ABSTRACT
Human lymphocytes in the quiescent state were exposed to UVC radiation. After irradiation the cells were allowed to repair for various times in the presence of [3H]thymidine or [3H]deoxycytidine in the culture medium. Hydroxyurea was not used to suppress semiconservative DNA replication in the small number of growing cells. After incubation DNA strand breaks were detected by the DNA-unwinding method and the amount of 3H incorporation in DNA was measured by liquid scintillation counting. The results show that the yield of DNA strand breaks and the amount of unscheduled DNA synthesis (UDS) can be measured from the same lymphocyte sample. A low background 3H incorporation in untreated cells could be achieved even in the absence of hydroxyurea. This requires, however, that 3H incorporation is measured only in the double-stranded DNA and that [3H]dCyd is used instead of [3H]dThd as the labelled deoxynucleoside.
