Cell Division by Meiosis [H]

This lesson covers meiosis as required by AQA GCSE Biology specification 4.1.1 (Higher Tier only). You need to understand the process of meiosis, how it differs from mitosis, and why it is essential for sexual reproduction and genetic variation.

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What Is Meiosis?

Meiosis is a type of cell division that produces gametes (sex cells). In animals, the gametes are sperm cells and egg cells (ova). In flowering plants, the gametes are pollen and egg cells.

Meiosis is sometimes called reduction division because it reduces the chromosome number from diploid (2n) to haploid (n).

Term	Meaning	Chromosome Number (Humans)
Diploid (2n)	A cell with two complete sets of chromosomes (one from each parent)	46 chromosomes (23 pairs)
Haploid (n)	A cell with one complete set of chromosomes	23 chromosomes

When two haploid gametes fuse during fertilisation, the resulting zygote is diploid again (23 + 23 = 46). This maintains the correct chromosome number from generation to generation.

Exam Tip: Remember: meiosis = gamete production = haploid cells = genetic variation. Mitosis = growth/repair = diploid cells = genetically identical cells. This comparison is tested very frequently at Higher Tier.

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Where Does Meiosis Occur?

Organism	Location of Meiosis	Gametes Produced
Human male	Testes	Sperm cells
Human female	Ovaries	Egg cells (ova)
Flowering plant	Anthers (male) and ovules (female)	Pollen grains and egg cells

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The Process of Meiosis

Meiosis involves two successive divisions, producing four genetically different haploid cells from one diploid parent cell.

Before Meiosis: DNA Replication

Just as in mitosis, the DNA is replicated during interphase before meiosis begins. Each chromosome is copied to form two identical sister chromatids joined at the centromere.

Meiosis I (First Division) — Reduction Division

1. The replicated chromosomes condense and become visible.
2. Homologous pairs of chromosomes (one from the mother and one from the father) line up together along the equator of the cell. This pairing is called synapsis.
3. During synapsis, sections of DNA can be exchanged between the paired homologous chromosomes — this is called crossing over, and it creates new combinations of alleles.
4. The homologous pairs are then separated — one chromosome from each pair is pulled to each pole of the cell.
5. The cell divides into two cells, each with half the original number of chromosomes (haploid). However, each chromosome still consists of two sister chromatids.

Meiosis II (Second Division) — Similar to Mitosis

1. The chromosomes line up along the equator of each cell.
2. The sister chromatids are separated — one chromatid is pulled to each pole.
3. Each cell divides again, producing a total of four haploid daughter cells.
4. Each of the four cells is genetically different from the others and from the parent cell.

flowchart TD
    A["Parent Cell (Diploid, 2n = 46)"] --> B["DNA Replication in Interphase"]
    B --> C["MEIOSIS I: Homologous pairs separate"]
    C --> D["Cell 1 (Haploid, n = 23)<br/>Each chromosome = 2 chromatids"]
    C --> E["Cell 2 (Haploid, n = 23)<br/>Each chromosome = 2 chromatids"]
    D --> F["MEIOSIS II: Sister chromatids separate"]
    E --> G["MEIOSIS II: Sister chromatids separate"]
    F --> H["Gamete 1"]
    F --> I["Gamete 2"]
    G --> J["Gamete 3"]
    G --> K["Gamete 4"]

    style A fill:#8e44ad,color:#fff
    style C fill:#e74c3c,color:#fff
    style F fill:#3498db,color:#fff
    style G fill:#3498db,color:#fff
    style H fill:#27ae60,color:#fff
    style I fill:#27ae60,color:#fff
    style J fill:#27ae60,color:#fff
    style K fill:#27ae60,color:#fff

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How Meiosis Produces Genetic Variation

Meiosis produces genetically unique gametes through two main mechanisms:

1. Independent Assortment

During Meiosis I, when homologous pairs line up at the equator, the orientation of each pair is random. The maternal chromosome could go to either pole, independently of all other pairs. With 23 pairs of chromosomes in humans, there are 2^23 = 8,388,608 possible combinations of chromosomes in the gametes from independent assortment alone.

2. Crossing Over

During Meiosis I, when homologous chromosomes are paired together during synapsis, sections of chromatids can break and swap between the homologous chromosomes. This creates chromatids with new combinations of alleles that were not present in either parent chromosome. Crossing over dramatically increases genetic variation beyond what independent assortment alone can produce.

3. Random Fertilisation

Although not part of meiosis itself, the random fusion of any sperm with any egg further increases genetic variation in the offspring.

Exam Tip: If asked how meiosis produces genetic variation, you must mention both independent assortment and crossing over. Many students only mention one and miss marks. Also note that random fertilisation adds further variation.

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Comparison: Mitosis vs Meiosis

Feature	Mitosis	Meiosis
Number of divisions	1	2
Number of daughter cells	2	4
Chromosome number of daughter cells	Diploid (2n = 46) — same as parent	Haploid (n = 23) — half of parent
Genetic variation	None — daughter cells are identical clones	Yes — daughter cells are all genetically different
Where it occurs	All body cells (somatic cells)	Reproductive organs (testes, ovaries, anthers, ovules)
Purpose	Growth, repair, asexual reproduction	Production of gametes for sexual reproduction
Crossing over	No	Yes (during Meiosis I)
Independent assortment	No	Yes
Homologous pairing	No	Yes (during Meiosis I)

Exam Tip: A very common exam question is to compare mitosis and meiosis. Learn this table thoroughly and be ready to state at least four clear differences. Always state the specific number of daughter cells (2 vs 4) and the specific chromosome number (diploid vs haploid) for full marks.

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The Importance of Meiosis in Sexual Reproduction

Meiosis is essential because: