Fig. 9: Replication of Leading and Lagging DNA Strands

The leading strand is made continuously in a 5' to 3' direction by DNA polymerase III as the DNA helicase unwinds the parental DNA helix. However, because the parental DNA strands are antiparallel, the lagging strand must be made in short fragments. RNA polymerase (primase) synthesizes a short RNA primer which is extended by DNA polymerase III. DNA polymerase II then digests the RNA primer and replaces it with DNA. Finally, DNA ligase joins the fragments of the lagging strand together.

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