How cells copy themselves through mitosis and meiosis
The cell cycle has interphase, when the cell grows and copies its DNA, followed by the M phase, when it divides. Interphase includes G₁ (growth), S (DNA synthesis) and G₂ (preparation for division). Cells that stop dividing enter a resting G₀ stage. Mitosis produces two genetically identical daughter cells and has four stages: prophase, when chromosomes condense; metaphase, when they line up at the equator; anaphase, when sister chromatids separate; and telophase, when new nuclei form. Cytokinesis then divides the cytoplasm, by furrowing in animal cells and by a cell plate in plant cells.
Meiosis occurs in reproductive cells and halves the chromosome number, producing four haploid cells from one diploid cell. In prophase I, homologous chromosomes pair up in synapsis and exchange segments through crossing over at chiasmata, a key source of genetic variation. Meiosis I separates homologous chromosomes, and meiosis II separates sister chromatids, much like mitosis. Independent assortment of chromosomes adds further variation.
Why it matters
Mitosis allows growth, healing of wounds and replacement of worn-out cells, and uncontrolled mitosis causes cancer. Meiosis keeps chromosome numbers constant across generations and creates the variation that makes each person unique. Errors in meiosis can cause conditions such as Down’s syndrome. This topic links cell biology with genetics and evolution.
Key terms
- Interphase
- The phase of the cell cycle in which the cell grows and replicates its DNA.
- Mitosis
- Cell division that produces two daughter cells genetically identical to the parent.
- Meiosis
- Cell division that halves the chromosome number to form gametes.
- Crossing over
- The exchange of segments between homologous chromosomes during prophase I of meiosis.