Question

Describe, in detail, how synthesis of a lagging strand of DNA occurs and WHY it is discontinuous. Be as descriptive as possible.

Answer 1

- A famous Japanese molecular biologist, Reiji Okazaki claimed that DNA backbones run in opposite directions. Okazaki found that a significant proportion of newly synthesized DNA exists as small fragments. These units of about a thousand nucleotides (called Okazaki fragments) are present briefly in the vicinity of the replication fork. During replication process, Okazaki fragments become covalently joined through the action of DNA ligase to form one of the daughter strands called lagging strand.  

- Explaining briefly, DNA strands are oriented antiparallel to one another with the arrowhead of one strand matching up with the tail of the other strand. Researchers labeled the ends of the DNA strands according to the carbons in the sugar ring. One end is called the 3' end, and the other is called the 5' end. So on any complete molecule of DNA, one strand will run from 3' to 5', and the other will run from 5' to 3'. The parent strand of DNA which runs in the 3' to 5' direction toward the fork, and it's able to be replicated continuously by DNA polymerase is called the leading strand; and the parent strand which runs in the 5' to 3' direction toward the fork is called the lagging strand. A lagging strand requires a slight delay before undergoing replication, and it must undergo replication discontinuously in small fragments.

- Lagging strand is discontinous because the lagging strand needs to be replicated in the opposite direction of the way the replication fork is proceeding since DNA polymerase can proceed only 5' to 3'. Thus, short fragments are produced as the replication fork expands. Otherwise, DNA polymerase would need to wait until the fork reached the end of the molecule to replicate the lagging strand. Okazaki found a beautiful solution which explains Okazaki fragments are joined by ligation.