By the end of this lesson
- Describe the outcome of meiosis.
- Compare meiosis with mitosis.
- Explain how meiosis produces haploid gametes.
BIG QUESTION
How are chromosome number and variation managed across generations?
One DNA replication is followed by two divisions, separating homologues then sister chromatids.
Meiosis includes two nuclear divisions
DNA replicates once, then meiosis IThe first division of meiosis, which separates the homologous chromosomes of each pair, so the two new cells are haploid. separates homologous chromosomes and meiosis IIThe second division of meiosis, which separates the sister chromatids, giving four genetically different haploid cells from the original diploid cell. separates sister chromatids.
One diploid parent cell produces four haploid daughter cells, each with one chromosome from every pair.
Halving chromosome number prevents doubling
In humans, meiosis produces gametes with 23 chromosomes. Fertilisation combines two haploid gametes to restore 46.
Without meiosis, chromosome number would double in each generation.
Meiosis creates genetic variation
Independent assortment of homologous chromosomes and crossing over create new allele combinations. Random fertilisation adds further variation.
Gametes are genetically different from one another and from the original diploid cell.
Mitosis and meiosis
Compare purpose, divisions and outcomes.
| Feature | Mitosis | Meiosis |
|---|---|---|
| Nuclear divisions | One. | Two. |
| Daughter cells | Two. | Four. |
| Chromosome number | Maintained. | Halved. |
| Genetic identity | Usually identical. | Genetically varied. |
| Main role | Growth/repair/cloning. | Gamete production. |
Language that earns precision
- meiosis
- Two divisions producing haploid genetically varied cells from a diploid cell.
- gamete
- Haploid reproductive cell that can fuse at fertilisation.
- crossing over
- Exchange of DNA between homologous chromosomes during meiosis.
- independent assortment
- Random orientation and separation of homologous pairs in meiosis.
Common misconceptions
Claim: Meiosis produces two gametes.
Correction: One meiotic event normally produces four haploid cells.
Claim: Meiosis copies chromosome number.
Correction: It halves chromosome number.
Claim: All gametes are identical because they came from one cell.
Correction: Meiosis generates new allele combinations.
Match the answer to the command word
Four haploid genetically varied gametes.
Use divisions, number, ploidy, identity and purpose.
Recall, apply, then justify
Compare mitosis and meiosis.
Hint
Number of divisions/cells, ploidy and variation.
Suggested marking points
- Mitosis has one division whereas meiosis has two.
- Mitosis produces two cells whereas meiosis produces four.
- Mitosis maintains chromosome number whereas meiosis halves it.
- Mitosis produces genetically identical cells whereas meiosis produces varied gametes.
Explain why meiosis is needed before fertilisation.
Hint
Haploid gametes and restored diploid number.
Suggested marking points
- Meiosis produces haploid gametes with one chromosome set.
- Fertilisation combines two haploid gametes.
- The diploid chromosome number is restored rather than doubled.
Predict consequences if homologous chromosomes failed to separate in meiosis I.
Planning hint
Think about all later gametes.
What a strong design includes
- A homologous pair would move to the same pole.
- The two cells after meiosis I would have unequal chromosome sets.
- After meiosis II, gametes would have an extra or missing chromosome.
- Fertilisation could produce aneuploid zygotes.
- Gene dosage may be altered.
- Development, fertility or survival may be affected.