Meiosis
High-Yield Summary
- Meiosis occurs only in germline cells: 1 diploid cell → 4 genetically unique haploid gametes, via two rounds (Meiosis I, Meiosis II).
- Meiosis I = reductional division (diploid → haploid). Prophase I: homologous chromosomes pair (synapsis, forming a tetrad) and cross over at chiasmata → recombinant chromosomes.
- Anaphase I separates homologous chromosomes (sister chromatids stay together). Anaphase II separates sister chromatids — the single most-tested distinction in this topic.
- Meiosis II = equational division (haploid → haploid); just separates the sister chromatids Meiosis I left intact.
- Nondisjunction (failure to separate properly) at Anaphase I or II produces gametes with abnormal chromosome number — e.g. Down syndrome (trisomy 21).
Key Terms
- Haploid (n)
- One copy of each chromosome — 23 total in humans.
- Diploid (2n)
- Two copies of each chromosome — 46 total in humans.
- Synapsis
- Pairing of homologous (maternal + paternal) chromosomes side by side in Prophase I, forming a tetrad.
- Crossing over
- Exchange of DNA segments between homologous chromosomes at chiasmata during Prophase I, producing recombinant chromosomes.
- Nondisjunction
- Failure of chromosomes (Anaphase I) or sister chromatids (Anaphase II) to separate properly, producing gametes with an abnormal chromosome number.
Meiosis I → Meiosis II
- 1Prophase I — synapsis pairs homologous chromosomes into tetrads; crossing over at chiasmata creates recombinant chromosomes.
- 2Metaphase I — tetrads (not individual chromosomes) align at the metaphase plate, in random orientation.
- 3Anaphase I — homologous chromosomes separate to opposite poles; sister chromatids remain joined.
- 4Telophase I / Cytokinesis — cell splits into 2 haploid cells, each chromosome still made of 2 sister chromatids.
- 5Prophase II — spindle re-forms in each haploid cell.
- 6Metaphase II — individual chromosomes line up at each cell's center.
- 7Anaphase II — sister chromatids finally separate to opposite poles.
- 8Telophase II / Cytokinesis — yields 4 genetically unique haploid daughter cells.
Mitosis vs. Meiosis
| Mitosis | Meiosis |
|---|---|
| 2n → two identical 2n cells | 2n → four unique haploid (n) cells |
| All dividing somatic cells | Germline cells only |
| No homologous pairing | Synapsis pairs homologs in Meiosis I |
| No crossing over | Crossing over in Prophase I |
| Sister chromatids separate (single anaphase) | Homologs separate (Anaphase I), then sister chromatids (Anaphase II) |
Common MCAT Trap
- Anaphase I separates HOMOLOGOUS CHROMOSOMES, not sister chromatids — sister chromatids don't split until Anaphase II. This exact distinction is the highest-yield trap in the topic.
- Mitosis and meiosis share stage names (prophase/metaphase/anaphase/telophase) but different outcomes — don't assume shared name means shared behavior.
- Nondisjunction can occur at either Anaphase I or Anaphase II — know which chromosome/chromatid error corresponds to which stage.
Quick Recall
What separates during Anaphase I vs. Anaphase II?
Where and when does crossing over occur?
Why is Meiosis I called the "reductional division"?
What chromosomal condition results from nondisjunction of chromosome 21?