Question
Why is meiosis called reductional division? Explain the events in Meiosis I that result in this reduction.
Solution — Step by Step
Meiosis is called reductional division because the chromosome number is halved (reduced) in the daughter cells.
A diploid parent cell (2n) divides to produce cells with haploid chromosome number (n). In humans: a 2n = 46 cell produces cells with n = 23 chromosomes.
This reduction is essential for sexual reproduction — when two gametes (each n) fuse at fertilisation, the resulting zygote is 2n, maintaining the species-specific chromosome number across generations.
Meiosis has two divisions:
- Meiosis I — the reductional division (reduces 2n to n)
- Meiosis II — the equational division (like mitosis, separates sister chromatids, n → n)
The chromosome number is halved specifically during Meiosis I, when homologous chromosomes are separated.
Prophase I: Homologous chromosomes pair up (synapsis) to form bivalents. Crossing over occurs at chiasmata — exchange of segments between non-sister chromatids of homologous chromosomes.
Metaphase I: Bivalents (homologous pairs) align at the metaphase plate — BOTH members of each homologous pair together (unlike mitosis where individual chromosomes align).
Anaphase I: Homologous chromosomes separate to opposite poles (kinetochore microtubules pull one chromosome of each pair to each pole). Sister chromatids remain attached at their centromeres.
Telophase I: Two cells form, each with n chromosomes (but each chromosome is still a double chromatid — so the DNA content is 2C, while chromosome number is n).
The key event: separation of homologous chromosomes in Anaphase I is what causes reduction.
In mitosis: parent cell (2n) → 2 daughter cells (2n). No reduction. Sister chromatids separate in Anaphase.
In Meiosis I (reductional): parent cell (2n) → 2 cells (n). Homologous chromosomes separate in Anaphase I.
| Feature | Mitosis | Meiosis I | Meiosis II |
|---|---|---|---|
| What separates | Sister chromatids | Homologous chromosomes | Sister chromatids |
| Chromosome count | 2n → 2n | 2n → n | n → n |
| Type | Equational | Reductional | Equational |
Why This Works
The reduction in chromosome number is made possible by the unique pairing of homologous chromosomes in Prophase I (synapsis). No such pairing occurs in mitosis — chromosomes remain as individual entities throughout.
Because homologs pair up in Meiosis I, the cell can separate ONE chromosome from each pair to each daughter cell. Each daughter receives only one representative of each homologous pair — not both. This is the physical basis of Mendel’s Law of Segregation.
Alternative Method — DNA Content Tracking
| Stage | Chromosome number | DNA content |
|---|---|---|
| Before S phase | 2n | 2C |
| After S phase | 2n | 4C |
| After Meiosis I | n | 2C |
| After Meiosis II | n | C |
DNA content is halved twice: once in Meiosis I (2C per cell) and once in Meiosis II (C per gamete). Chromosome number reduces only once — in Meiosis I.
Common Mistake
Saying meiosis II is the reductional division. Meiosis II is equational (like mitosis) — it separates sister chromatids without changing chromosome number. The reduction happens in Meiosis I when HOMOLOGOUS CHROMOSOMES (not sister chromatids) separate. Some students confuse the two divisions because both result in cells with half the DNA of the original, but chromosome count reduction is only in Meiosis I.