Meiosis

Definition

A specialized, two-stage nuclear division that reduces chromosome number by half, converting a diploid germ cell into four haploid gametes. During prophase I, homologous chromosomes pair and undergo crossing over, exchanging genetic material between paternal and maternal chromatids. The first meiotic division (meiosis I) separates homologous chromosomes; the second (meiosis II) separates sister chromatids. In arthropods, meiosis produces spermatozoa in males and oocytes in females, with fertilization restoring diploidy in the zygote. Variations include achiasmate meiosis in some Drosophila males, where crossing over is absent.

Etymology

From Greek meiosis, 'diminution, lessening', referring to the halving of chromosome number

Example

In female Lepidoptera, meiosis arrests at metaphase I until ovulation, a strategy that synchronizes egg maturation with environmental cues such as host plant availability or temperature.

Synonyms

• reduction division

Related Terms

• Mitosis
• gametogenesis
• Spermatogenesis
• Oogenesis
• Crossing over
• synapsis
• haploid
• Diploid
• Parthenogenesis
• Polyploidy

Usage Notes

Distinguished from mitosis by the pairing of homologous chromosomes and two successive divisions yielding haploid products. In entomology, meiotic mechanisms vary widely: some insects (e.g., certain Hymenoptera) exhibit arrhenotoky, where unfertilized haploid eggs develop into males via mitotic rather than meiotic development. The term is sometimes misapplied to describe any cell division; specialists reserve it for the reductional division preceding gamete formation. Arthropod cytogeneticists often specify 'meiosis I' or 'meiosis II' when discussing chiasma frequency or segregation patterns.