Question
Describe the stages of meiosis I and meiosis II, emphasising crossing over and independent assortment as sources of genetic variation.
Solution — Step by Step
flowchart TD
A[Parent Cell 2n] --> B[Meiosis I - Reductional]
B --> C[Prophase I - Crossing Over]
C --> D[Metaphase I - Bivalents align]
D --> E[Anaphase I - Homologs separate]
E --> F[Telophase I - 2 cells each n]
F --> G[Meiosis II - Equational]
G --> H[Prophase II]
H --> I[Metaphase II]
I --> J[Anaphase II - Chromatids separate]
J --> K[Telophase II - 4 cells each n]Prophase I has five sub-stages: Leptotene (chromosomes become visible), Zygotene (homologous chromosomes pair up = synapsis, forming bivalents), Pachytene (crossing over occurs — exchange of chromatid segments between homologs at chiasmata), Diplotene (homologs begin to separate but remain attached at chiasmata), Diakinesis (terminalization of chiasmata, nuclear envelope breaks down). Crossing over during pachytene creates recombinant chromosomes — a major source of genetic variation.
In Metaphase I, bivalents align at the metaphase plate. The orientation of each bivalent is random — this is independent assortment (each pair of homologs can face either pole independently). In Anaphase I, homologous chromosomes separate (NOT sister chromatids — this is the key difference from mitosis). Each pole gets one chromosome from each homologous pair. Telophase I produces two cells, each with n chromosomes (but each chromosome still has two chromatids). This is the reductional division — chromosome number is halved.
Meiosis II proceeds like a normal mitosis. In Anaphase II, the centromeres split and sister chromatids separate. The result is 4 haploid cells (n), each with unique combinations of alleles due to crossing over and independent assortment. This is the equational division — chromosome number stays n.
Why This Works
Meiosis generates genetic diversity through three mechanisms: (1) Crossing over (pachytene of prophase I) creates new allele combinations on individual chromosomes. (2) Independent assortment (metaphase I) randomly distributes maternal and paternal chromosomes. For humans with 23 pairs, this gives = 8.4 million possible combinations per gamete. (3) Random fertilization further doubles the combinations. Together, these ensure that no two gametes (and hence no two offspring) are genetically identical.
Alternative Method
| Feature | Meiosis I | Meiosis II |
|---|---|---|
| Type | Reductional | Equational |
| What separates | Homologous chromosomes | Sister chromatids |
| Crossing over | Yes (prophase I) | No |
| Chromosome number change | 2n to n | n to n |
| Result | 2 haploid cells | 4 haploid cells |
Common Mistake
The most tested distinction: in Anaphase I, homologous chromosomes separate (each chromosome still has 2 chromatids). In Anaphase II, sister chromatids separate. Students who write “chromatids separate in Anaphase I” are describing mitosis, not meiosis I. Also, crossing over happens in Prophase I (pachytene), not metaphase or anaphase.