Biology 101 · Chapter 5
Cell division
Try this first
A single fertilised egg becomes a body of roughly 37 trillion cells — and nearly every one carries an identical copy of the same 3-billion-letter manual. Copy a book that many times and even a microscopic error rate would be a disaster. So how does one cell split into two without either half losing, scrambling, or shorting its instructions?
A cut on your skin closes over because the cells at its edge start dividing, each one making a faithful copy of itself to fill the gap. You replace hundreds of billions of cells a day this way — gut lining, blood, skin — every division handing down a complete, correct genome. Division isn't tearing in half; it's copy, then split, in that strict order, with checks along the way.
The one idea
Cell division is controlled copying. First the cell duplicates all its DNA (replication — the strand-unzipping from Chapter 2), then it partitions one full copy into each of two daughters. The cell cycle is the ordered, checkpointed program that makes sure copy-then-split happens in the right sequence, exactly once.
The cell cycle: a gated loop
A dividing cell runs a repeating four-phase program. Most of its life is interphase — split into G1 (grow and work), S (synthesis: copy the DNA), and G2 (grow more, double-check the copy) — and only a brief finale is M (mitosis), the actual split. The S phase is where Chapter 2's unzip-and-template trick runs across all 3 billion letters.
Mitosis: halving the copy evenly
By the time mitosis begins, every chromosome has already been copied in S phase, so it exists as two identical sister chromatids joined at the middle (the familiar X shape). Mitosis is the careful business of pulling one chromatid of each pair to opposite ends, so both daughters get one complete set. The cell condenses its chromosomes, lines them up across the middle, hauls the sisters apart on protein cables, then pinches into two.
| Phase | What happens | ~Duration |
|---|---|---|
| G1 | Cell grows, does its everyday job | ~11 h |
| S | DNA replicated — every chromosome copied | ~8 h |
| G2 | Grows more; checks the copy for errors | ~4 h |
| M | Mitosis: chromosomes split, cell pinches in two | ~1 h |
Why the checkpoints matter
The cycle isn't a free-running clock; it's gated. At each checkpoint the cell asks: is it big enough? Is the DNA undamaged? Was every chromosome copied exactly once and lined up correctly? Only a "yes" lets it proceed. These brakes are the difference between healthy growth and catastrophe: cancer is, at its core, cells whose checkpoints have failed, dividing when they shouldn't. (A second kind of division, meiosis, instead halves the chromosome count to make sperm and eggs and shuffles the genes — the engine of variation we'll pick up when the track reaches genetics and evolution.)
Work one, then finish one
Worked: A human body cell has 46 chromosomes. In S phase the DNA is copied, so each of the 46 now consists of two sister chromatids — twice the DNA, but still 46 chromosomes. Mitosis sends one sister of each to each daughter. So each daughter ends with 46 chromosomes — identical to the parent, not half. Mitosis conserves the count.
Your turn: A cell with 8 chromosomes divides by mitosis. How many does each daughter cell have? (Answer: 8. By meiosis, instead, it would be 4 — half.)
Why this earns a place in your toolkit
The cell cycle is a textbook control system: a state machine whose transitions only fire when validation passes — biology's version of committing a transaction only after every check succeeds, with cancer as the failure mode when the guards are bypassed. That framing drives real modelling: tumour-growth dynamics, and machine-vision tools that score dividing cells in pathology slides. The repeated doubling (1 → 2 → 4 → 8 …) is exponential growth in the flesh, the same curve behind compounding and runaway processes. And development — one cell's descendants self-organising into a whole body — is a north star for generative and self-organising systems, from cellular automata to morphogenesis.
Recall check · no peeking
- In what order must a cell copy and split its DNA — and why does the order matter?
- What happens in S phase? What happens in M phase?
- Why does a cell have checkpoints, and what goes wrong when they fail?
- A human cell starts with 46 chromosomes; how many does each daughter have after mitosis?
- In one line, how does meiosis differ from mitosis?
Explain it back
In one plain sentence, tell a friend why a cell must copy all of its DNA before it divides, never after.