Meiosis vs. Mitosis

Mitosis is an equational division where a diploid parent cell produces two identical diploid daughter cells, primarily for growth and development. Meiosis is a reductional division where a diploid germ cell divides into four genetically distinct haploid daughter cells, essential for sexual reproduction. The two processes differ in chromosome numbers, genetic recombination, and daughter cell identity.

Prophase I is the longest meiotic phase because it consists of five substages during which homologous chromosomes pair to form a synaptonemal complex and undergo recombination, producing four genetically diverse chromatids. In contrast, mitotic prophase is shorter and does not involve homologous chromosome pairing or recombination, resulting in genetically identical daughter cells.

In mitosis, cohesin complexes holding sister chromatids are completely removed by metaphase, allowing chromatids to separate and move to opposite poles. In meiosis I, only cohesin complexes on chromosome arms detach at anaphase I, while those at the kinetochore remain intact, keeping sister chromatids attached as homologous chromosomes separate.

Before mitosis and meiosis I, cells synthesize DNA to create two homologous copies of each chromosome. High cyclin-dependent kinase activity prevents DNA replication during these divisions by blocking pre-replicative complex formation. No DNA synthesis occurs before meiosis II because CDK activity remains elevated throughout both meiotic divisions.

In mitosis, accidental double-strand breaks are repaired by homologous recombination. In meiosis, double-strand breaks are intentionally created at selected chromosome sites by the endonuclease Spo11 as part of meiosis and crossing over. These programmed breaks are essential for genetic diversity, whereas mitotic breaks are random damage requiring repair.

Female meiosis is more error-prone due to several factors. A single oocyte meiosis cycle can take up to 40 years, causing cohesin deterioration and premature sister chromatid release. Chiasmata holding homologous chromosomes can slip with time, and aging decreases spindle-assembly checkpoint proteins like Mad2, increasing chromosome segregation errors.

Chromosome segregation errors produce aneuploid cells with incorrect chromosome numbers, leading to genetic diseases like Down syndrome and Turner syndrome. These errors are more likely in meiosis than mitosis. Maternal age significantly increases risk; women under 30 have less than 0.1% chance of Down syndrome, while women at 45 have a 3.5% chance.