Gametes are not made by mitosis. Sperm and eggs are made by meiosis, a different kind of nuclear division. Extended: meiosis is a reduction division that halves the chromosome number from diploid to haploid and makes gametes that are genetically different from each other.
Meiosis is involved in the production of gametes. Extended: it is a reduction division in which the chromosome number is halved from diploid to haploid, resulting in genetically different cells.
Fig. 1: meiosis turns one diploid cell into four haploid gametes, each with one chromosome from each pair
Section 01
Making gametes.
The sim cell is in an ovary or testis. Red chromosomes came from the mother and blue from the father. Press Next step to the end, then press Start again and repeat: compare the gametes each time.
Meiosis is the nuclear division involved in the production of gametes: sperm and eggs in animals, pollen nuclei and egg cells in plants.
Extended.Reduction division: the chromosome number is halved from diploid to haploid. In the sim, 4 chromosomes become 2 per gamete; in humans, 46 become 23.
Extended.One of each pair: each gamete gets one chromosome from every pair, never both and never neither.
Extended.Genetically different: the pairs line up at random, so the gametes carry different mixtures of the mother's and father's chromosomes.
Examiner trap
Haploid does not mean half a chromosome or half of each chromosome. It means a single set: one whole chromosome of each type.
Stage 1 · Learn
Check what the sim just showed you
Four quick checks. Each correct answer earns XP and records this skill on your profile.
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In the sim, how many chromosomes does each gamete have at the end?
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How many gametes are made from the one cell in the sim?
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You press Start again and run meiosis twice. Why do the gametes contain different mixtures of red and blue chromosomes?
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Extended: what happens at the step First division in the sim?
Section 02
Mitosis or meiosis?
Many exam questions ask you to compare the two divisions. The key differences are where they happen, how many chromosomes the new cells get and whether the cells are identical.
Mitosis
Meiosis
Used for
growth, repair, replacement, asexual reproduction
making gametes
Chromosome number in new cells
the same as the parent cell (diploid stays diploid)
halved (diploid to haploid)
New cells are genetically
identical
different
Human example
skin cell, 46 → 46
cell in testis, 46 → sperm, 23
Why halve? At fertilisation a haploid sperm (23) and a haploid egg (23) fuse to form a diploid zygote (46). Without meiosis, the chromosome number would double in every generation.
Why variety? Genetically different gametes, combined at random at fertilisation, give offspring that differ from their parents and from each other.
Examiner trap
The zygote divides by mitosis, not meiosis. Meiosis happens only when gametes are being made.
Stage 2 · Exam
Exam-style questions
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Finish the four checks above to unlock the exam questions
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Meiosis is involved in the production of:
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Extended: a dog has 78 chromosomes in its body cells. How many are there in a dog sperm?
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Extended: which statement describes meiosis?
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Extended: why is it important that gametes are haploid?
Skill unlocked
Meiosis, mastered.
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