# DNA Replication Fork

> Before a cell divides it copies its DNA. At the replication fork the double helix is unwound and each strand serves as a template. One new strand is made continuously, and the other in short pieces.

Web page: https://cellnaut.com/topics/dna-replication-fork

![Teaching illustration: DNA Replication Fork](https://cellnaut.com/texture-references/gpt-image-2-biology-more-teaching-2026-05-31/jpg/dna-replication-fork-teaching.jpg)

## Labelled parts

1. **Replication fork**: The Y-shaped region where the double helix is being opened and copied.
2. **Helicase**: Breaks the hydrogen bonds between the strands to unwind the helix.
3. **Single-strand binding proteins**: Coat the exposed single strands so they do not pair up again.
4. **Leading strand**: Copied continuously in the same direction as the fork opens.
5. **DNA polymerase**: Adds nucleotides to the growing strand, following the template.
6. **Sliding clamp**: A ring-shaped protein that holds the polymerase on the DNA.
7. **Lagging strand**: Copied in short pieces, because polymerase can only build in one direction.
8. **Okazaki fragment**: One of the short pieces on the lagging strand, later joined together by ligase.
9. **RNA primer**: A short RNA starter that polymerase needs to begin each new piece.

## Key points

- Helicase unwinds the helix and single-strand binding proteins keep the strands apart.
- DNA polymerase can only add nucleotides in one direction, so the lagging strand is made as Okazaki fragments.
- Each daughter DNA molecule has one old strand and one new strand (semi-conservative replication).

## Check yourself

**1. Which enzyme unwinds the DNA double helix?**

- A. DNA ligase
- B. Helicase
- C. RNA polymerase
- D. Ribosome

**Answer:** B. Helicase - Helicase breaks the hydrogen bonds between the strands.

**2. Why is the lagging strand made in fragments?**

- A. Polymerase works in one direction, away from the fork on this strand
- B. It has a different sequence
- C. It is made by ribosomes
- D. It is not copied at all

**Answer:** A. Polymerase works in one direction, away from the fork on this strand - The short pieces are Okazaki fragments, joined later.

## Related lessons

- [From DNA to Protein](https://cellnaut.com/learn/dna-to-protein)
- [The Cell Cycle and Mitosis](https://cellnaut.com/learn/cell-division)

## See it in 3D

- [Animal Cell](https://cellnaut.com/studio/animal)