The dampening of DNA replication cycles after x-irradiation or ultraviolet irradiation.

Escherichia coli strains 15T(-) (555-7) and B/r were grown in the presence of thymine-(14)C to label all DNA. The ability of these parental DNA's to undergo cycles of replication subsequent to cellular irradiation with either X-ray or ultraviolet light (UV) was followed with density labels. Exposed cells were shifted into the density medium at times which were approximately multiples of normal rounds of DNA replication. A portion of the parental DNA, replicated semiconservatively once during an initial cycle following UV or X-irradiation in E. coli, failed to replicate again within the time studied. The time course of semiconservative parental DNA replication is altered.