Meiosis and Genetic Variation — AQA A-level Biology

SPEC 3.4.3 - GENETIC INFORMATION

Meiosis

Process, independent segregation, crossing over & genetic variation

What Is Meiosis?

Meiosis

A type of nuclear division that produces four genetically different haploid daughter cells from one diploid parent cell.

Meiosis occurs in organisms that reproduce sexually. In humans it takes place in the ovaries and testes to produce gametes. It involves two successive nuclear divisions — meiosis I and meiosis II — producing cells with half the original chromosome number.

Meiosis I — Reduction Division

Homologous chromosomes separate. The chromosome number is halved: diploid (2n) → two haploid cells. Crossing over occurs at prophase I.

Meiosis II — Equational Division

Sister chromatids separate — similar to mitosis. Each of the two haploid cells divides to give four haploid daughter cells in total.

Where does meiosis occur?

Meiosis occurs only where haploid cells are produced — in gonads (ovaries, testes) in animals, and in specialised reproductive structures in plants. When given an unfamiliar life cycle, identify where haploid cells are formed — that is where meiosis occurs.

Haploid & Diploid

Diploid (2n)

A cell containing two complete sets of chromosomes — one set inherited from each parent. Human body cells: 2n = 46 (23 pairs).

Haploid (n)

A cell containing one complete set of chromosomes. Human gametes: n = 23.

Homologous Chromosomes

A pair of chromosomes — one maternal, one paternal — that carry genes for the same characteristics at the same loci. They may carry different alleles of those genes.

Cell typeChromosome number (human)Produced by

Body (somatic) cells46 (2n)Mitosis

Gametes (eggs, sperm)23 (n)Meiosis

Zygote46 (2n)Fertilisation (n + n)

Why halve the chromosome number?

When two gametes fuse at fertilisation, the diploid chromosome number is restored. Without meiosis, the chromosome number would double with every generation.

Meiosis I — The Reduction Division

Meiosis I separates homologous chromosomes. Before meiosis I begins, DNA replication has occurred, so each chromosome consists of two identical sister chromatids joined at the centromere.

StageKey events

Prophase IHomologous chromosomes pair up as bivalents . Crossing over occurs: non-sister chromatids of homologous chromosomes exchange segments at points called chiasmata .

Metaphase IBivalents line up at the equator of the cell. Independent segregation : each bivalent aligns randomly — maternal or paternal chromosome faces either pole. Spindle fibres attach to centromeres.

Anaphase IHomologous chromosomes separate and move to opposite poles. Sister chromatids remain joined at centromere.

Telophase ITwo haploid cells formed — each with one chromosome from each homologous pair (but each chromosome still has two chromatids).

Key Point

After meiosis I, the cells are haploid in terms of chromosome number — but each chromosome still consists of two chromatids. The DNA content has been halved, not quartered.

Meiosis II — Separation of Chromatids

Meiosis II is similar to mitosis. It separates sister chromatids in each of the two cells produced by meiosis I. No further DNA replication occurs between meiosis I and meiosis II.

StageKey events

Prophase IIChromosomes condense. New spindle forms in each cell.

Metaphase IIChromosomes (each still consisting of two chromatids) align at the equator. Spindle fibres attach to centromeres.

Anaphase IICentromeres split. Sister chromatids separate and move to opposite poles.

Telophase IIFour haploid daughter cells formed. Each contains a single set of chromosomes, each consisting of one chromatid. The cells are genetically different from each other.

End result of meiosis

One diploid cell → four haploid daughter cells, each genetically different from the parent cell and from each other.

Meiosis — Overview Diagram

Crossing Over

The exchange of sections of chromatids between homologous chromosomes during prophase I of meiosis, producing new combinations of alleles (recombinant chromosomes).

Chiasma (pl. chiasmata)

The point at which crossing over occurs — where non-sister chromatids of homologous chromosomes break and rejoin, exchanging segments.

Common Error

Crossing over occurs between non-sister chromatids of homologous chromosomes at prophase I. It does NOT occur between sister chromatids (which are identical — no new combinations possible), and it does NOT occur at meiosis II.

Independent Segregation

The random alignment of homologous chromosome pairs at the equator during metaphase I, so that either chromosome of each homologous pair can move to either pole. This results in different combinations of maternal and paternal chromosomes in the daughter cells.

At metaphase I, each bivalent lines up independently. For each pair, there is an equal chance of the maternal or paternal chromosome facing either pole. With n pairs of homologous chromosomes, there are 2ⁿ possible chromosome combinations.

Possible combinations from independent segregation = 2ⁿ

Human example

Humans have 23 pairs of homologous chromosomes (n = 23).

2²³ = 8,388,608 (over 8 million) possible chromosome combinations from independent segregation alone — even before crossing over is considered.

Exam Tip

The spec requires you to use the expression 2ⁿ to calculate possible chromosome combinations after meiosis (without crossing over), and to derive the formula for random fertilisation: 2ⁿ × 2ⁿ = 2²ⁿ.

Random Fertilisation

Each gamete produced by meiosis is genetically unique (due to crossing over and independent segregation). When two gametes fuse at fertilisation, the combination is also random — any sperm can fertilise any egg.

Combinations from random fertilisation = 2ⁿ × 2ⁿ = (2ⁿ)²

Human example (independent segregation only)

Possible gamete combinations: 2²³ ≈ 8.4 million per individual.

Possible zygote combinations from random fertilisation: 2²³ × 2²³ = 2⁴⁶ ≈ 70 trillion genetically distinct combinations — before crossing over is even considered.

Three sources of variation — summary

Crossing over (prophase I) — produces new allele combinations on individual chromosomes

Independent segregation (metaphase I) — random assortment of whole maternal/paternal chromosomes → 2ⁿ combinations per gamete

Random fertilisation — any sperm fuses with any egg → 2²ⁿ possible zygote combinations

Mitosis vs Meiosis

FeatureMitosisMeiosis

Number of divisions12 (meiosis I + II)

Daughter cells produced24

Chromosome number in daughtersSame as parent (2n)Half parent (n)

Genetically identical?Yes (barring mutation)No — genetically different

Crossing over?NoYes — prophase I

Independent segregation?NoYes — metaphase I

PurposeGrowth, repair, asexual reproductionSexual reproduction — gamete/spore production

Where?All dividing cellsGonads (ovaries, testes) and reproductive structures

Examiner Alert

A common error is describing meiosis as producing "genetically identical" cells. It does not — genetic variation arises through crossing over and independent segregation. Only mitosis (under normal conditions) produces genetically identical daughters.

Chromosome Diagrams — Exam Skill

AQA regularly asks you to complete diagrams showing chromosome content of cells after meiosis I or meiosis II. Work through this systematically using a simplified example with 2n = 4 (two pairs of homologous chromosomes).

Exam Technique

When completing such diagrams: after meiosis I, each cell has the haploid chromosome number (n), but each chromosome still has two chromatids. After meiosis II, chromatids separate — each final cell still has n chromosomes, but now each is a single chromatid. The chromosome number does not change in meiosis II; what changes is that each chromosome goes from two chromatids to one.

DEEP Framework: Sources of Genetic Variation

A common 4–6 mark question: "Explain how meiosis produces genetically different daughter cells."

DEEP CHAIN — "Explain how meiosis produces genetic variation"

D

Define Meiosis produces four genetically different haploid daughter cells from one diploid parent cell. Two mechanisms during meiosis I create genetic variation: crossing over and independent segregation.

E

Explain: Crossing Over (Prophase I) During prophase I, homologous chromosomes pair up as bivalents. Non-sister chromatids of homologous chromosomes exchange segments at points called chiasmata. This produces new combinations of alleles on individual chromosomes — recombinant chromosomes that did not exist in the parent cell.

E

Explain: Independent Segregation (Metaphase I) At metaphase I, bivalents align randomly at the equator — either chromosome of each homologous pair may face either pole. This random assortment means that different combinations of maternal and paternal chromosomes end up in daughter cells. With n pairs of chromosomes, 2ⁿ combinations are possible. In humans (n = 23): 2²³ ≈ 8.4 million combinations.

P

Practical Extension: Random Fertilisation Genetic variation is further increased by random fertilisation. Any of ~8.4 million possible sperm can fertilise any of ~8.4 million possible eggs: 2²³ × 2²³ ≈ 70 trillion unique combinations. This ensures that every individual produced by sexual reproduction (except identical twins) is genetically unique.

Common Exam Errors

Error 1 — Crossing over at wrong stage or between wrong structures

Crossing over occurs at prophase I, between non-sister chromatids of homologous chromosomes. Common wrong answers: "meiosis II," "sister chromatids," or "between any two chromosomes." Sister chromatids are genetically identical — exchanging them creates no new combinations.

Error 2 — Calling meiosis products "genetically identical"

Meiosis produces cells that are genetically different from each other and from the parent cell. Only mitosis produces genetically identical daughters. Meiosis generates variation through crossing over and independent segregation.

Error 3 — Confusing the two divisions

Meiosis I: homologous chromosomes separate (reduction division — 2n→n). Meiosis II: sister chromatids separate (equational division). Do not describe chromatids separating in meiosis I or homologous chromosomes separating in meiosis II.

Error 4 — 2ⁿ formula confusion

For combinations from one gamete (independent segregation only): 2ⁿ where n = number of homologous pairs. For combinations from random fertilisation: 2ⁿ × 2ⁿ = 2²ⁿ. Students often apply the wrong formula to the wrong question.

Practice Question

A cell has a diploid chromosome number of 2n = 8 (four pairs of homologous chromosomes). It undergoes meiosis.

(a) How many chromosomes will each daughter cell contain after meiosis is complete? [1 mark]

(b) Using the formula 2ⁿ, calculate the number of different chromosome combinations possible in the gametes of this organism due to independent segregation alone. [1 mark]

(c) Explain how crossing over increases genetic variation beyond that produced by independent segregation alone. [3 marks]

a)

4 chromosomes — meiosis halves the chromosome number: 2n = 8, so n = 4.

b)

n = 4, so 2⁴ = 16 different combinations.

c)

Independent segregation only rearranges whole chromosomes. Crossing over produces new combinations of alleles within individual chromosomes. During prophase I, non-sister chromatids of homologous chromosomes exchange segments at chiasmata. This creates recombinant chromosomes with allele combinations that did not exist in either parent chromosome — variation that independent segregation alone cannot produce.

Synoptic Links

Meiosis connects directly to evolution, genetics, inheritance and chromosome mutations across the specification.

→ Topic 4.03 (Chromosome Mutations)

Non-disjunction during meiosis I or II produces gametes with abnormal chromosome numbers. This directly arises from errors in the same division process studied here.

→ Topic 4.05 (Genetic Diversity)

Meiosis is a major mechanism generating genetic diversity within sexually reproducing populations. Crossing over and independent segregation ensure that no two offspring are genetically identical.

→ Topic 7.01 (Evolution)

The genetic variation generated by meiosis provides the raw material for natural selection. Without this variation, allele frequencies could not change across generations in response to selective pressures.

→ Topic 7.02 (Inheritance)

Independent segregation of chromosomes at meiosis I is the physical basis of Mendel's law of independent assortment. Linkage occurs when genes are on the same chromosome — crossing over can separate linked alleles.

Summary

Key Definitions

TermDefinition

MeiosisA type of nuclear division producing four genetically different haploid daughter cells from one diploid parent cell

Crossing overExchange of chromatid segments between homologous chromosomes at prophase I, producing new allele combinations

ChiasmaThe point at which crossing over occurs between non-sister chromatids of homologous chromosomes

Independent segregationRandom alignment of bivalents at metaphase I — either chromosome of each pair may face either pole; produces 2ⁿ gamete combinations

Three sources of variation

Crossing over (prophase I)

Independent segregation (metaphase I): 2ⁿ

Random fertilisation: 2ⁿ × 2ⁿ

Meiosis I vs Meiosis II

Meiosis I: homologous chromosomes separate (2n→n)

Meiosis II: sister chromatids separate (n→n, single chromatids)

Result: 4 haploid, genetically different cells