Cell Division: Mitosis & Meiosis
How cells reproduce — from healing a paper cut to creating the next generation
🔑 Big Idea
There are two types of cell division: mitosis, which makes new body cells (somatic cells), and meiosis, which creates egg and sperm cells (gametes). Understanding both is essential for grasping growth, reproduction, genetic diversity, and diseases like cancer.
1. The Cell Cycle
Before a cell divides, it must prepare. Actively dividing eukaryotic cells pass through a series of stages known collectively as the cell cycle:
- G₁ Gap 1 — Cell grows, carries out normal functions, and prepares for DNA replication.
- S Synthesis — DNA is replicated (each chromosome is copied).
- G₂ Gap 2 — Cell continues to grow and prepares proteins needed for division.
- M Mitotic Phase — The cell divides its nucleus (mitosis) and cytoplasm (cytokinesis).
Checkpoints act as quality-control gates. The G₁ checkpoint (restriction point) is a "point of no return" — once passed, the cell is committed to dividing. The G₂ checkpoint verifies DNA replication is complete, and the spindle-assembly checkpoint ensures chromosomes are properly attached before separation.
A typical human cell spends most of its life in interphase (G₁ + S + G₂).