CorpusHuman Anatomy
Cells & TissuesCell process

Meiosis

Also known as: meiosis I, meiosis II, reduction division, crossing over, synapsis, chiasma, independent assortment, nondisjunction, gametogenesis

Two successive divisions after a single round of DNA replication that halve the chromosome number (2n → n) and shuffle genetic material, producing haploid gametes.

Overview

Products4 haploid cells (n = 23)
Independent-assortment combinations2²³ ≈ 8.4 × 10⁶
Oocyte arrestsProphase I (diplotene) until ovulation; metaphase II until fertilisation

Meiosis I (reductional division) separates homologous chromosomes. Its long prophase I has five stages: leptotene (chromosomes condense), zygotene (homologues pair — synapsis — zipped by the synaptonemal complex into bivalents/tetrads), pachytene (crossing over: programmed double-strand breaks are repaired using the non-sister chromatid of the homologue, exchanging segments), diplotene (homologues pull apart but remain joined at chiasmata, the visible sites of crossovers; primary oocytes arrest here — dictyotene) and diakinesis. In metaphase I bivalents align at the plate with homologues facing opposite poles; their orientation is random (independent assortment: 2²³ ≈ 8.4 million combinations of maternal and paternal chromosomes). In anaphase I homologues separate while sister chromatids stay together (centromeric cohesin is protected by shugoshin). Each daughter cell is haploid (n) but each chromosome still has two chromatids.

Meiosis II (equational division) follows without DNA replication and resembles mitosis: sister chromatids separate, giving four haploid cells with single-chromatid chromosomes (n, 1C). Crossing over plus independent assortment (plus random fertilisation) make every gamete genetically unique.

Spermatogenesis (from puberty, continuous) produces four sperm from each primary spermatocyte. Oogenesis begins before birth: primary oocytes arrest in prophase I (diplotene) for up to ≈50 years; meiosis I is completed just before ovulation (producing the first polar body), and the secondary oocyte arrests in metaphase II until fertilisation, when meiosis II completes (second polar body). One ovum results from each primary oocyte.

Parts & subdivisions

  1. Prophase I
    Leptotene, zygotene (synapsis), pachytene (crossing over), diplotene (chiasmata; oocyte arrest), diakinesis.
  2. Meiosis I
    Homologues separate → two haploid cells (2 chromatids per chromosome).
  3. Meiosis II
    Sister chromatids separate → four haploid cells.
  4. Crossing over
    Reciprocal exchange between non-sister chromatids; also needed for correct segregation.

Function

  • Produces haploid gametes so that fertilisation restores the diploid number.
  • Generates genetic diversity.

Clinical significance

Nondisjunction and aneuploidy

Failure of homologues (meiosis I) or sister chromatids (meiosis II) to separate gives gametes with an extra or missing chromosome. Trisomy 21 (≈95 % of Down syndrome cases) is mostly maternal meiosis I nondisjunction, increasing with maternal age; others: trisomy 18, trisomy 13, 47,XXY (Klinefelter), 45,X (Turner — often paternal loss). Uniparental disomy can follow "trisomy rescue".

Maternal age effect

Cohesion between homologues weakens during the decades-long prophase I arrest, explaining the steep rise in aneuploid pregnancies after ≈35 years.

Mnemonics

Prophase I substages

"Lazy Zebras Prefer Dozing Daily" — Leptotene, Zygotene, Pachytene, Diplotene, Diakinesis.

Test yourself

Crossing over between non-sister chromatids of homologous chromosomes occurs during:
  1. Prophase I (pachytene)
  2. Metaphase I
  3. Anaphase II
  4. S phase

Answer: Prophase I (pachytene)

Homologues synapse via the synaptonemal complex (zygotene) and exchange segments in pachytene; the crossover sites become visible as chiasmata in diplotene.

Down syndrome is most often caused by:
  1. Paternal nondisjunction in meiosis II
  2. Maternal nondisjunction in meiosis I
  3. Robertsonian translocation
  4. Mitotic nondisjunction after fertilisation (mosaicism)

Answer: Maternal nondisjunction in meiosis I

About 95 % of cases are free trisomy 21 from nondisjunction, ≈90 % of which is maternal and mostly in meiosis I — linked to the decades-long prophase I arrest of oocytes. Translocations account for ≈4 % and mosaicism ≈1 %.

More cells & tissues questions

Compiled from standard anatomy and physiology references — see the Cells & Tissues sources. For education only; not medical advice.