Short on time? This page is the recap. For explanations, figures and worked examples, use the full study guide.
Recap
- Meiosis turns one diploid (2n) cell into four haploid (n) cells. In animals these become gametes; fertilization restores 2n.
- Homologous chromosomes: one from each parent, same genes in the same order, possibly different alleles. Sister chromatids: identical copies made in S phase, joined at the centromere.
- DNA replicates once (S phase before meiosis). There is no S phase between meiosis I and II.
- Meiosis I: prophase I (condensing, synapsis of homologues, chiasmata may form, spindle forms, nuclear envelope breaks down); metaphase I (homologous pairs at the plate); anaphase I (homologues separate, sister chromatids stay joined); telophase I and cytokinesis (two haploid cells, chromosomes still duplicated).
- Meiosis II: prophase II (new spindle); metaphase II (chromosomes in single file, the kinetochores of the two sister chromatids attached to microtubules from opposite poles); anaphase II (centromere proteins break down, sister chromatids separate); telophase II and cytokinesis (four haploid cells, single-chromatid chromosomes).
Key relationships
| Stage (per cell), start 2n | Chromosomes | Chromatids | DNA (G1 = x) |
|---|---|---|---|
| G1 | 2n | 2n | x |
| Prophase I to metaphase I | 2n | 4n | 2x |
| End of meiosis I | n | 2n | x |
| End of meiosis II | n | n | x ÷ 2 |
| Mitosis | Meiosis | |
|---|---|---|
| Divisions | 1 | 2 |
| Cells made | 2 | 4 |
| Daughter chromosome number | 2n | n |
| Daughter cells | genetically identical | genetically different |
| Shared features | spindle moves chromosomes; same stage names; cytokinesis by furrow or cell plate |
Assumptions behind the counts
- The table assumes chromosomes separate correctly. Errors (nondisjunction) are covered in Topic 5.2.
- “DNA content” means nuclear DNA per cell; values are relative.
- The cleavage furrow applies to animal cells; plant cells form a cell plate.
Mistakes to avoid
- Saying sister chromatids separate in anaphase I.
- Calling cells at the end of meiosis I diploid because their chromosomes have two chromatids.
- Adding an S phase between meiosis I and II.
- Counting a duplicated chromosome as two chromosomes.
- Saying homologues are identical copies.
- Saying meiosis gives two cells.
Quick self-check
- A species has 2n = 16. How many chromosomes and chromatids are in a cell at metaphase I? (16 chromosomes, 32 chromatids)
- How many cells, with how many chromosomes each, does that cell give at the end of meiosis II? (4 cells of 8 chromosomes)
- Which stage halves the chromosome number, and why? (Anaphase I with telophase I: homologues go to different cells, so each cell gets one of each pair.)
Next: practice questions.