# Revision on Topic 12 · The Cell Cycle and Cell Division

F.5 HKDSE Biology · Topic XII · Lesson 2 (revision of 12.1–12.4)

The whole topic in one set: chromosomes, the cell cycle, mitotic and meiotic cell division, and how to read a DNA-content graph.

## Figure

DNA content per cell against time: a gradual rise is replication, a vertical drop is the cell dividing.

## Key points

- **Chromosomes** = DNA + proteins, seen as **chromatin** when the cell is not dividing. **Homologous chromosomes (同源染色體)**: a pair, one paternal, one maternal.
- **Body cells** are **diploid** (two sets); **gametes** are **haploid** (one set). The number is the same within a species.
- **Interphase**: DNA replicates, new organelles and proteins are synthesized, the cell grows.
- **Mitotic cell division** = mitosis + **cytokinesis (胞質分裂)**: genetically identical cells for growth, repair and asexual reproduction.
- **Meiotic cell division**, in gamete-producing cells: the 1st division separates **homologous chromosomes**, the 2nd separates **sister chromatids**; no replication between them.
- Haploid gametes restore the diploid number at **fertilization**; **independent assortment (獨立分配)** and **crossing over (互換)** cause **genetic variations**.
- **Both** start with replicated DNA and involve nuclear and cytoplasmic division.

## Must know

| | Mitotic cell division | Meiotic cell division |
|---|---|---|
| Place | Body cells | Gamete-producing cells |
| Number of cell divisions | One | Two |
| Pairing, separation of homologous chromosomes | Does not occur | Occurs |
| Crossing over | Does not occur | May occur |
| Daughter cells | Two, same chromosome number as parent (2n) | Four, half of parent (n) |
| Genetic make-up | Same as parent and among daughter cells | Different from parent and among daughter cells |

## Common mistakes

- ✗ "The chromosome number is halved in meiosis II." ✓ It is halved in **meiosis I**, when homologous pairs separate; meiosis II separates sister chromatids.
- ✗ "The vertical drop from 4 to 2 is DNA replication finishing." ✓ A vertical drop is the **cell dividing**: each daughter cell gets half the DNA. Replication is the **gradual rise** in interphase.
- ✗ "The instant jump from 1 to 2 is DNA replication." ✓ An instant doubling with no slope is **fertilization**; replication takes time and shows as a slope.
- ✗ "Making new organelles and proteins raises the DNA content." ✓ Only **DNA replication** raises the DNA content; making new organelles and proteins does not.
- ✗ "Mitosis includes splitting the cytoplasm." ✓ **Mitosis** is nuclear division only; **cytokinesis** (cytoplasmic division) follows it.

## Quick check

1. A body cell has 2n = 8. How many chromosomes are in each daughter cell after mitotic cell division, and in each gamete?
2. State two events in interphase that prepare the cell for division.
3. In which stage do sister chromatids separate in mitotic cell division? In which stage do homologous chromosomes separate in meiotic cell division?
4. A DNA-content graph reads 2 → 4 (gradual) → 2 → 1 (vertical drops). Name the type of cell division and say what the gradual rise is.
5. Give two reasons why meiotic cell division is important.

## Answers

1. 8 in each daughter cell; 4 in each gamete.
2. Any two: DNA replicates; new organelles and proteins are synthesized; the cell grows.
3. Anaphase; anaphase I.
4. Meiotic cell division; the gradual rise is DNA replication in interphase.
5. It forms haploid gametes, so the diploid number is restored at fertilization; independent assortment and crossing over cause genetic variations.

## Did you know?

The 46 chromosomes in one of your body cells hold about 2 metres of DNA, coiled into a nucleus only about 6 micrometres across.