# Lagging strand

DNA polymerase only builds a new strand in one direction (5' to 3'). On one template strand it builds continuously (leading strand). On the other it builds short pieces called Okazaki fragments (lagging strand), joined by ligase.

Biology (university) · Part 04: DNA, transcription and translation · Card 3 of 9 · Last updated 2026-10-19

## Analogy

Writing a line backwards by moving your pen a short step at a time.

## In practice

Without DNA ligase to seal the gaps, the lagging strand would stay in broken pieces.

## Practice

### Flashcards

- **What does DNA ligase do?** It joins the Okazaki fragments on the lagging strand into a continuous strand.

### Interview questions

**Describe the steps of DNA replication and explain why it is called semi-conservative.**

- Helicase unwinds and separates strands
- Primase starts a primer
- DNA polymerase adds nucleotides 5' to 3'
- Leading continuous, lagging in Okazaki fragments
- Each helix has one old and one new strand

## Questions people ask

### Why does the lagging strand need Okazaki fragments?

DNA polymerase can only add nucleotides in one direction, 5' to 3'. As the helix unzips, one template lets the polymerase follow the fork. The other runs the opposite way, so the enzyme must repeatedly work away from the fork in short pieces, which are joined later.

### What is a primer and why is it needed?

DNA polymerase cannot start a new strand from nothing. It needs a short starting piece with a free 3' end. An enzyme called primase builds a short RNA primer on the template. Polymerase extends it, and later the RNA primers are replaced with DNA.

## Related concepts

- [DNA structure](https://studyvisually.com/learn/science/biology-university/biology-university-part-4/biology-university-part-4-concept-1)
- [Semi-conservative replication](https://studyvisually.com/learn/science/biology-university/biology-university-part-4/biology-university-part-4-concept-2)

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