Meiosis and Genetic Variation

Meiosis is the specialised cell division that produces gametes — haploid sex cells carrying half the chromosome number of the parent cell. Unlike mitosis, which faithfully reproduces the parental genome in every daughter, meiosis combines a reductive chromosomal division with two distinct mechanisms for generating genetic variation: independent assortment of homologous chromosomes and crossing over between non-sister chromatids. The genetic novelty produced at each meiotic event, multiplied by random fertilisation, is what makes every sexually reproduced organism (except identical twins) genetically unique — and provides the raw material on which natural selection acts.

Spec mapping: This lesson covers AQA 7402 Section 3.4.4 — Genetic diversity can arise as a result of mutation or during meiosis, with synoptic links to Section 3.2.2 (mitosis and the cell cycle), Section 3.7.1 (the genetic basis of inheritance) and Section 3.7.2 (populations and evolution). The relevant content covers the chromosomal events of meiosis I and meiosis II, the mechanisms by which meiosis generates genetic variation (independent assortment, crossing over, random fertilisation), and the comparison between meiosis and mitosis. (Refer to the official AQA specification document for exact wording.)

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.

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Key Terminology

Before working through the stages, the precise meanings of these terms must be locked in. Many candidates lose marks at A-Level by using "chromosome", "chromatid" and "bivalent" interchangeably — they are distinct.

• 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. Gametes (sperm, egg) are haploid.
• 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. The X and Y chromosomes are sometimes called homologous despite their substantial size and gene-content differences because they pair during male meiosis.
• Bivalent (tetrad): the pair of homologous chromosomes that come together as a four-chromatid unit during prophase I of meiosis. A bivalent has four chromatids — two from each homologue.
• Sister chromatids: the two identical copies of a chromosome formed by DNA replication during S phase, joined at the centromere by cohesin protein. Sister chromatids are genetically identical at the moment of formation but may diverge due to crossing over.
• Chiasma (plural chiasmata): the physical point at which crossing over occurs in a bivalent. Chiasmata are visible by microscopy in late prophase I / diakinesis and are essential for accurate segregation as well as for genetic recombination.

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Overview of Meiosis

Meiosis consists of two successive divisions, each with its own prophase, metaphase, anaphase and telophase. Crucially, DNA is replicated once (in the S phase of interphase, before meiosis I) but the cell divides twice, so the chromosome number is halved.

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 a single chromatid per chromosome

Note the distinction between the two divisions:

• Meiosis I is the reductive division because the chromosome number halves (from 2n to n). The separating units are homologous chromosomes — pairs that came together as bivalents in prophase I.
• Meiosis II is the equational division because the chromosome number stays the same (n to n). The separating units are sister chromatids — the units joined at the centromere since the preceding S phase.

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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.

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Meiosis II — Separation of Sister Chromatids

Meiosis II is mechanically similar to mitosis but starts with a haploid cell (n) rather than a diploid one. No DNA replication occurs between meiosis I and meiosis II — the cells enter meiosis II with chromosomes that already have two chromatids each, as a consequence of the S phase that preceded meiosis I.

Prophase II

• Chromosomes condense (if they partially decondensed in telophase I).
• The nuclear envelope breaks down (if it reformed in telophase I — this is species-specific).
• 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. Note that bivalents have already been resolved in meiosis I — only single chromosomes (with paired chromatids) are present at this stage.

Anaphase II

• The centromeres divide, and sister chromatids are pulled to opposite poles by the spindle.
• Because of crossing over in prophase I, the sister chromatids may no longer be genetically identical to one another — they may carry recombinant segments derived from the homologous chromosome.

Telophase II and Cytokinesis II

• Chromosomes decondense.
• Nuclear envelopes reform around each haploid set of chromosomes.
• The two cells from meiosis I each divide, producing a total of four haploid daughter cells.
• Each of the four cells is genetically unique due to the combined effects of crossing over and independent assortment.

Gametogenesis: from meiotic products to gametes

The four products of meiosis undergo further differentiation to become functional gametes:

• Spermatogenesis (male): all four products mature into motile sperm, each with a head (containing the haploid nucleus and acrosome), midpiece (mitochondria for motility) and tail (flagellum).
• Oogenesis (female): asymmetric cytokinesis at both meiotic divisions concentrates the cytoplasm into one large egg, with three small polar bodies that degenerate. Only one of the four meiotic products becomes a functional egg.

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Sources of Genetic Variation in Meiosis

Meiosis generates genetic variation through three independent mechanisms, all interacting to produce essentially unlimited diversity in the gamete pool.

1. Crossing Over (Prophase I)

• Non-sister chromatids of homologous chromosomes exchange sections of DNA at points called chiasmata (singular: chiasma).
• This produces recombinant chromatids — chromatids with new combinations of alleles that differ from either parent chromosome.
• Crossing over is initiated by programmed double-strand breaks (catalysed by the Spo11 enzyme), which are then repaired using the homologous chromosome as a template — a mechanism that ensures genetic exchange but maintains chromosomal integrity.
• Multiple chiasmata (typically 1–3 per bivalent in humans) form along each homologous pair, and different chiasmata occur at different positions in different meiotic divisions, producing enormous variability between meioses in the same individual.
• The further apart two genes are on a chromosome, the more likely a chiasma will form between them — this principle underlies gene mapping: recombination frequency is roughly proportional to physical distance over short ranges.

2. Independent Assortment (Metaphase I)

• The random orientation of bivalents at metaphase I means that each gamete receives a random mixture of maternal and paternal chromosomes.
• The orientation of one bivalent has no influence on the orientation of any other bivalent — they assort independently.
• With n haploid chromosomes there are 2ⁿ possible combinations. In humans (n = 23), this gives 2²³ ≈ 8.4 × 10⁶ different chromosome combinations per gamete from independent assortment alone.
• Independent assortment is the chromosomal mechanism behind Mendel's second law — and is therefore the reason that unlinked genes assort independently in dihybrid crosses.

3. Random Fertilisation

• Any one of the genetically distinct sperm can fuse with any one of the genetically distinct eggs at fertilisation.
• In humans, this gives approximately 8.4 × 10⁶ × 8.4 × 10⁶ ≈ 7 × 10¹³ different zygote combinations from independent assortment alone, before crossing over is considered.
• Adding crossing over makes the number of distinguishable combinations effectively unbounded — far in excess of the total number of humans who have ever lived (~1.2 × 10¹¹).
• This ensures that, with the exception of identical twins, every individual produced by sexual reproduction is genetically unique.

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Comparing Mitosis and Meiosis

Feature	Mitosis	Meiosis
Number of divisions	One	Two (meiosis I and II)
Number of daughter cells	Two	Four
Ploidy of daughter cells	Diploid (2n)	Haploid (n)
Genetic variation	Daughter cells genetically identical to parent	Daughter cells genetically different from parent and each other
Crossing over	Does not normally occur	Occurs during prophase I
Independent assortment	Does not occur	Occurs at metaphase I
Pairing of homologues	Does not occur	Bivalents form during prophase I
Where it occurs	Somatic (body) cells	Reproductive organs (gonads)
Purpose	Growth, repair, asexual reproduction	Production of gametes for sexual reproduction
Chromosome behaviour at metaphase	Individual chromosomes (sister chromatids) align singly	Bivalents (homologous pairs, four chromatids) align in meiosis I
Anaphase	Sister chromatids separate	Homologues separate in meiosis I; sister chromatids separate in meiosis II

flowchart TD
  A["Diploid parent cell (2n) with DNA replicated in S phase"] --> B{"Type of division?"}
  B -->|Mitosis| C["Single division"]
  C --> D["Two diploid (2n) daughter cells, genetically identical to parent"]
  B -->|Meiosis| E["Meiosis I: reductive division"]
  E --> F["Two haploid (n) cells, each chromosome still 2 chromatids"]
  F --> G["Meiosis II: equational division"]
  G --> H["Four haploid (n) gametes, genetically unique"]

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Significance of Meiosis for Evolution