Identify the region in prokaryotic DNA, where replication starts.

The correct answer is Option (4) → Origin of replication

Replication does not start at a random place in DNA. In E. coli there is a definite region where it begins. This region is the origin of replication.

(1) Lagging strand — Incorrect. This is the new strand that is made as short pieces (Okazaki fragments) because DNA polymerase can work only in the 5′ → 3′ direction. It is a strand, not a place where replication starts.

(2) Leading strand — Incorrect. This is the new strand that is made continuously, in the same direction in which the fork opens. It is also a strand, not a starting site.

(3) Promoter region — Incorrect. This is the site where RNA polymerase binds to start transcription, not replication.

(4) Origin of replication — Correct. Replication begins here, and the fork then moves along the DNA.

NCERT Extract:
The Machinery and the Enzymes – “… The DNA polymerases on their own cannot initiate the process of replication. Also the replication does not initiate randomly at any place in DNA. There is a definite region in E. coli DNA where the replication originates. Such regions are termed as origin of replication. It is because of the requirement of the origin of replication that a piece of DNA if needed to be propagated during recombinant DNA procedures, requires a vector.”