# Explain why cells in S phase must complete DNA replication before entering mitosis.

> IB DP Biology Standard Level (2023 syllabus) — D2.1 Cell and nuclear division · Explain · 4 marks

> During the cell cycle, a cell in S phase replicates its entire genome. Regulatory checkpoints monitor the completion of this process before the cell is permitted to enter mitosis.

## Mark scheme (4 marks)

1. Each chromosome must consist of two identical sister chromatids (joined at the centromere) before mitosis begins.
2. If replication is incomplete, daughter cells would receive insufficient or unequal genetic material.
3. During mitosis, sister chromatids are separated so that each daughter cell receives one copy of each chromosome; this only produces genetically identical cells if replication was complete.
4. The G2 checkpoint (DNA damage / replication checkpoint) detects unreplicated or damaged DNA and prevents entry into mitosis until replication is verified as complete.

## Key terms

- [S phase](https://www.gradenine.co.uk/glossary/s-phase)
- [DNA replication](https://www.gradenine.co.uk/glossary/dna-replication)
- [mitosis](https://www.gradenine.co.uk/glossary/mitosis)

## Related

- [Revision notes for IB DP Biology Standard Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-why-cells-in-s-phase-3bde6e88) · Published by Druglandscape Ltd.