Both mitosis and meiosis are ways a eukaryotic cell divides its nucleus. Before either one starts, the cell copies its DNA during the S phase of interphase, so each chromosome is made of two identical sister chromatids.
Mitosis: copies for growth and repair
- Purpose: growth, replacing worn-out cells, healing, and asexual reproduction.
- Divisions: one.
- Result: two daughter cells that are genetically identical to the parent and to each other.
- Chromosome number: stays the same. A human body cell (2n = 46) produces two cells with 46 chromosomes each.
The phases, in order: prophase (chromosomes condense, spindle forms), metaphase (chromosomes line up at the middle), anaphase (sister chromatids are pulled to opposite poles), and telophase (new nuclei form). Cytokinesis then splits the cytoplasm. The memory aid is PMAT.
Meiosis: gametes with half the chromosomes
- Purpose: making gametes (sperm and egg cells) for sexual reproduction.
- Divisions: two, meiosis I and meiosis II.
- Result: four cells that are genetically different from each other.
- Chromosome number: halved. Human gametes are haploid (n = 23), so fertilization restores 46.
Where the genetic variety comes from
- Crossing over in prophase I: homologous chromosomes swap segments.
- Independent assortment in metaphase I: each homologous pair lines up independently. With 23 pairs, one person can make 2²³, or about 8.4 million, different chromosome combinations, before counting crossing over.
The key difference in anaphase
In anaphase I of meiosis, homologous chromosomes separate and the sister chromatids stay together. In anaphase II and in mitotic anaphase, the sister chromatids separate.
Quick comparison
- Mitosis: 1 division, 2 cells, identical, diploid to diploid, body cells.
- Meiosis: 2 divisions, 4 cells, varied, diploid to haploid, gametes.