased into high molecular weight material. These findings are analogous to what is observed in microorganisms and suggest a basically common organization of the growing point of DNA in bacterial and in mammalian cells.