The big idea: Before mitosis or meiosis, every DNA molecule is replicated. Each chromosome is then two identical chromatids held together until anaphase.
The points to remember
- Before replication, a chromosome is one long DNA molecule.
- Replication happens in interphase, before the nucleus divides.
- After replication, each chromosome is two sister chromatids: two identical DNA molecules.
- The chromatids are held together at the centromere until anaphase.
- When they separate, each daughter nucleus gets one copy of every chromosome.
Remember it as: Copy, keep together, then pull apart.
Real example: copying your genome. Before one of your cells divides, it copies all 46 of its DNA molecules: about 6 billion base pairs in total, which would be roughly 2 metres of DNA if laid end to end.
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Reading the DNA graph: If you measure the DNA in one cell over time, it doubles during replication and halves when the cell divides.
| Part of the graph | What is happening |
|---|---|
| Flat, low | The cell grows; DNA not yet copied |
| Rising | DNA replication: the DNA is being copied |
| Flat, high (double) | Each chromosome is now two chromatids |
| Sudden drop by half | Division: the DNA is shared between two cells |
Both divisions need it: Mitosis and meiosis both start with replicated chromosomes.
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How it is asked: A 1A graph of DNA per cell (which point is replication?), a one-mark 'distinguish the DNA before and after replication', or as the first steps of a long answer on how mitosis keeps the chromosome number.
IB-style questionExplain[2 marks]
Explain why the DNA of a cell must be replicated before the cell divides by mitosis.
Model answer plan
See the answer broken into clear points, with guidance on how each point earns marks.