# Meiosis

> CIE A-Level Biology · Inheritance
> Source: https://www.owlsprep.com/study/cie-9700-u17-meiosis/

This module covers the key stages of meiosis, its role in producing haploid gametes for sexual reproduction, and how it generates genetic variation. We also compare meiosis to mitosis to prepare for common exam questions.

**Prerequisites:** [Understanding of mitosis and the cell cycle](https://www.owlsprep.com/study/cie-9700-u05-mitosis/)

## Learning objectives

- Describe the key stages of meiosis in terms of chromosome behaviour
- Explain how meiosis generates genetic variation via crossing over and independent assortment
- Compare and contrast meiosis and mitosis for exam questions
- Identify stages of meiosis from descriptions or diagrams

## Overview and Biological Role of Meiosis

**Meiosis** — A type of cell division that reduces chromosome number by half, producing four haploid daughter cells from one diploid parent cell, required for sexual reproduction.

*Example:* Meiosis produces sperm and egg cells in humans.

In sexually reproducing organisms, meiosis is required to maintain a constant chromosome number across generations. If gametes were diploid, the chromosome number would double every generation after fertilization, leading to developmental abnormalities.

**Worked example:** Red wood ants have 22 chromosomes in their somatic (body) cells. How many chromosomes are present in their gametes, and why is this number necessary for sexual reproduction?

1. 1. Meiosis reduces chromosome number by half to produce haploid gametes. Calculate the gamete chromosome number:
2. $$\frac{22}{2} = 11$$
3. 2. When two gametes fuse during fertilization, the resulting zygote gets one set of chromosomes from each parent, returning to the diploid number of 22 that matches the parent somatic cells. This maintains a constant chromosome number across generations.

## Stages of Meiosis

Meiosis occurs in two sequential divisions: meiosis I (reduction division) and meiosis II (equational division, similar to mitosis). DNA replication occurs once in interphase before meiosis I begins.

**Bivalent** — A pair of homologous chromosomes that have paired up (synapsed) during prophase I of meiosis, consisting of four total chromatids.

1. Prophase I: Homologous chromosomes synapse, crossing over occurs at chiasmata between non-sister chromatids
2. Metaphase I: Bivalents align randomly at the metaphase plate
3. Anaphase I: Homologous chromosomes separate, sister chromatids remain attached at the centromere
4. Telophase I: Two haploid cells form, each chromosome still has two sister chromatids
5. Meiosis II: Separates sister chromatids, producing four final haploid daughter cells

**Worked example:** A cell undergoing meiosis has visible chiasmata, the nuclear envelope is breaking down, and no chromosomes have aligned at the cell equator yet. What stage of meiosis is this?

1. 1. Recall the order of events in meiosis: crossing over and synapsis occur before chromosome alignment.
2. 2. Chiasmata are the physical sites of crossing over, which form exclusively during prophase I, before metaphase I alignment.
3. Conclusion: This cell is in prophase I of meiosis.

## Meiosis and Genetic Variation

Meiosis generates enormous genetic diversity in gametes, which is the raw material for natural selection and evolution. There are two core mechanisms that generate new genetic combinations.

**Independent Assortment** — The random alignment of homologous chromosome pairs at the metaphase plate in metaphase I, leading to random segregation of maternal and paternal chromosomes into daughter cells.

Crossing over (recombination) also generates variation: during prophase I, non-sister chromatids of homologous chromosomes exchange segments of DNA. This creates new combinations of alleles on each chromosome that are not present in either parent. The number of possible unique gametes from independent assortment alone is $2^n$, where $n$ is the haploid chromosome number.

**Worked example:** Name and explain two processes during meiosis that generate genetic variation in gametes.

1. 1. Crossing over: During prophase I, non-sister chromatids of homologous chromosomes exchange segments of DNA. This shuffles alleles between maternal and paternal chromosomes, creating new recombinant chromosomes that are genetically distinct from the parent chromosomes.
2. 2. Independent assortment: During metaphase I, each homologous chromosome pair aligns randomly relative to all other pairs. Each gamete therefore receives a random mix of maternal and paternal chromosomes, creating almost unlimited unique combinations of chromosomes.

## Comparing Meiosis and Mitosis

CIE exams regularly ask for comparisons between meiosis and mitosis. For full marks, you must compare the same feature for both processes, rather than just listing separate features.

| Feature | Meiosis | Mitosis |
| --- | --- | --- |
| Number of nuclear divisions | 2 | 1 |
| Number of daughter cells produced | 4 | 2 |
| Ploidy of daughter cells | Haploid (n) | Diploid (2n) |
| Homologous pairing and crossing over | Occurs in prophase I | Does not occur |
| Genetic similarity to parent cell | Genetically distinct | Genetically identical |
| Biological function | Gamete production for sexual reproduction | Growth, repair, asexual reproduction |

**Worked example:** State three valid differences between meiosis and mitosis.

1. For full marks, each difference should compare the same feature for both processes. Three accepted comparison answers are:
2. 1. Meiosis involves two nuclear divisions, while mitosis involves one nuclear division.
3. 2. Meiosis produces four daughter cells per parent cell, while mitosis produces two daughter cells per parent cell.
4. 3. Meiosis produces genetically distinct daughter cells, while mitosis produces genetically identical daughter cells.

## Common pitfalls

- **Wrong:** Claiming meiosis produces four genetically identical daughter cells
  - Why it fails: Confuses the outcome of meiosis with mitosis, ignoring genetic variation from crossing over and independent assortment
  - Correct: Meiosis produces four genetically distinct haploid daughter cells
- **Wrong:** Claiming crossing over occurs between sister chromatids of homologous chromosomes
  - Why it fails: Mixes up identical sister chromatids with genetically distinct non-sister chromatids from separate parents
  - Correct: Crossing over occurs between non-sister chromatids of homologous chromosomes
- **Wrong:** Stating DNA replication occurs between meiosis I and meiosis II
  - Why it fails: Assumes replication precedes every division, as in mitotic cell cycles
  - Correct: DNA replication occurs once in interphase before meiosis I; no replication occurs between the two meiotic divisions
- **Wrong:** Claiming homologous chromosomes separate in anaphase II
  - Why it fails: Confuses the separation events of the two meiotic divisions
  - Correct: Homologous chromosomes separate in anaphase I; sister chromatids separate in anaphase II

## Cheatsheet

| Feature | Meiosis | Mitosis |
| --- | --- | --- |
| Number of divisions | 2 | 1 |
| Daughter cells | 4 | 2 |
| Ploidy | Haploid (n) | Diploid (2n) |
| Homologous pairing | Yes | No |

## What's next

Meiosis is the core cellular process that underpins all genetic inheritance in sexually reproducing organisms. The genetic variation generated by crossing over and independent assortment explains the patterns of inheritance observed in monohybrid and dihybrid crosses, and is the foundation for understanding how linked genes are inherited. Meiosis also provides the mechanism for the genetic variation that drives evolution by natural selection, and errors during meiosis are the cause of many chromosomal genetic disorders. Next, you will apply your understanding of meiosis to solve inheritance problems and explore how recombination affects gene linkage.

- [Monohybrid and dihybrid inheritance](https://www.owlsprep.com/study/cie-9700-u17-monohybrid-and-dihybrid-inheritance/)
- [Codominance and multiple alleles](https://www.owlsprep.com/study/cie-9700-u17-codominance-and-multiple-alleles/)
- [Sex linkage](https://www.owlsprep.com/study/cie-9700-u17-sex-linkage/)

---

From [OwlsPrep](https://www.owlsprep.com) — free study guides for A-Level, IB, AP and IGCSE, written against the official syllabus. Canonical page: https://www.owlsprep.com/study/cie-9700-u17-meiosis/