Cell Division (B1.2)
Chromosomes
Human body cells contain 46 chromosomes in 23 homologous pairs. Each chromosome is a long DNA molecule. One of each pair came from each parent at fertilisation. Chromosomes are found in the nucleus of eukaryotic cells.
The cell cycle
The cycle has two main phases:
- Interphase (G1 → S → G2): cell grows, DNA is replicated (copied exactly), extra organelles made.
- Mitosis + cytokinesis: nucleus divides, then cytoplasm divides.
Mitosis — stages
| Stage | Key event |
|---|---|
| Prophase | Chromosomes condense; nuclear envelope breaks down |
| Metaphase | Chromosomes line up at the cell equator |
| Anaphase | Chromatids pulled to opposite poles by spindle fibres |
| Telophase | Two new nuclei form; chromosomes relax |
| Cytokinesis | Cytoplasm divides → 2 daughter cells |
Result: 2 genetically identical daughter cells, each with 46 chromosomes (diploid).
Uses of mitosis: growth, repair/replacement of cells, asexual reproduction.
Cancer
A mutation in genes that control the cell cycle removes normal regulation — cells divide uncontrollably, forming a tumour. Risk factors include ionising radiation, UV light, carcinogens.
Stem cells
- Embryonic stem cells (blastocyst): pluripotent — can form any cell type. Therapeutic potential for Parkinson's, diabetes, spinal cord injury. Ethical concern: destruction of an embryo.
- Adult stem cells (e.g. bone marrow): multipotent — limited to related lineages (blood cells). Used in bone marrow transplants for leukaemia.
- Plant meristems (root/shoot tips): totipotent — can form a whole new plant. Used in tissue culture for rapid cloning.
Common exam errors
- Saying chromosomes are copied during mitosis — they are copied during interphase (S phase).
- Confusing mitosis (2 identical diploid cells) with meiosis (4 haploid cells — covered in B6.1).
- Forgetting that daughter cells are genetically identical to the parent cell.
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