Cell Division Guide

What should I know about Mitosis: Identical Cell Copies?

Mitosis produces two genetically identical daughter cells from one parent cell. Each division doubles the cell count: n divisions produce 2ⁿ cells from a single starting cell. Starting from 1 cell: 10 divisions = 1,024 cells. 20 divisions = 1,048,576 cells (~1 million). 37 divisions ≈ 1 × 10¹¹ cells — approximately the number of cells in the human body (starting from a single fertilised egg). DNA content: G1 phase: 2C (diploid DNA content). S phase: replicating, 2C→4C. G2 phase: 4C. After division completes, each daughter cell returns to 2C, ready to begin the cycle again — this precise halving back to the starting DNA content after doubling during S phase is what keeps chromosome number constant across successive mitotic divisions.

What's the key thing to understand about Cell Cycle Phases?

The cell cycle has two major stages: Interphase (approximately 90% of cycle time): G1 (Gap 1) — cell grows, synthesises proteins, 8-11 hours. S (Synthesis) — DNA replication, 6-8 hours. G2 (Gap 2) — cell prepares for division, checks DNA, 4-5 hours. M phase (Mitosis + Cytokinesis) approximately 1-2 hours: Prophase, Metaphase, Anaphase, Telophase, then Cytokinesis. G0 (quiescent): cells that have exited the cycle (e.g. neurons, muscle cells). Cell cycle checkpoints: G1/S checkpoint — checks cell size, nutrient availability and DNA damage before allowing entry into S phase. G2/M checkpoint — verifies DNA replication completed correctly before mitosis begins. Spindle assembly checkpoint (metaphase) — ensures all chromosomes are correctly attached to spindle fibres before anaphase proceeds, preventing unequal division of genetic material.

What's the difference between Mitosis and Meiosis?

Mitosis: one division, two daughter cells, genetically identical to parent (except mutation), diploid→diploid. Used for: growth, repair, asexual reproduction. Meiosis: two divisions, four daughter cells (gametes), genetically different from parent and each other, diploid→haploid. Used for: sexual reproduction. Key differences: meiosis I separates homologous chromosomes (crossing over occurs in prophase I). Meiosis II separates sister chromatids (like mitosis). Crossing over (recombination): homologous chromosomes exchange segments of DNA during prophase I, creating new combinations of alleles on each chromosome — this, combined with the random orientation of homologous pairs at metaphase I (independent assortment), is why meiosis generates genetically unique gametes rather than exact copies.

What should I know about Cancer and Cell Division?

Cancer is fundamentally a disease of uncontrolled cell division — mutations in cell cycle regulatory genes allow cells to divide without normal controls. Proto-oncogenes (normal): promote cell division when needed (e.g. growth factor receptors). Become oncogenes when mutated — permanently active, drive constant division. Tumour suppressor genes (normal): brake the cell cycle (e.g. p53, Rb). When mutated and inactive, division is not stopped when it should be. p53: the most commonly mutated gene in human cancers, found mutated in over half of all tumour types. When both tumour suppressor genes and proto-oncogenes accumulate mutations in the same cell line, the combination of a stuck-on accelerator (oncogene) and a failed brake (tumour suppressor) is what drives uncontrolled tumour growth.

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