RANDOM DISTRIBUTION OF HIGHLY REPETITIVE AND INTERMEDIATE FREQUENCY MOUSE L‐929 CELL DNA SEQUENCES SYNTHESIZED AFTER UV LIGHT EXPOSURE

Abstract— The DNA precursor ³H‐thymidine incorporation rate (dpm/pg DNA) in mouse L‐929 cells decreases immediately after exposure to UV light. This decrease is initially dose dependent, but at exposures greater than 22.5 J/m² appears to be radio‐resistant. This was not explained by measurements of uptake of ³H‐thymidine into the acid soluble pool (dpm/pg DNA). The radio‐resistant incorporation amounted to approximately 35% of the control rate. DNA reassociation (“C₀T”) studies were performed with DNA labeled 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 sequences 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 labeled for short intervals and isolated. The DNA reassociation studies showed that this DNA synthesis was also of all classes of repetitious frequency.