Q.What is the cause of discontinuous synthesis of DNA on one of the parental strands of DNA? What happens to these short stretches of synthesised DNA?

DNA polymerase can only add nucleotides in the 5' → 3' direction, which creates a problem during replication. Because the two parental strands run antiparallel (one 3' → 5', the other 5' → 3'), and the replication fork opens in one direction, only one strand—the leading strand—can be synthesized continuously toward the fork. The other strand, called the lagging strand, runs 3' → 5' in the direction of fork movement, so DNA polymerase must work away from the fork in short bursts. …

The heart of this puzzle lies in understanding how DNA polymerase works. This enzyme, responsible for building new DNA strands, has a fundamental directional constraint: it can only add nucleotides to the 3' end of a growing chain, meaning synthesis always proceeds in the 5' → 3' direction. This wouldn't be a problem if DNA were a simple linear molecule, but the two strands of the double helix run antiparallel—one goes 5' → 3' while its partner runs 3' → 5'.

When the replication fork opens up, exposing both template strands, DNA polymerase faces a dilemma. On one template strand (the 3' → 5' strand), the enzyme can move smoothly in the same direction as the fork is opening, laying down nucleotides continuously. This is the leading strand, and its synthesis is straightforward.

But the other template strand runs in the opposite direction. DNA polymerase still must work 5' → 3', yet the fork is moving away from that direction. The solution? The enzyme synthesizes this lagging strand in short, backwards segments. As the fork opens a bit more, polymerase jumps back to the newly exposed region and synthesizes another short stretch in the 5' → 3' direction, again moving away from the fork's overall direction of travel.

These short stretches are called Okazaki fragments, named after the Japanese scientist Reiji Okazaki who discovered them. In prokaryotes they're roughly 1000–2000 nucleotides long; in eukaryotes they're shorter, about 100–200 nucleotides. …

Instead of starting from the definition of Okazaki fragments, start from the fork geometry alone: at a Y-shaped fork only one exposed 3'-OH end points toward the fork (continuous synthesis), while the other template's exposed end points away from it -- a polymerase that can only extend a 3'-OH end has no choice but to synthesise that s …