Random distribution of highly repetitive and intermediate frequency mouse L-929 cell DNA sequences synthesized after UV light exposure

The DNA precursor 3H-thymidine incorporation rate (dpm/μ g DNA) in mouse L-929 cells decreases immediately after exposure to UV light. This decrease is initially dose dependent, but at exposures greater than 22.5J/m/2 appears to be radio-resistant. This was not explained by measurements of uptake of 3H-thymidine into the acid soluble pool (dpm/μg DNA). The radio-resistant incorporation amounted to approximately 35% of the control rate. DNA reassociation studies were performed with DNA labelled with 3H-thymidine immediately after exposure of L-929 cells with a dose of UV resulting in radio-resistant incorporation to determine whether this radio-resistant incorporation was occurring in sequence of a particular repetitious frequency. These studies, performed in the highly repetitive to intermediate range, showed that the radio-resistant incorporation was occurring in DNA of all classes of repetitious frequency. DNA synthesized at different times after UV exposure, during the period when post-replication repair can occur, was similarly labelled for short intervals and isolated. The DNA reassociation studies showed that this DNA synthesis was also of all classes of repetitious frequency. (author)