Question
Describe the stages of meiosis I and meiosis II. Where do crossing over and independent assortment occur? How does meiosis produce four genetically different haploid cells from one diploid cell?
Solution — Step by Step
Prophase I is the longest and most complex phase. It has five sub-stages:
- Leptotene — chromosomes condense
- Zygotene — homologous chromosomes pair up (synapsis), forming bivalents
- Pachytene — crossing over occurs between non-sister chromatids at points called chiasmata
- Diplotene — homologues begin to separate; chiasmata visible
- Diakinesis — bivalents fully condensed, nuclear envelope breaks down
Metaphase I — bivalents align at the metaphase plate. The orientation of each bivalent is random — this is independent assortment.
Anaphase I — homologous chromosomes separate (not sister chromatids). Each pole gets one chromosome from each homologous pair. Chromosome number is halved (2n → n).
Telophase I — two haploid cells form, each with n chromosomes (still as sister chromatids).
Prophase II — chromosomes condense again, spindle forms.
Metaphase II — chromosomes align at equator.
Anaphase II — sister chromatids separate (centromere splits), pulled to opposite poles.
Telophase II — four haploid daughter cells formed, each with n chromosomes (single chromatids).
flowchart TD
A[Parent cell 2n] --> B[Meiosis I]
B --> C[Prophase I: Crossing over at chiasmata]
C --> D[Metaphase I: Independent assortment]
D --> E[Anaphase I: Homologues separate]
E --> F[2 haploid cells n]
F --> G[Meiosis II]
G --> H[Anaphase II: Sister chromatids separate]
H --> I[4 haploid cells n]Why This Works
Meiosis achieves two goals: (1) halving the chromosome number (essential for sexual reproduction — gametes must be haploid so fertilisation restores the diploid number), and (2) generating genetic diversity through crossing over (new allele combinations) and independent assortment (random orientation of bivalents).
Alternative Method
To count chromosomes at each stage: start with 2n = 46 (human). After meiosis I: each cell has 23 chromosomes (each with 2 chromatids). After meiosis II: each cell has 23 chromosomes (single chromatids). Total cells = 4 haploid.
Common Mistake
The most common error: “crossing over occurs in meiosis II.” Crossing over happens during prophase I (pachytene stage), when homologous chromosomes are paired as bivalents. In meiosis II, there are no homologues to cross over with.
Meiosis I is the reductional division (2n → n) — homologues separate. Meiosis II is equational (like mitosis) — sister chromatids separate. The key difference from mitosis is that meiosis I separates homologues, not sister chromatids.