CB3.1 — Reproduction and meiosis (Edexcel 1SC0)
Sexual vs asexual reproduction
| Feature | Sexual | Asexual |
|---|---|---|
| Gametes required | Yes | No |
| Genetic variation | Yes | No (clones) |
| Number of parents | 2 | 1 |
| Examples | Humans, flowers | Bacteria, runners in strawberries |
Advantages of sexual reproduction: genetic variation → better adapted to changing environments. Advantages of asexual reproduction: fast; no need for a mate; all offspring can reproduce.
Meiosis
Meiosis produces gametes (sex cells) — sperm and eggs in animals; pollen and ovules in plants.
Key features:
- Two divisions (meiosis I and II).
- Produces four haploid cells (half the chromosome number) — in humans: 23 chromosomes each.
- Each cell is genetically unique due to independent assortment and crossing over.
Crossing over: homologous chromosomes exchange segments in prophase I → increases variation. Independent assortment: chromosomes separate randomly in meiosis I → further variation.
At fertilisation, two haploid gametes fuse → diploid zygote (46 chromosomes).
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