Fig. 4. Accumulation of DNA replication intermediates in DNA-ligase-I-deficient cells. Cultures were subjected to a short pulse with [³H]-thymidine followed by a chase of up to 30 minutes. Cells were then harvested, lysed in agarose plugs and subjected to alkaline gel electrophoresis. Fractions from <0.2 kb to >23 kb were collected and counted. The figure shows the percentage of total radioactivity present in single-stranded DNA fragments <2.0kb for each cell line. Values shown are means±s.e.m. , mouse wild-type (PF20); , human wild-type (MRC5); , mouse Lig1 null (PFL13); [UNK], human LIG1 point mutant (46BR).