Meiosis & Genetic Variation Revision Notes | A Level AQA Biology

1. Why Meiosis Matters

Every human being is genetically unique — yet we all share the same basic genome. Meiosis is the mechanism that makes this possible. It is the division that generates the genetic diversity on which natural selection acts.

Without meiosis, sexual reproduction would double the chromosome number every generation. Meiosis solves that problem and, in doing so, shuffles the genetic deck in two distinct ways. Understanding both mechanisms — and being able to draw the chromosome content at each stage — is a core AQA A Level requirement.

2. The Big Picture: Two Divisions, Four Cells

Picture a diploid parent cell — it carries two complete sets of chromosomes, one inherited from each parent. Meiosis runs this cell through two successive nuclear divisions, labelled meiosis I and meiosis II, to produce four haploid daughter cells.

Meiosis I is the reductive division: homologous chromosome pairs are separated, halving the chromosome number. Meiosis II resembles mitosis — sister chromatids are pulled apart — but it acts on cells that are already haploid. The result is four cells, each with half the chromosome number of the original parent.

Here is a useful way to track the numbers. Start with a diploid parent cell (2n = 4 for a simple model organism with four chromosomes). After meiosis I, you have two cells each with n = 2 chromosomes, but each chromosome still consists of two sister chromatids joined at the centromere. After meiosis II, those chromatids are separated, giving four cells each with n = 2 single chromosomes.

3. Meiosis I — The Reductive Division

Before meiosis begins, DNA replication occurs during interphase — so each chromosome now consists of two identical sister chromatids. During prophase I, homologous chromosomes pair up in a process called synapsis, forming structures called bivalents (or tetrads). This pairing is unique to meiosis and is the stage at which crossing over can occur.

During metaphase I, bivalents line up at the cell equator. Crucially, the orientation of each bivalent is random — the maternal chromosome of one pair is equally likely to face either pole. This random orientation is the basis of independent segregation.

At anaphase I, homologous chromosomes are pulled to opposite poles by spindle fibres. Sister chromatids remain joined at their centromeres. By telophase I, two cells form, each containing one chromosome from each homologous pair — the chromosome number has been halved.

4. Meiosis II — Separating Sister Chromatids

Meiosis II follows without further DNA replication. In metaphase II, chromosomes (each still consisting of two sister chromatids) line up at the equator of each cell. At anaphase II, centromeres split and sister chromatids are pulled to opposite poles.

By the end of meiosis II, four haploid cells have been produced. Each contains single-chromatid chromosomes. In males, all four cells develop into functional gametes; in females, typically one cell becomes the egg and the others become polar bodies.

Worked example: Drawing Chromosome Content After Each Meiotic Division

A diploid parent cell has 2n = 4 (two pairs of homologous chromosomes: one long pair and one short pair). Draw and describe the chromosome content of cells after meiosis I and after meiosis II.

1. Parent cell: 4 chromosomes arranged as 2 homologous pairs. Each chromosome = 2 sister chromatids after DNA replication. Total chromatids = 8. — Always start by establishing 2n and confirming that DNA replication has occurred before meiosis begins.
2. Homologous pairs synapse: long pair forms one bivalent (4 chromatids), short pair forms another bivalent (4 chromatids). — A bivalent = two homologous chromosomes held together. Each bivalent contains 4 chromatids total — this is sometimes called a tetrad.
3. Each cell receives: 1 long chromosome (2 chromatids) + 1 short chromosome (2 chromatids). Chromosome number per cell = 2 (n = 2). DNA content = half the parent cell. — Homologous chromosomes have separated. Sister chromatids are still joined. Draw each chromosome as an X-shape to show the two chromatids.
4. Each of the two cells divides again. Each daughter cell receives: 1 long chromatid + 1 short chromatid. Chromosome number per cell = 2 (n = 2). Each chromosome is now a single chromatid. — The chromosome number does not change between meiosis I and meiosis II — it is still n = 2. What changes is that each chromosome is now a single chromatid rather than two joined chromatids.
5. Parent: 1 cell, 4 chromosomes. After meiosis I: 2 cells × 2 chromosomes = 4 chromosomes total. After meiosis II: 4 cells × 2 chromosomes = 8 chromatids total (now called chromosomes). DNA has been halved twice. — The total chromosome count across all daughter cells doubles from the parent because you have more cells, but each individual cell is haploid.

Answer: After meiosis I: 2 cells, each with n = 2 chromosomes (each chromosome = 2 sister chromatids). After meiosis II: 4 cells, each with n = 2 single-chromatid chromosomes.

5. Source of Variation I — Independent Segregation

Here is where meiosis becomes genuinely powerful. When bivalents line up at the equator during metaphase I, the orientation of each pair is entirely random. For any given bivalent, the maternal chromosome is equally likely to face either pole.

Because each pair orients independently of every other pair, the number of possible chromosome combinations in the daughter cells is $2^n$, where $n$ is the haploid number. For humans, with $n = 23$, this gives $2^{23} = 8{,}388{,}608$ possible combinations from independent segregation alone — before crossing over is even considered.

To picture this with the 2n = 4 model: the long bivalent can orient with the maternal chromosome facing the top or bottom pole; the short bivalent can do the same. That gives $2^2 = 4$ possible combinations of chromosomes in the daughter cells. Each combination produces genetically different daughter cells.

6. Source of Variation II — Crossing Over

Independent segregation shuffles whole chromosomes. Crossing over goes further — it reshuffles the alleles within chromosomes. During prophase I, when homologous chromosomes are paired as bivalents, non-sister chromatids (one from each homologue) can break at corresponding points and rejoin with each other.

The points where this exchange occurs are called chiasmata (singular: chiasma). Each chiasma represents a physical crossover event. The result is that chromatids now carry a new combination of alleles — some from the maternal chromosome, some from the paternal chromosome. These recombinant chromatids are passed into daughter cells, producing genetic variation that could not arise from independent segregation alone.

A critical precision point: crossing over occurs between non-sister chromatids of homologous chromosomes. Sister chromatids — the two identical copies joined at the centromere — do not cross over with each other. Examiners test this distinction directly.

Together, independent segregation and crossing over mean that every gamete produced is genetically unique. When two gametes fuse at fertilisation, the resulting zygote has a combination of alleles that has almost certainly never existed before — this is the foundation of genetic diversity within a species.

7. Comparing Meiosis and Mitosis

Mitosis and meiosis are both nuclear divisions, but their outcomes are fundamentally different. Mitosis produces two daughter cells that are genetically identical to the parent cell and to each other — it is used for growth, repair, and asexual reproduction. Meiosis produces four daughter cells that are genetically different from each other and from the parent cell — it is used to produce gametes.

A common exam question asks you to explain the different outcomes, not merely list them. The key causal chain is: meiosis involves two divisions rather than one, homologous chromosomes pair and then separate in meiosis I, and crossing over reshuffles alleles — none of these events occur in mitosis.

8. Exam Technique and Common Mistakes

AQA questions on meiosis tend to fall into three categories: diagram completion, explanation questions, and comparison questions. Each rewards a slightly different approach.

For diagram questions, always establish the parent cell's chromosome number first, then halve it for post-meiosis I cells, and keep it the same (but as single chromatids) for post-meiosis II cells. Draw chromosomes as X-shapes after DNA replication and as single lines after meiosis II. Label maternal and paternal chromosomes differently if the question asks you to show variation.

For explanation questions about genetic variation, you need two distinct mechanisms: independent segregation and crossing over. Stating only one will not gain full marks. Use the phrase "non-sister chromatids" when describing crossing over — this is the precise AQA terminology and distinguishes a high-quality answer from a vague one.

For comparison questions between mitosis and meiosis, structure your answer around the key differences: number of divisions, chromosome number of daughter cells, genetic identity, and whether homologous chromosomes pair. Always include a specific reason for each difference, not just a description.

Meiosis is therefore the cellular mechanism underpinning genetic diversity: two nuclear divisions reduce a diploid parent cell to four haploid daughter cells, and two distinct processes — independent segregation and crossing over — ensure those daughter cells are genetically unique, satisfying the core AQA specification requirements for this topic.