Module 2: Meiosis
These free OCR A Level Biology Meiosis revision notes have been written for specification points 2.1.6(f) and 2.1.6(g).
Meiosis
Meiosis is a type of nuclear division that produces haploid gametes.
Haploid means they only have one of each type of chromosome, whilst diploid means they have 2 of each.
Humans have 46 chromosomes (their diploid number) in their body cells, whilst their gametes (sperm and ova) have 23 (their haploid number).
Gametes are sex cells, and will typically be haploid so that when fertilisation occurs, the newly formed organism will have a ‘full’ genome with the correct number of chromosomes. This is true of most animals and plants, whilst some fungi can have more complex reproductive lifecycles.
Sexual Reproduction: Genetic Variation
Meiosis enables sexual reproduction to occur, and is important because it:
- Maintains chromosome numbers across generations.
- Introduces genetic variation (crossing over, independent assortment, and random fertilisation).
- Enables natural selection (and long-term species survival).
The table below outlines the mechanisms that generate genetic variation:
| Source of Variation | Explanation |
|---|---|
| Crossing over | Non-sister chromatids swap DNA sections during prophase I → shuffling alleles → creating new allele combinations → more potential genotypes |
| Independent assortment | The side of the cell that homologous chromosomes (in metaphase I) and sister chromatids (in metaphase II) line up on the equator is random → random segregation → more potential genotypes |
| Random fertilisation | Haploid gametes containing a random set of chromosomes can fuse with another gamete in numerous genetic possibilities. |
*It is important to note that independent assortment occurs during metaphase (I or II), whilst random segregation occurs during anaphase (I or II).
Stages of Meiosis
The table below outlines the events which occur during each stage of meiosis I and II, the differences between each stage’s I and II are indicated in bold:
| Stage | Events and Notes |
|---|---|
| Prophase I |
– DNA supercoils and chromosomes condense – Nuclear envelope breaks down – Homologous chromosomes pair up and form a bivalent: non-sister chromatids cross arms at the chiasmata – Crossing over occurs: alleles are shuffled – Spindle begins to form (from centrioles in animals; from cytoplasm in plants) |
| Metaphase I |
– Bivalents (homologous pairs of crossed-over chromosomes) line up at the equator – Spindle fibres attach to centromeres – Chromosome arrangement is random: independent assortment occurs |
| Anaphase I |
– Homologous chromosomes are pulled to opposite poles – Bivalents separate: allele shuffling has occurred – Independent segregation: Homologous chromosomes are pulled to opposite sides |
| Telophase I (animals only) |
– Nuclear envelope (may) reform around each set of chromosomes (in animals) – Most plant cells skip telophase I and go to prophase II |
| Cytokinesis (animals only) | 1 cell splits into 2 haploid cells, but the chromosomes consist of two sister chromatids |
| Interphase (animals only) | Short interphase: chromosomes uncoil |
| Prophase II |
– Reformed nuclear envelopes break down (if they reformed) – DNA supercoils and chromosomes condense |
| Metaphase II |
– Chromosomes line up on the equator – Spindle fibres attach to centromeres – Chromatid arrangement is random: independent assortment occurs |
| Anaphase II |
– Sister chromatids are pulled to opposite poles – Centromeres are pulled apart (as sister chromatids separate) – Independent segregation: sister chromatids are pulled to opposite sides |
| Telophase II | Nuclear envelopes form around each set of chromosomes |
| Cytokinesis | Each cell divides, producing 4 haploid gametes. |