Okazaki DNA fragments contain equal amounts of lagging-strand and leading-strand sequences.

The question of whether discontinuous DNA replication operates only for the lagging strand or for both strands in Escherichia coli is still not resolved. Most in vitro data indicate that DNA replication is semi-discontinuous. In contrast, the in vivo data indicate that DNA replication is discontinuous in both strands. Proponents for the semi-discontinuous DNA replication model argued that the failure of demonstrating semi-discontinuous DNA replication in vivo may be due to the breakdown of a continuously synthesized strand by DNA repair processes. In this article, we present further evidence that the apparent discontinuous DNA replication in E. coli is not the result of DNA repair processes. Furthermore, we show that the Okazaki fragments contain about equal amounts of leading- and lagging-strand sequences. These results are most consistent with the discontinuous DNA replication model as originally proposed by Okazaki et al.