Question

Explain how the synthesis of the lagging strand occurs in E. coli. Include a description of the role of each of the key enzymes involved. [12 marks]

Answer 1

The lagging strand is the strand of nascent DNA whose direction of synthesis is opposite to the direction of the growing replication fork. Because of its orientation, replication of the lagging strand is more complicated as compared to that of the leading strand. As a consequence, the DNA polymerase on this strand is seen to "lag behind" the other strand.

The lagging strand is synthesized in short, separated and discontinuous segments. On the lagging strand template, a primase "reads" the template DNA and initiates the synthesis of a short complementary RNA primer. A DNA polymerase extends the primed segments, forming Okazaki fragments. The RNA primers are then removed and replaced with DNA, and the fragments of DNA are joined together by DNA ligase.