# 12.1 Chromosomes · 12.2 The Cell Cycle and Mitotic Cell Division

F.5 HKDSE Biology · Topic XII · Lesson 1 (12.1–12.2)

What chromosomes are, how to count them, and how a body cell copies itself exactly through the cell cycle and mitotic cell division.

## Figure

Mitosis in an animal cell with 2n = 4: the four stages every exam asks you to name, draw or put in order.

## Key points

- **Chromosomes** = DNA coiled around proteins. Not dividing: long, thin **chromatin (染色質)** fibres, invisible under the microscope. Dividing: fibres **coil up** into **short, thick** chromosomes, visible after staining.
- A dividing chromosome is **duplicated**: two identical **sister chromatids (姊妹染色單體)** joined at the **centromere (着絲點)**, one DNA molecule each; once separated, each is a chromosome.
- **Homologous chromosomes (同源染色體)**: a matching pair of similar size and shape, one **paternal**, one **maternal**.
- **Diploid (2n)**: two sets, in **body (somatic) cells**, made by mitotic cell division. **Haploid (n)**: one set, in **gametes**, made by meiotic cell division. Human: 46 and 23.
- **Cell cycle** = **interphase** (the cell **synthesizes proteins**, makes **organelles**, **grows**, **replicates DNA**) → **mitotic cell division** = **mitosis** (nuclear division) + **cytokinesis** (cytoplasmic division).
- **Prophase**: chromosomes shorten and thicken, nuclear membrane breaks down. **Metaphase**: line up at the **equator**, spindle fibres attach. **Anaphase**: spindle fibres **contract**, sister chromatids separate to opposite poles. **Telophase**: nuclear membranes form, chromosomes uncoil.
- **Cytokinesis**: an animal cell's membrane **constricts** inwards; a plant cell forms a **cell plate (細胞板)** (new wall + membrane) that **grows outwards**.
- Daughter cells are **genetically identical** to the parent cell. Roles: **growth**, **repair** and **asexual reproduction**.

## Must know

| Stage (2n = 4 cell) | Chromosomes | Chromatids per chromosome | DNA content (arbitrary unit) |
|--------------------|--------|-----------|-----------|
| Interphase, before DNA replication | 4 | 1 | 2 |
| Prophase, metaphase (DNA replicated) | 4 | 2 | 4 |
| Anaphase (chromatids separated) | 8 | 1 | 4 |
| Each daughter cell | 4 | 1 | 2 |

For a human cell (2n = 46): 46 chromosomes and 92 chromatids at metaphase of mitosis; 23 chromosomes in each gamete.

## Common mistakes

- ✗ "A human cell at metaphase has 92 chromosomes." ✓ 46 chromosomes × 2 chromatids = 92 chromatids; a chromatid counts as a chromosome only after anaphase separates it.
- ✗ "Interphase is the resting stage." ✓ The cell grows, makes proteins and organelles and replicates DNA; interphase takes most of the cycle.
- ✗ "In mitosis, homologous chromosomes pair up and then separate." ✓ Mitosis separates **sister chromatids**; homologous chromosomes never pair up in mitosis.
- ✗ "DNA is replicated during prophase." ✓ DNA is replicated in **interphase**, before mitosis begins; DNA content doubles then, halves at nuclear division.
- ✗ "A plant cell divides by pinching in the cell membrane." ✓ Only animal cells constrict; a plant cell builds a **cell plate** that grows outwards.

## Quick check

1. A dog body cell has 78 chromosomes. How many chromatids does it have at metaphase of mitosis, and how many chromosomes does each daughter cell receive?
2. In which stage of the cell cycle is DNA replicated, and what does each chromosome consist of afterwards?
3. Name the stage: (a) chromosomes line up at the equator; (b) sister chromatids move to opposite poles; (c) nuclear membranes form around each group of chromosomes.
4. Give two reasons why interphase is not a "resting stage".
5. A chemical stops spindle fibres from forming. At which stage do the dividing cells stop, and why?

## Answers

1. 156 chromatids; 78 chromosomes.
2. Interphase; two identical sister chromatids joined at the centromere.
3. (a) metaphase; (b) anaphase; (c) telophase.
4. Any two: DNA is replicated; the cell grows; proteins are synthesized; new organelles are made.
5. Metaphase: without spindle fibres the sister chromatids cannot be pulled to opposite poles, so anaphase cannot proceed.

## Did you know?

Red blood cells have no nucleus and cannot divide, so stem cells in your bone marrow make about 2 million new ones every second by mitotic cell division.