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2.3Unit 2 — Multicellular Organisms

Reproduction — SQA National 5 Biology — SQA National 5 Biology Revision Notes & Topical Questions

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Section 1 of 2Gametes and chromosome numbers

  • Haploid gametes:
    • Gametes are specialised sex cells that contain a single set of chromosomes (n).
    • In humans, normal body cells are diploid (46 chromosomes, 2n); gametes are haploid (23 chromosomes, n).
  • Comparison of animal and plant gametes:
    OrganismMale GameteSite of Male ProductionFemale GameteSite of Female Production
    AnimalsSperm (motile, has tail and mitochondria)TestesEgg / Ovum (large, non-motile, contains food store)Ovaries
    Flowering PlantsPollen grain (lightweight or sticky)Anthers (part of stamen)OvuleOvary (part of carpel)

Section 2 of 2Fertilisation

  • Fertilisation is the fusion of the nucleus of a haploid male gamete with the nucleus of a haploid female gamete to produce a diploid zygote:
    • Haploid Sperm Nucleus (n) + Haploid Egg Nucleus (n) → Diploid Zygote Nucleus (2n)
  • The diploid zygote divides repeatedly by mitosis to develop into an embryo, ensuring all subsequent somatic cells retain the identical diploid chromosome complement.

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