Meiosis is a specialized type of cell division that reduces the chromosome number by half, producing four genetically different haploid daughter cells from a single diploid cell. It is also called reductional division and occurs in reproductive cells.
Meiosis consists of two sequential divisions: meiosis I (reductional division) and meiosis II (equational division). Unlike mitosis, meiosis involves two rounds of division but only one round of DNA replication.
Meiosis I is the actual reductional division where chromosome number is halved. It has four stages:
Prophase I is the longest and most complex phase, divided into five sub-stages: leptotene (chromosomes condense), zygotene (homologous chromosomes pair up — synapsis), pachytene (crossing over occurs between non-sister chromatids of homologous chromosomes, forming chiasmata), diplotene (homologous chromosomes begin to separate but remain attached at chiasmata), and diakinesis (chiasmata terminalize, nuclear membrane disappears).
During metaphase I, bivalents (paired homologous chromosomes) align at the equator. In anaphase I, homologous chromosomes separate (not sister chromatids), reducing the chromosome number to half. Telophase I completes the first division.
Meiosis II is similar to mitosis — sister chromatids separate. The four stages (prophase II, metaphase II, anaphase II, telophase II) result in four haploid cells.
Crossing over during pachytene of prophase I is crucial for genetic variation. It involves the exchange of genetic material between non-sister chromatids of homologous chromosomes, creating new combinations of alleles.
Meiosis is significant because it: maintains the chromosome number constant across generations in sexually reproducing organisms, introduces genetic variation through crossing over and independent assortment, and produces haploid gametes that fuse during fertilization to restore the diploid number.
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