Condensation and movement at Higher Level: The same statement as SL. The 1A items on 'both mitosis and meiosis' appear at both levels; at HL also explain how microtubules and their motors move chromosomes.
Practise this as you read
- Identify a process shared by mitosis and meiosis.
- Outline how chromosomes are moved to the poles.
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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: a drug that stops the spindle. The cancer drug paclitaxel (Taxol) binds to microtubules and stops them from shortening. Chromosomes can no longer be pulled apart, so dividing cancer cells get stuck in mitosis.
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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.
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[3 marks]
Outline how chromosomes are moved to opposite poles during nuclear division.
Model answer plan
See the answer broken into clear points, with guidance on how each point earns marks.