question_answer

DNA replication occurs in a small replication fork and not in its entire length.

(i) Why is DNA replication continuous and discontinuous on the two strands of DNA?

(ii) Explain the importance of 'origin of replication' in a replication fork.

Answer:

(i) Polymerisation by DNA dependent DNA polymerase occurs only in 5'\[\to \]3' direction only on the 3'\[\to \]5' strand. This is because it adds bases at 3'-end. Since, the two strands of DNA run in antiparallel directions, the two templates provide different ends for replication. Thus, replication on two templates proceeds in opposite directions. One strand with 3'\[\to \]5' direction forms its complementary strand continuously because 3' - end of the latter is always open for elongation.

On the 3?\[\to \]5' template the replication is discontinuous because only a short segment of DNA strand can be built in 5?\[\to \]3' direction due to exposure of a small stretch of template at one time.

(ii) ?Origin of replication' is a sequence of DNA at which replication is initiated. Any piece of DNA, when linked to this sequence can be made to replicate within the host cells. This sequence is responsible for controlling the copy number of the linked DNA.