Mitosis vs Meiosis

The body needs two completely different outcomes:

• Growth and repair — new cells must be identical to parent cells
• Reproduction — gametes (egg/sperm) must have HALF the chromosomes so fertilisation restores the full count

Where: All body (somatic) cells — skin, muscle, liver, etc.
Result: 2 daughter cells, identical to parent, same chromosome number (diploid → diploid)
Purpose: Growth, repair, asexual reproduction

Stages of Mitosis

Memory trick: PMAT — Prophase, Metaphase, Anaphase, Telophase

Where: Gonads (testes, ovaries) — germ cells only
Result: 4 daughter cells, genetically unique, half the chromosome number (diploid → haploid)
Purpose: Sexual reproduction, genetic diversity

Meiosis I — Reducing Chromosome Number

• Homologous pairs align together (unique to meiosis)
• Crossing over occurs at chiasmata → genetic recombination (big source of variation)
• Homologous chromosomes separate → 2 cells with n chromosomes each (but still sister chromatids joined)

Meiosis II — Separating Chromatids

• Similar to mitosis
• Sister chromatids separate
• Final result: 4 haploid cells (n)

Meiosis creates variation in two ways:

1. Crossing over — homologous chromosomes exchange segments → new allele combinations
2. Independent assortment — chromosome pairs align randomly → 2²³ = ~8 million possible combinations in humans

This is why siblings are genetically different even with the same parents.

• Meiosis I is the reductional division (chromosome number halves)
• Meiosis II is the equational division (like mitosis — chromatids separate)
• Crossing over = prophase I, at chiasmata (points of crossover)
• Errors in meiosis → aneuploidy: Down syndrome = trisomy 21 (non-disjunction in meiosis I)
• In females, meiosis completes only when egg is fertilised — it is arrested in metaphase II
• Stages in males: primary spermatocyte → secondary spermatocyte → spermatid → sperm