Q.Discontinuous synthesis of DNA occurs in one strand, because:

DNA polymerase can only add nucleotides to the 3′-OH end of a growing strand, which means synthesis always proceeds in the 5′ → 3′ direction. Because the two strands of the DNA double helix run antiparallel, when the replication fork opens, one template strand (3′ → 5′) allows continuous synthesis in the direction of fork movement — this is the leading strand. The other template strand (5′ → 3′) runs opposite to the fork direction, so the polymerase must work backwards in short bursts, creating Okazaki fragments that are later sealed by DNA ligase. This discontinuous synthesis on the lagging strand is not a choice for efficiency or a consequence of DNA length; it is an unavoidable result of the enzyme's directional constraint. …

The heart of this question lies in understanding a fundamental constraint of the enzyme that builds DNA. When the double helix unwinds during replication, two template strands are exposed running in opposite directions—one oriented 3' → 5' and the other 5' → 3'. The replication machinery must copy both simultaneously as the fork moves forward.

DNA polymerase, the enzyme responsible for adding new nucleotides, has an absolute directional requirement: it can only synthesize DNA in the 5' → 3' direction. This means it can only add a new nucleotide to the 3'-OH end of a growing chain. There is no version of this enzyme in nature that works the other way around.

On the leading strand, where the template runs 3' → 5', this poses no problem. The polymerase simply moves continuously in the same direction as the replication fork, smoothly adding nucleotides one after another. The synthesis is continuous and straightforward.

But on the lagging strand, the template runs 5' → 3'—the "wrong" way relative to fork movement. The polymerase still must work 5' → 3', which is now opposite to the direction the fork is opening. The enzyme cannot work backward toward the fork. Instead, it must wait for a stretch of template to be exposed, then synthesize a short fragment away from the fork. As more template is exposed, another fragment is started, and another. These short pieces—Okazaki fragments—are synthesized discontinuously, each in the 5' → 3' direction but collectively covering a template that runs the opposite way. …

Picture the replication fork as a zipper opening left to right, exposing two template strands running in opposite directions. The strand running 3'→5' (in the fork's opening direction) lets the 5'→3'-only polymerase move continuously toward the fork — no problem. The other exposed strand runs 5'→3' in the fork's opening direction, so continuous synthesis on it would require the enzyme to move 3'→5', which no DNA polymerase can do. It must …