Knowledge Organiser: Mitosis and the Cell Cycle
Key Definitions
- Mitosis: Division producing 2 identical daughter cells
- Cell cycle: Interphase + mitosis + cytokinesis
- Interphase: ~90% of cycle; growth and DNA replication
- Chromatid: One of two identical strands of a duplicated chromosome
- Centromere: Where two chromatids are joined
Stages of Mitosis (PMAT)
- Prophase: Chromosomes condense; nuclear envelope breaks down
- Metaphase: Chromosomes line up at the equator
- Anaphase: Chromatids pulled to opposite poles
- Telophase: Two new nuclei form
- Cytokinesis: Cytoplasm divides → two daughter cells
Why Mitosis?
- Growth — increase in cell number in multicellular organisms
- Repair and replacement of damaged or worn-out cells (e.g. skin, gut lining)
- Asexual reproduction (e.g. in bacteria, yeast, plants via runners)
- Grade 7+: all daughter cells are genetically identical to the parent cell and to each other — this is essential for maintaining the organism's genetic information in every body cell
Mitosis vs Meiosis
- Mitosis → 2 genetically identical daughter cells, both diploid (same chromosome number as parent)
- Meiosis → 4 genetically different daughter cells, all haploid (half the chromosome number)
- Mitosis = growth and repair of body cells; Meiosis = production of gametes (sex cells)
- Human body cells: 46 chromosomes (diploid, 2n); human gametes: 23 chromosomes (haploid, n)
Calculation
- Number of cells after n divisions = 2ⁿ
- Number of divisions = total time ÷ time per division
- Example: after 4 divisions, 2⁴ = 16 cells
Memory Aid
- IPMAT: Interphase, Prophase, Metaphase, Anaphase, Telophase
- "I Picked My Apples Today"
- M = chromosomes line up at the Middle; A = chromatids pulled Apart to opposite poles; T = Two new nuclei form
Common Mistakes
- Writing "chromosomes are pulled apart" in anaphase: In anaphase it is the chromatids (sister chromatids) that are pulled to opposite poles — not whole chromosomes. Each chromatid becomes a chromosome in the new cell. This distinction is a mark-scheme requirement.
- Confusing mitosis with meiosis: Mitosis produces 2 genetically identical diploid cells for growth and repair. Meiosis produces 4 genetically different haploid cells for gamete production. Writing "mitosis produces haploid cells" is a common and penalised error.
- Forgetting DNA replication before division: DNA is replicated during interphase, before mitosis begins. If asked what must happen before a cell can divide, always state: "DNA is copied so each daughter cell receives a full copy of the genetic information."
- Stating daughter cells have half the chromosomes: After mitosis, each daughter cell is diploid and has the same chromosome number as the parent cell (46 in humans). It is meiosis that halves the chromosome number.
- Saying chromosomes "appear" in prophase: Chromosomes exist as chromatin throughout the cell cycle. In prophase they condense and become visible under a microscope — they do not appear from nowhere.