Question

What is the basis for the difference in the synthesis of the leading and lagging strand of DNA molecules?

Options

• Origin of replication occurs only at the 5’ end of the molecules.

• DNA ligase works only in the 3’ → 5’ direction.

• DNA polymerase can join new nucleotides only to the 3’ end of the growing strand.

• Helicases and single-strand binding proteins that work at the 5’ end.

Answer 1

DNA polymerase can join new nucleotides only to the 3’ end of the growing strand.

Explanation:

The primary reason for the difference in synthesis is the directional constraint of the enzyme DNA polymerase, which can only add new nucleotides in the 3’ → 5’ direction. Because the two strands of the DNA double helix are antiparallel (running in opposite directions), only one strand, the leading strand, can be synthesised continuously as the replication fork opens. The other strand, known as the lagging strand, must be synthesised in short, discontinuous segments called Okazaki fragments because its template runs in a direction that forces the polymerase to move away from the expanding replication fork to find a free 3’ end.