Sexual and Asexual Reproduction

This lesson covers sexual reproduction, asexual reproduction, and the cell division processes of meiosis and mitosis as required by the Edexcel GCSE Combined Science specification (1SC0). You need to compare the two types of reproduction, explain how each type of cell division works and understand their significance.

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Mitosis

Mitosis is the type of cell division used for growth, repair and asexual reproduction. It produces two genetically identical daughter cells, each with the same number of chromosomes as the parent cell.

The Cell Cycle

Before a cell divides by mitosis, it goes through the cell cycle:

1. Interphase — the cell grows, organelles are replicated, and DNA is copied (each chromosome is duplicated to form two identical copies joined at a centromere).
2. Mitosis — the nucleus divides; the chromosomes line up, separate and move to opposite ends of the cell.
3. Cytokinesis — the cytoplasm divides, producing two genetically identical daughter cells.

graph LR
    A["Interphase: cell grows, DNA replicates"] --> B["Mitosis: nucleus divides"]
    B --> C["Cytokinesis: cytoplasm divides"]
    C --> D["2 genetically identical daughter cells"]

Key Features of Mitosis

Feature	Detail
Number of divisions	1
Number of daughter cells	2
Chromosome number	Same as parent (diploid, 2n)
Genetic variation	None — daughter cells are identical
Purpose	Growth, repair, asexual reproduction

Exam Tip: Mitosis produces two identical cells. If the parent cell has 46 chromosomes, each daughter cell also has 46. There is no genetic variation.

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Meiosis

Meiosis is the type of cell division that produces gametes (sex cells) — sperm and egg cells in animals, pollen and egg cells in plants. It produces four genetically different daughter cells, each with half the number of chromosomes.

Steps of Meiosis

1. Before meiosis: DNA replicates during interphase.
2. First division (meiosis I): Homologous chromosome pairs line up and separate — each daughter cell receives one chromosome from each pair. Crossing over can occur, where sections of DNA are exchanged between homologous chromosomes.
3. Second division (meiosis II): The chromosomes line up again and the sister chromatids separate — producing four cells.

graph TD
    A["Parent cell (diploid, 2n = 46)"] --> B["DNA replicates"]
    B --> C["Meiosis I: homologous pairs separate"]
    C --> D["2 cells (haploid, n = 23)"]
    D --> E["Meiosis II: sister chromatids separate"]
    E --> F["4 genetically different haploid cells (gametes)"]

Key Features of Meiosis

Feature	Detail
Number of divisions	2
Number of daughter cells	4
Chromosome number	Half of parent (haploid, n)
Genetic variation	Yes — daughter cells are all different
Purpose	Production of gametes

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Comparing Mitosis and Meiosis

Feature	Mitosis	Meiosis
Number of divisions	1	2
Daughter cells produced	2	4
Chromosome number in daughter cells	Diploid (2n)	Haploid (n)
Genetically identical?	Yes	No
Where it occurs	Body (somatic) cells	Reproductive organs
Purpose	Growth, repair, asexual reproduction	Producing gametes

Exam Tip: A common exam question asks you to compare mitosis and meiosis in a table. Learn the six key differences above — they are worth several marks.

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Sexual Reproduction

Sexual reproduction involves the fusion of two gametes (sex cells) — one from each parent. The fusion of gametes is called fertilisation, which produces a zygote.

• Gametes are haploid (n = 23 chromosomes in humans).
• When two gametes fuse, the zygote is diploid (2n = 46 chromosomes).
• The offspring inherits a mixture of genetic material from both parents.
• This produces genetic variation in the offspring.

Advantages of Sexual Reproduction

• Produces genetic variation in offspring.
• Variation means the population is more likely to survive environmental changes.
• Natural selection can act on variation, driving evolution.

Disadvantages of Sexual Reproduction

• Requires two parents — finding a mate takes time and energy.
• Slower than asexual reproduction.
• Not all offspring will have favourable combinations of alleles.

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Asexual Reproduction

Asexual reproduction involves only one parent. The offspring are produced by mitosis and are genetically identical to the parent — they are clones.

Examples of Asexual Reproduction

Organism	Method
Bacteria	Binary fission
Plants	Runners (strawberries), tubers (potatoes), bulbs
Fungi	Spore production, budding
Starfish	Fragmentation and regeneration

Advantages of Asexual Reproduction

• Only one parent needed — no time or energy spent finding a mate.
• Faster than sexual reproduction — rapid population increase.
• If the environment is stable, all offspring are well-suited to survive.

Disadvantages of Asexual Reproduction

• No genetic variation — all offspring are clones.
• If the environment changes, the entire population may be affected.
• Vulnerable to disease — if one individual is susceptible, all clones are too.

Exam Tip: Some organisms can reproduce both sexually and asexually. For example, strawberry plants produce runners (asexual) and also produce flowers for sexual reproduction. This is a common 6-mark question topic.

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Summary

• Mitosis produces 2 genetically identical diploid cells — used for growth, repair and asexual reproduction.
• Meiosis produces 4 genetically different haploid cells (gametes) — used for sexual reproduction.
• Sexual reproduction involves fertilisation of two gametes, producing genetic variation.
• Asexual reproduction involves one parent and mitosis, producing clones (no variation).
• Genetic variation from sexual reproduction helps populations survive changing environments.

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Deeper Look: Why Meiosis Produces Variation

Meiosis is not simply "mitosis with half the chromosomes" — it contains two deliberate steps that generate genetic variation before fertilisation occurs: