The replication of DNA in murine lymphoma cells (L5178Y): II. Size of replicating units

Abstract

The size of the replicating units of the DNA from the murine lymphoma cell L5178Y has been studied by direct measurement of the size of the growing DNA strands in sucrose gradients. Cells were pulse-labelled with [ 3H]thymidine and then irradiated with low doses of X-rays in order to introduce a small amount of fragmentation into the DNA, thus overcoming entanglement effects. Detailed analysis of the radioactivity profiles obtained on sedimentation of the resulting labelled DNA fragments in alkaline sucrose gradients has provided information about the size of the growing DNA strands. The results suggest that most of the replicating units have a single-strand molecular weight of about 2 · 10 8. Sometimes a much smaller population with molecular weight of about 4 · 10 7 is also observed. On completion of replication the newly-synthesized strands are linked end-to-end, so that the molecular weight of the DNA strands in the G2 phase of the cell cycle is greater than 10 9 ( i.e., larger than can be estimated by the methods available at present).