Meiosis
Every sperm and egg is the product of meiosis — two back-to-back divisions that halve the chromosome count and shuffle the genetic deck. Learn how one diploid cell becomes four unique haploid gametes.
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Interphase
S Phase Complete
The cell has completed DNA replication during S phase — every chromosome now consists of two identical sister chromatids. The cell grows and duplicates its centrosomes in preparation for two rounds of division.
- DNA replicated: each chromosome is now 2 sister chromatids
- Homologous chromosome pairs each contain 4 chromatids total
- Cell will divide TWICE to produce 4 haploid cells
Two very different divisions
Homologous chromosomes separate. Chromosome number halves: 2n → n. Sister chromatids stay joined. This division is unique to meiosis.
Sister chromatids separate — essentially mitosis on haploid cells. Chromosome count stays at n. Two cells → four cells.
Why meiosis matters
Meiosis is the engine of sexual reproduction and genetic diversity. Without it, chromosome counts would double with every fertilization and allele combinations would never shuffle — evolution would grind to a halt.
2n → n
Meiosis halves the chromosome count from diploid (2n = 46) to haploid (n = 23), so that fertilization restores the diploid number without doubling it every generation.
Crossing Over
In Prophase I, non-sister chromatids of homologous chromosomes exchange segments at chiasmata — physically shuffling alleles and creating chromosomes that are neither fully maternal nor fully paternal.
8 Million+
Independent assortment alone yields 2²³ ≈ 8 million possible chromosome combinations per gamete. Combined with crossing over, no two human gametes are genetically identical.
Meiosis vs. Mitosis
Mitosis produces 2 identical diploid cells for growth and repair — no mixing, no reduction. Meiosis produces 4 genetically unique haploid cells for reproduction. The key differences: meiosis has two divisions, includes synapsis and crossing over in Prophase I, and separates homologs (not sister chromatids) in the first division. Never confuse them on an exam.
Quick recap
- 1Interphase — DNA replicated; centrosomes duplicated — ready for two divisions.
- 2Prophase I — Homologs pair (synapsis); crossing over shuffles alleles.
- 3Metaphase I — Bivalents align at equator; orientation is random (independent assortment).
- 4Anaphase I — Homologs separate to poles; chromosome number halves (2n → n).
- 5Telophase I — Two haploid cells form — each chromosome still has 2 chromatids.
- 6Metaphase II — Individual chromosomes align in both cells — like mitosis.
- 7Anaphase II — Sister chromatids finally separate in both cells.
- 84 Haploid Cells — Cytokinesis II → 4 genetically unique haploid gametes.