Meiosis
மியாசிஸ்
Do not confuse Meiosis with Mitosis. Mitosis maintains ploidy; Meiosis halves it. If your germ cells failed this reduction, every generation would double its chromosome count, leading to immediate genetic collapse.
The idea
Meiosis is a specialized cell division reducing chromosome number by half, producing four haploid daughter cells. It involves two sequential nuclear divisions, Meiosis I and Meiosis II, but only a single cycle of DNA replication during the S-phase. This ensures genetic continuity across generations while promoting variation through recombination.
Why it matters
It ensures the production of haploid gametes for sexual reproduction, allowing the restoration of the diploid phase upon fertilization.
NEET lens
NEET focuses heavily on the phases of Prophase I: Leptotene, Zygotene, Pachytene, Diplotene, and Diakinesis. Memorize the specific event of each stage, especially crossing over in Pachytene. Expect questions on the comparison between Meiosis I and II; remember that Meiosis II is essentially equational division, similar to Mitosis.
Clinical significance
Nondisjunction during Meiosis I or II leads to aneuploidy. Trisomy 21, or Down Syndrome, is the direct clinical result of this failure. When homologous chromosomes or sister chromatids fail to separate, the resulting gametes carry an abnormal chromosome count.
Introduction
Meiosis is a two-division cell cycle (Meiosis I + Meiosis II) that reduces chromosome number by half (2n → n), producing 4 haploid cells from one diploid parent cell with genetic recombination.
Core concept
Meiosis I (reductive division): Prophase I (LZPDD: Leptotene→Zygotene→Pachytene→Diplotene→Diakinesis; synapsis and crossing over occur), Metaphase I (bivalents align), Anaphase I (homologs separate), Telophase I → 2 haploid cells. Meiosis II (equational division, like mitosis): sister chromatids separate → 4 haploid cells. Key events: crossing over (Prophase I), independent assortment (Metaphase I).
Go deeper
Meiosis I is REDUCTIVE: 2n→n because homologous chromosomes separate. Meiosis II is EQUATIONAL: n→n because sister chromatids separate (chromosome number unchanged). Together they generate 4 haploid genetically unique cells.
From Class 10
Mitosis makes identical cells for growth. Meiosis is different — it makes sex cells (gametes) with half the chromosome number, enabling sexual reproduction.