Cell Division: Mitosis and the Cell Cycle
The Cell Cycle
The cell cycle is the series of stages a cell goes through as it grows and divides. It consists of three main stages:
1. Interphase (growth phase) — the cell spends most of its life here
2. Mitosis — the nucleus divides
3. Cytokinesis — the cytoplasm divides, producing two daughter cells
Interphase
During interphase, the cell prepares for division:
- The cell grows and increases in size
- The number of organelles (mitochondria, ribosomes) increases
- The DNA replicates — each chromosome makes an identical copy of itself, forming two sister chromatids joined at the centromere
- The cell carries out normal metabolic functions
Interphase is not a resting phase — it is the longest and most active part of the cell cycle.
Mitosis
Mitosis produces two genetically identical diploid daughter cells. It is used for growth, repair of damaged tissue, and asexual reproduction.
The stages of mitosis can be remembered as PMAT:
| Stage | What happens |
|---|---|
| Prophase | Chromosomes condense and become visible. The nuclear membrane breaks down. Spindle fibres form. |
| Metaphase | Chromosomes line up along the equator (middle) of the cell. Spindle fibres attach to the centromeres. |
| Anaphase | Sister chromatids are pulled apart to opposite poles of the cell by the spindle fibres. |
| Telophase | Two new nuclear membranes form around each set of chromosomes. Chromosomes uncoil. |
After telophase, cytokinesis occurs — the cytoplasm splits and two identical daughter cells are formed.
Key Features of Mitosis
- Produces 2 daughter cells
- Daughter cells are genetically identical to each other and to the parent cell
- Daughter cells are diploid (have the full number of chromosomes — 46 in humans)
- Used for growth and repair, not for producing sex cells
Mitosis vs Meiosis
| Feature | Mitosis | Meiosis |
|---|---|---|
| Number of divisions | 1 | 2 |
| Daughter cells produced | 2 | 4 |
| Genetically identical? | Yes | No |
| Chromosome number | Diploid (46) | Haploid (23) |
| Where it happens | Body cells | Reproductive organs |
| Purpose | Growth and repair | Producing gametes |
Cell Growth and Cancer
Normal cells divide in a controlled way. Cancer occurs when cells divide uncontrollably, forming a mass of abnormal cells called a tumour.
- Benign tumours — grow slowly, do not invade other tissues, are not cancerous
- Malignant tumours — grow rapidly, invade neighbouring tissues, and can metastasise (spread to other parts of the body through the blood or lymph)
Risk factors for cancer include:
- Genetic — inherited faulty genes (e.g. BRCA genes for breast cancer)
- Lifestyle — smoking (lung cancer), UV exposure (skin cancer), obesity
- Carcinogens — chemicals or radiation that damage DNA
Exam Tips
- Always state that mitosis produces genetically identical cells — this is a common mark point
- Remember that DNA replication happens in interphase, not during mitosis itself
- If asked why organisms need mitosis, mention both growth and repair