Okazaki fragments are produced during lagging-strand DNA synthesis. On which strand do they occur, and which enzyme covalently joins the fragments into a continuous strand?

Introduction / Context:
Replication forks copy the two antiparallel strands differently. The leading strand is synthesized continuously, whereas the lagging strand is synthesized discontinuously as Okazaki fragments that later must be covalently joined. This question checks that mechanistic pairing: which strand and which enzyme.

Given Data / Assumptions:

• Okazaki fragments indicate discontinuous synthesis.
• Final joining requires ligation of the sugar–phosphate backbone.

Concept / Approach:
Okazaki fragments arise exclusively on the lagging strand because DNA polymerases synthesize 5'→3', yet fork movement forces discontinuous synthesis on one template. DNA ligase seals the nicks between fragments after primer removal and gap filling.

Step-by-Step Solution:

Verification / Alternative check:
Biochemical assays show ligase-deficient extracts accumulate unsealed fragments on the lagging template.

Why Other Options Are Wrong:

• Leading strand, DNA polymerase: leading strand is not fragmentary; polymerase extends but does not seal nicks.
• mRNA, anticodons: irrelevant to DNA replication.
• tRNA, DNA polymerase: unrelated to fragment ligation.
• Lagging strand, DNA gyrase: gyrase relieves supercoiling, not ligation.

Common Pitfalls:
Confusing gap filling (polymerase) with backbone sealing (ligase); thinking both strands form Okazaki fragments—only lagging does.

Final Answer:
Lagging strand, DNA ligase