Why cells divide
Cells divide for growth, repair and, in many organisms, reproduction. Before dividing a cell must copy its DNA and then share it out accurately.
The cell cycle
The cycle has two big parts:
- Interphase (most of the cycle): G1 (growth), S (DNA is replicated, so each chromosome becomes two identical sister chromatids), G2 (preparing to divide).
- M phase: mitosis followed by cytokinesis, the division of the cytoplasm.
Checkpoints make sure DNA is undamaged and complete. Cancer is uncontrolled division, often because mutations disable these controls.
Mitosis: PMAT
- Prophase: chromosomes condense and the nuclear envelope breaks down.
- Metaphase: chromosomes line up along the cell's equator.
- Anaphase: sister chromatids are pulled apart toward opposite poles.
- Telophase: nuclei re-form; cytokinesis follows.
Result: two genetically identical diploid cells. In humans a diploid (2n) cell has 46 chromosomes.
Meiosis
Meiosis makes gametes (sperm and eggs) using two divisions after one round of DNA replication.
- Meiosis I: homologous chromosome pairs are separated. Before this, in prophase I, homologs pair up and swap segments by crossing over, creating new combinations of alleles. The number of chromosomes is halved.
- Meiosis II: sister chromatids separate, much like mitosis.
Result: four genetically different haploid (n) cells. Human gametes have 23 chromosomes; fertilisation restores 46.
Variation arises in two ways: crossing over and independent assortment, the random way homologous pairs line up in metaphase I.
Comparison
| Mitosis | Meiosis | |
|---|---|---|
| Divisions | 1 | 2 |
| Cells produced | 2 | 4 |
| Chromosome number | stays 2n | halves to n |
| Genetic result | identical | varied |
| Purpose | growth, repair | gametes |
Worked example
A cell with 2n = 8 chromosomes undergoes mitosis and another undergoes meiosis. How many chromosomes are in the daughter cells?
- Mitosis: each of the two daughter cells has 8 chromosomes (same as the parent).
- Meiosis: each of the four daughter cells has 4 chromosomes (n = 4).
- Counting chromatids: after S phase the 8 chromosomes each have two chromatids, so the cell has 16 chromatids but still 8 chromosomes. A chromosome is counted by its centromere, not its chromatids.
Errors
If chromosomes fail to separate (nondisjunction) a gamete may have an extra or missing chromosome. Trisomy 21, with three copies of chromosome 21, causes Down syndrome.