The big idea: In both mitosis and meiosis, chromosomes first condense: the DNA coils up, helped by histone proteins, until each chromosome is short and thick.
The points to remember
- Histones are proteins that DNA wraps around, like thread on beads.
- The DNA-histone beads coil again and again: supercoiling.
- Condensed chromosomes are short and thick, so they can be moved without tangling or breaking.
- Condensing makes chromosomes visible under a light microscope.
Remember it as: Wind it up before you move it.
Real example: packing it in. The DNA in one human cell would stretch about 2 metres if laid end to end, yet it fits inside a nucleus only about 6 micrometres across. Wrapping round histones and supercoiling make this possible.
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Moving chromosomes: Chromosomes are moved by microtubules, protein fibres that form the spindle, and by microtubule motors: proteins that pull chromosomes along the microtubules.
| Shared feature | In mitosis and meiosis |
|---|---|
| Condensation | Histones and supercoiling make chromosomes short and thick |
| Spindle | Microtubules grow from the poles and attach at the centromeres |
| Movement | Motor proteins pull chromosomes; microtubules shorten |
| Separation | Chromatids separate (mitosis, and the second division of meiosis) |
Not shared: Pairing of homologous chromosomes, crossing over and halving the chromosome number happen in meiosis only.
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How it is asked: Mostly 1A items: 'what happens in both mitosis and meiosis?' (condensation by histones, microtubules moving chromatids) and 'what is a function of histones?'. Sometimes a one-mark 1B on histones.
IB-style questionOutline[2 marks]
Outline the role of histones in cell division.
Model answer plan
See the answer broken into clear points, with guidance on how each point earns marks.