The big idea: In eukaryotes the nucleus divides before the cell does, so that no daughter cell is left anucleate (without a nucleus).
The points to remember
- The nucleus holds the cell's genes, which control the cell's activities.
- An anucleate cell cannot make new proteins for long and can never divide.
- So nuclear division (mitosis or meiosis) comes first.
- Then cytokinesis splits the cytoplasm between the new nuclei.
Remember it as: Divide the nucleus, then the cell.
Real example: red blood cells. Human red blood cells lose their nucleus as they mature. Without a nucleus they cannot divide, so new red blood cells must be made all the time from stem cells in the bone marrow, which still have nuclei.
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Two kinds of nuclear division: Mitosis keeps the chromosome number and the genome the same. Meiosis halves the chromosome number and makes every cell genetically different.
| Mitosis | Meiosis | |
|---|---|---|
| Chromosome number | Kept the same | Halved |
| Genome of daughter cells | Identical to the parent | Different: genetic diversity |
| Daughter cells | Two | Four |
| Roles | Growth, repair, replacement, asexual reproduction | Making gametes for sexual reproduction |
Why halve the number?: At fertilisation two gametes fuse. If they were not haploid, the chromosome number would double in every generation.
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How it is asked: A Paper 2 'distinguish between mitosis and meiosis', a 1B 'distinguish between the cells produced', or a 1A item on what each division does to the chromosome number.
IB-style questionOutline[2 marks]
Outline why the nucleus of a eukaryotic cell divides before the cell divides.
Model answer plan
See the answer broken into clear points, with guidance on how each point earns marks.