Mitosis & Meiosis
One cell becomes two - or four. How life copies itself, repairs a body, and shuffles the next generation.
How one cell becomes two
Junior level — plain language, no maths
You started as a single cell. Now you're made of tens of trillions - and every one came from that first cell dividing, over and over. This copying is called cell division, and it's happening in you right now: healing a cut, replacing worn-out skin and gut lining, growing your hair and nails. The everyday version, which makes two identical copies, is called mitosis.
The hard part is the instructions. Every cell carries a full set of DNA, packed into chromosomes, and before it divides it must copy every single one perfectly - then share the copies out so each new cell ends up with a complete set, no more and no less. Mitosis is the carefully choreographed dance that lines the copies up and pulls them apart evenly. Get it wrong and cells end up with too many or too few chromosomes.
There's a second kind of division, meiosis, reserved for making eggs and sperm. Instead of two identical cells it makes four, each with half the DNA and shuffled into fresh combinations - which is why you resemble your parents but are a clone of neither. Mitosis builds and repairs a body; meiosis makes the cells that hand life on to the next generation.
Things worth knowing
- You shed and replace tens of thousands of skin cells every minute - all made by mitosis copying existing cells.
- Before dividing, a human cell copies all 2 metres of DNA packed into its 46 chromosomes - with astonishingly few mistakes.
- Meiosis shuffles your parents' genes into new combinations, which is why siblings differ - you're not a copy of either parent.
The phases of mitosis, and how meiosis differs
Student level — the core equations
A dividing cell runs a cycle: it grows and copies its DNA during interphase, then splits in the mitotic phase. Mitosis itself has four choreographed stages. Prophase: the copied chromosomes condense into visible X-shapes - two identical sister chromatids joined at a centromere - and a spindle of microtubules assembles. Metaphase: the chromosomes line up single-file across the cell's equator. Anaphase: the spindle hauls the sister chromatids apart to opposite poles. Telophase: nuclei reform around each set, and the cell pinches in two (cytokinesis).
The outcome is two genetically identical diploid daughter cells, each with the full chromosome count (46 in humans). Checkpoints police the whole process - the cell won't proceed unless its DNA is undamaged and every chromosome is correctly attached. When those brakes fail, cells divide unchecked, which is the essence of cancer.
Meiosis, which makes gametes, runs the machinery twice with a twist. It begins by pairing up homologous chromosomes (one from each parent) and swapping segments between them - crossing over - then separates the homologs in meiosis I and the sister chromatids in meiosis II. The result is four haploid cells, each with half the chromosomes (23) and a unique reshuffle of genes. That shuffling, together with the random assortment of chromosomes, is the engine of genetic variety.
Key formulas
| Mitosis product | \(2n \to 2n + 2n\) | two identical diploid cells |
|---|---|---|
| Meiosis product | \(2n \to n + n + n + n\) | four unique haploid cells |
| Human chromosomes | \(2n = 46,\quad n = 23\) | |
Things worth knowing
- Mitosis produces two genetically identical cells; meiosis produces four genetically unique ones, each with half the DNA.
- Cancer is uncontrolled mitosis: cells that ignore the checkpoints meant to halt damaged or unwanted division.
- Crossing over swaps DNA between paired chromosomes in meiosis, mixing your parents' genes into brand-new combinations.
Chromosome mechanics, the cell cycle, variation and cancer
Scholar level — full mathematical depth
01The cell cycle and its checkpoints
Division is one stage of a cycle: \(G_1 \to S\) (DNA replication) \(\to G_2 \to M\), driven by cyclins and cyclin-dependent kinases. Checkpoints (G1/S, G2/M, and the spindle-assembly checkpoint) halt the cycle until conditions are right, and p53 - "the guardian of the genome" - triggers repair or programmed death when DNA is damaged.
02The spindle and chromosome segregation
A kinetochore on each centromere captures spindle microtubules, and the spindle-assembly checkpoint blocks anaphase until every chromosome is bi-oriented, attached to both poles. Only then is the cohesin holding the sisters together cleaved, and they snap apart. Mistakes here produce aneuploidy - the wrong chromosome number.
03Meiosis and the sources of variation
Meiosis I is reductional - homologs separate, halving the count; meiosis II is equational - sisters separate, as in mitosis. Two independent shufflers create the variety: crossing over recombines homologs, and independent assortment of the 23 pairs alone yields \(2^{23} \approx 8.4\) million combinations per gamete, before recombination is even counted.
04When division goes wrong
Cancer is fundamentally a disease of the cell cycle: mutations in oncogenes (the accelerators) and tumour-suppressors (the brakes, such as p53 and RB) let cells divide without permission and ignore their checkpoints. Aneuploidy from mis-segregation - trisomy 21, for example - shows the steep cost of even a single extra chromosome.
05Beyond the textbook
Cells can also die on purpose (apoptosis), stop dividing for good (senescence, tied to the shortening of telomeres each division), and repurpose the same machinery for regeneration. The balance between dividing and holding back is exactly what keeps a trillion-cell body from either wasting away or running amok.
Key formulas
| The cell cycle | \(G_1 \to S \to G_2 \to M\) | |
|---|---|---|
| Mitosis vs meiosis | \(2n \to 2n\ (\times 2) \quad\text{vs}\quad 2n \to n\ (\times 4)\) | |
| Independent assortment | \(2^{n} = 2^{23} \approx 8.4\times10^{6}\) | combinations per gamete |
Things worth knowing
- Independent assortment alone gives 2²³ ≈ 8.4 million chromosome combinations per human gamete - before crossing over adds even more.
- p53, the "guardian of the genome", halts division to repair DNA or triggers cell suicide. It's mutated in about half of all cancers.
- Each division shortens the telomere caps on your chromosomes; when they run out the cell stops dividing - a built-in limit tied to ageing.