4.4.5Additional lessons
Meiosis — IB Biology SL Revision Notes & Topical Questions
5 min read 4 parts Lesson 112 of 142
Part 1 of 4Reduction Division
Meiosis involves one round of DNA replication followed by two successive divisions, so the chromosome number is halved once.
Purpose
- Meiosis halves the chromosome number so that fertilisation restores the diploid number in the zygote.
Homologous chromosomes
- Pairs of chromosomes, one from each parent, carrying the same genes in the same order but possibly different alleles.
Human numbers
- A diploid human cell with 46 chromosomes forms haploid gametes with 23 chromosomes.
Outcome
- One diploid cell produces four haploid cells that are genetically different from each other and from the parent.
Where
- In humans meiosis occurs in the testes and ovaries, forming sperm and egg cells.
Part 2 of 4The Stages
The two divisions are called meiosis I and meiosis II, and each has prophase, metaphase, anaphase and telophase.
1
Before meiosis I, each chromosome replicates, so it consists of two identical sister chromatids.
2
In prophase I, homologous chromosomes pair up and crossing over occurs at points called chiasmata.
3
In metaphase I homologous pairs line up at the equator and orient randomly, which is independent assortment.
4
In anaphase I, the homologous chromosomes separate and move to opposite poles, halving the chromosome number.
5
In meiosis II, sister chromatids separate as in mitosis, producing four haploid cells.
| Feature | Meiosis I | Meiosis II |
|---|---|---|
| Chromosomes separated | Homologous pairs | Sister chromatids |
| Cells at end | 2 haploid | 4 haploid |
| Crossing over | Occurs | Does not occur |
Part 3 of 4Variation and Life Cycles
Meiosis is central to sexual reproduction because it keeps the chromosome number constant and creates variation.
Variation and Life Cycles
Crossing over
- Exchange of segments between homologous chromosomes produces chromosomes with new combinations of alleles.
Independent assortment
- Random orientation of pairs produces many different mixtures of maternal and paternal chromosomes.
Sexual life cycle
- Meiosis makes haploid gametes,
- and fertilisation restores the diploid number,
- so the chromosome number stays constant across generations.
Compared with mitosis
- Mitosis gives two identical diploid cells, while meiosis gives four different haploid cells.
Part 4 of 4Exam Checklist & Tips
4.4.5 – Key Points for the Exam
- Meiosis A reduction division that produces four genetically different haploid cells.
- Homologous chromosomes A pair of chromosomes with the same genes in the same order, one from each parent.
- Chiasma A point where crossing over has occurred between non-sister chromatids.
- Gamete A haploid sex cell that fuses with another at fertilisation.
- Haploid Having a single set of chromosomes, represented as n.
- Diploid Having two sets of chromosomes, represented as 2n.
Common Mistakes
- Saying homologous chromosomes separate in meiosis II; this happens in meiosis I.
- Stating that meiosis produces two cells; it produces four.
- Believing crossing over occurs between sister chromatids;
- it occurs between non-sister chromatids of homologues.
- Forgetting that DNA replicates only once for the two divisions.
Last updated:
\n