Why is the lagging strand synthesized in fragments?

DNA polymerase only works 5'→3'. The lagging strand template runs 5'→3' toward the fork, so polymerase must work away from the fork in short bursts.

What would happen if ligase were knocked out?

Okazaki fragments on the lagging strand would never be joined, leaving nicks in the new DNA that could lead to strand breaks.

Pro 🔒~25 min

DNA Replication (Detailed)

Helicase, primase, DNA polymerase, and leading/lagging strand synthesis

How it works

DNA replication is semi-conservative — each new DNA molecule contains one original and one new strand. Helicase unwinds the double helix at the replication fork. Single-strand binding proteins stabilize the separated strands. Primase synthesizes short RNA primers. DNA polymerase III extends primers in the 5'→3' direction. On the leading strand, synthesis is continuous. On the lagging strand, synthesis is discontinuous, producing Okazaki fragments (~1000-2000 bp in prokaryotes). DNA polymerase I replaces RNA primers with DNA. DNA ligase seals nicks between fragments. The result: two identical DNA molecules.

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Step-by-step

Press Play to watch the replication fork advance.
Observe the difference between leading (continuous) and lagging (discontinuous) strand synthesis.
Toggle enzyme labels and 5'→3' direction arrows.

Key formulas

$5' \rightarrow 3'$ DNA polymerase can only add nucleotides in the 5' to 3' direction

Frequently asked questions

Why is the lagging strand synthesized in fragments?: DNA polymerase only works 5'→3'. The lagging strand template runs 5'→3' toward the fork, so polymerase must work away from the fork in short bursts.
What would happen if ligase were knocked out?: Okazaki fragments on the lagging strand would never be joined, leaving nicks in the new DNA that could lead to strand breaks.