Meiosis and Genetic Variation

Meiosis is the form of cell division that produces gametes (sex cells) — cells with half the chromosome number of the parent cell. Unlike mitosis, meiosis introduces genetic variation, which is essential for natural selection and evolution. This lesson covers the stages of meiosis, the mechanisms that generate genetic variation, and the significance of sexual reproduction for AQA A-Level Biology (specification 3.2.2).

Key Definition: Meiosis is a type of cell division that produces four genetically different haploid (n) daughter cells from one diploid (2n) parent cell. It involves two successive divisions: meiosis I and meiosis II.

---

Key Terminology

• Diploid (2n): a cell containing two complete sets of chromosomes — one set inherited from each parent. In humans, 2n = 46.
• Haploid (n): a cell containing one complete set of chromosomes. In humans, n = 23.
• Homologous chromosomes: a pair of chromosomes (one from each parent) that have the same genes at the same loci but may carry different alleles. They are the same length and have the centromere in the same position.
• Bivalent: a pair of homologous chromosomes that come together during meiosis I.
• Sister chromatids: the two identical copies of a chromosome formed by DNA replication during S phase, joined at the centromere.

---

Overview of Meiosis

Meiosis consists of two divisions:

Division	What Separates	Result
Meiosis I (reduction division)	Homologous chromosomes	Two haploid cells, each with one chromosome from each homologous pair (but each chromosome still consists of two sister chromatids)
Meiosis II (similar to mitosis)	Sister chromatids	Four haploid cells, each with one chromatid from each chromosome

---

Meiosis I — The Reduction Division

Prophase I

• Chromosomes condense and become visible.
• Homologous chromosomes pair up (synapsis), forming bivalents. Each bivalent consists of four chromatids (a tetrad).
• Crossing over occurs: non-sister chromatids of homologous chromosomes exchange segments of DNA at points called chiasmata (singular: chiasma). This produces recombinant chromatids with new combinations of alleles.
• The nuclear envelope breaks down.
• The spindle forms, with fibres attaching to the centromeres of the bivalents.

Key Definition: Crossing over is the exchange of genetic material between non-sister chromatids of homologous chromosomes during prophase I of meiosis. The points at which crossing over occurs are called chiasmata. This produces new combinations of alleles on a single chromatid.

Metaphase I

• Bivalents line up at the metaphase plate (equator).
• The orientation of each bivalent is random — either the maternal or paternal homologue of each pair can face either pole. This is independent assortment (also called random orientation or random assortment).
• The number of possible arrangements = 2^n, where n is the haploid number. In humans (n = 23), this gives 2²³ = 8 388 608 possible combinations of maternal and paternal chromosomes in the gametes from independent assortment alone.

Anaphase I

• Homologous chromosomes are pulled to opposite poles by the spindle fibres.
• The chiasmata are the last points of contact between the homologues and are resolved (broken) as the chromosomes move apart.
• Note: sister chromatids remain attached at their centromeres — they do not separate in meiosis I.
• This is the step that achieves the reduction division: each pole now has a haploid set of chromosomes (one from each homologous pair).

Telophase I and Cytokinesis I

• The chromosomes may partially decondense.
• The nuclear envelope may reform (this varies between species).
• The cell divides into two haploid cells. Each cell contains one chromosome from each homologous pair, but each chromosome still consists of two sister chromatids.
• There is typically a brief interkinesis (gap) between meiosis I and meiosis II. No DNA replication occurs during interkinesis.

---

Meiosis II — Separation of Sister Chromatids

Meiosis II is similar to mitosis but starts with a haploid cell.

Prophase II

• Chromosomes condense.
• The nuclear envelope breaks down (if it reformed in telophase I).
• New spindle fibres form, oriented at 90° to the meiosis I spindle.

Metaphase II

• Individual chromosomes (each consisting of two sister chromatids) line up at the metaphase plate.