6.6: Lagging Strand Synthesis
During replication, the complementary strands in double-stranded DNA are synthesized at different rates. Replication first begins on the leading strand. Replication starts later, occurs more slowly, and proceeds discontinuously on the lagging strand.
There are several major differences between synthesis of the leading strand and synthesis of the lagging strand. 1) Leading strand synthesis happens in the direction of replication fork opening, whereas lagging strand synthesis happens in the opposite direction. 2) For leading strand synthesis, a single primer is needed, whereas multiple RNA primers are required for lagging strand synthesis. 3) After initial primer synthesis, the leading strand needs only DNA polymerase for replication to continue, whereas the lagging strand needs multiple enzymes, including DNA polymerase I, RNase H, and ligase. 4) The leading strand is synthesized as a continuous piece, whereas the lagging strand is synthesized as a series of shorter pieces called Okazaki fragments. Thus, lagging strand synthesis is a multistep process involving sophisticated coordination among different molecules.
Due to the different genome sizes of prokaryotes and eukaryotes, the process of lagging strand synthesis differs between them. The most prominent difference is the length of the Okazaki fragments. The average Okazaki fragment length is around 1000 to 2000 nucleotides in prokaryotes, but only 100 to 200 nucleotides in eukaryotes.