# 12.3 Meiotic Cell Division · 12.4 Mitotic vs Meiotic Cell Division

F.5 HKDSE Biology · Topic XII · Lesson 1 (12.3–12.4)

How gamete-producing cells halve the chromosome number in two divisions, why no two gametes are alike, and how meiotic cell division compares with mitotic cell division.

## Figure

Animal cell, 2n = 4: meiosis I separates homologous chromosomes, meiosis II separates sister chromatids.

## Key points

- **Sexual reproduction**: haploid **gametes** fuse at **fertilization** into a diploid **zygote**. Gametes come from **meiotic cell division (減數細胞分裂)** of **gamete-producing cells** in the testes and ovaries.
- It starts after interphase and **DNA replication**, then runs **two divisions** (meiosis I and II) with **no DNA replication between them**: **four haploid** daughter cells, **genetically different** from the parent cell and from each other.
- **First meiotic division** halves the chromosome number. **Prophase I**: **homologous chromosomes pair up**, **crossing over** may occur. **Metaphase I**: pairs line up along the equator (**double row**). **Anaphase I**: the **two members of each homologous pair separate**.
- **Second meiotic division** is like mitosis but starts from a haploid cell: **sister chromatids separate** (anaphase II), giving four haploid cells.
- **Importance 1**: haploid gametes mean fertilization **restores the diploid number**, so the chromosome number of a species stays constant from generation to generation.
- **Importance 2, genetic variations (遺傳變異)**: **independent assortment (獨立分配)**, pairs line up **randomly** at metaphase I and separate independently, giving humans $2^{23}$ = 8 388 608 chromosome combinations per gamete; **crossing over (互換)** in prophase I exchanges corresponding segments of **non-sister chromatids** of a homologous pair. Variation **increases the chance for the species to survive** when conditions change.
- **Both** types of cell division start with **replicated DNA** and use **spindles** to separate chromosomes. DNA content: one doubling, then one halving (mitotic) or **two** (meiotic).

## Must know

| | Mitotic cell division | Meiotic cell division |
|---|---|---|
| Nuclear divisions | One, separating chromatids | Two: first separates homologous chromosomes, second separates chromatids |
| Daughter cells | Two, diploid (2n) body cells | Four, haploid (n) gametes |
| Genetic materials | Identical to each other and to the parent cell | Differ from each other and from the parent cell |
| Homologous chromosomes | No pairing, no crossing over | Pair up and may cross over (prophase I) |
| At the equator | Single row | Double row (metaphase I) |
| Site | Body cells | Gamete-producing cells in sex organs |

Counting in a 2n = 4 cell: after meiosis I each cell has 2 chromosomes of 2 chromatids (DNA 2 units); each gamete has 2 chromosomes of 1 chromatid (DNA 1 unit). Human gamete: 23 chromosomes.

## Common mistakes

- ✗ "DNA is replicated again between meiosis I and meiosis II." ✓ DNA is replicated **once**, in interphase before meiosis I, never between the two divisions.
- ✗ "Sister chromatids separate in meiosis I." ✓ Meiosis I separates **homologous chromosomes**, each still made of two chromatids; sister chromatids separate in **anaphase II**.
- ✗ "Homologous chromosomes pair up in meiosis II." ✓ Pairing happens in **prophase I** only and lasts until anaphase I; meiosis II starts from a haploid cell, so there are no pairs left.
- ✗ "Crossing over swaps segments between sister chromatids." ✓ Between **non-sister chromatids** of a homologous pair, one maternal and one paternal; sister chromatids are identical, so swapping them would change nothing.
- ✗ "Meiotic cell division happens in all cells that reproduce sexually." ✓ Only in **gamete-producing cells** in the sex organs (testes, ovaries); body cells divide by mitotic cell division.

## Quick check

1. A cell with 2n = 4 undergoes meiotic cell division. Give the number of chromosomes, and chromatids per chromosome, in each cell at the end of meiosis I and in each gamete.
2. Which happens in the first meiotic division only: (a) sister chromatids separate; (b) homologous chromosomes pair up and separate; (c) DNA is replicated?
3. Name the two processes in meiotic cell division that produce genetic variation in gametes, and the stage in which each takes place.
4. A hybrid plant formed by crossing two different species cannot produce gametes. Explain with reference to meiotic cell division.
5. A graph of DNA content per cell shows one doubling followed by two halvings. Name the type of cell division and state what happens at each halving.

## Answers

1. End of meiosis I: 2 chromosomes, 2 chromatids each. Gamete: 2 chromosomes, 1 chromatid each.
2. (b). Sister chromatids separate in meiosis II; DNA is replicated in interphase, before meiosis I.
3. Independent assortment (metaphase I, pairs line up randomly) and crossing over (prophase I).
4. The chromosomes from the two parent species are not homologous, so they fail to pair up in meiosis I; the hybrid cannot make gametes.
5. Meiotic cell division. First halving: homologous chromosomes separate (end of meiosis I). Second: sister chromatids separate (end of meiosis II).

## Did you know?

A baby girl is born with all her egg cells already made; they pause in prophase I of meiosis, and some wait more than 40 years before they finish dividing.