Cell division & heredity · Topic 13 of 17

Meiosis & Sexual Reproduction

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Biology · sexual reproduction

Meiosis divides a diploid cell twice to make four genetically unique haploid gametes; fertilization restores the diploid number.

01 · MeiosisMeiosis → 4 unique haploid cells

02 · Human numbersDiploid 46 (2n) · haploid 23 (n)

03 · VariationCrossing over: homologs swap DNA

Meiosis: 1 diploid cell → 4 genetically unique haploid gametes must know

Fertilization: egg (n) + sperm (n) → diploid zygote (2n) must know

Crossing over: homologs swap segments in prophase I → new allele mixes

Independent assortment: random line-up in metaphase I → mixed gametes

Sexual reproduction → genetic variation; asexual → clones

Step by step

DNA is copied once, in interphase, before meiosis I

Prophase I: homologs pair up (tetrads) and cross over

Metaphase I: homologous pairs line up together at the middle

Anaphase I–telophase I: homologs separate → 2 haploid cells

Step by step

Meiosis II: sister chromatids separate → 4 haploid cells (no new DNA copy)

Diploid (2n) = 2 sets, 46 in humans · haploid (n) = 1 set, 23 must know

1 DNA copy, 2 divisions: meiosis I splits pairs, meiosis II splits chromatids

Homologs = 1 from each parent · sister chromatids = identical copies

Mitosis: 2 identical diploid cells · meiosis: 4 unique haploid gametes

Commonly confused

Chromosome number halves in meiosis I, not in meiosis II

Show 3 more facts

Pairing, crossing over and tetrads happen only in meiosis I — never mitosis

Independent assortment alone: 2²³ ≈ 8.4 million gamete combinations

Tetrad = 4 chromatids of a homologous pair · chiasma = crossover point

Pearl

Meiosis halves the number, fertilization restores it: 23 + 23 = 46.

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