Meiosis and Germ Cell Division

Meiosis is the specialised cell division that produces haploid gametes from diploid germ-cell precursors. A single round of DNA replication is followed by two successive divisions, halving the chromosome number and, through recombination and independent assortment, generating genetic diversity among the resulting eggs and sperm.

Definition

Meiosis is a form of germ-cell division in which one round of DNA replication is followed by two successive nuclear divisions, producing four haploid cells from one diploid precursor; homologous chromosomes pair and recombine and then segregate in meiosis I, while sister chromatids separate in meiosis II.

Scope

The entry describes the two meiotic divisions (meiosis I and II), the pairing and recombination of homologous chromosomes, the reductional segregation that halves ploidy, the role of meiosis in gametogenesis, and how errors in chromosome segregation give rise to aneuploidy. It treats meiosis as a cell-biology and germ-cell topic, not as clinical guidance.

Key concepts

• Reductional division (meiosis I) and equational division (meiosis II)
• Homologous chromosome pairing (synapsis)
• Crossing over and recombination
• Chiasmata and the synaptonemal complex
• Independent assortment
• Haploid gametes and fertilisation
• Aneuploidy and nondisjunction

Mechanisms

After a single round of DNA replication, meiosis carries out two divisions. In meiosis I, homologous chromosomes pair (synapsis), exchange segments by crossing over to form chiasmata, and are then segregated to opposite poles — a reductional division that halves the chromosome number. A defining molecular feature, reviewed by Petronczki and colleagues, is that sister kinetochores are oriented to the same pole (mono-orientation) and that cohesion along chromosome arms is released in meiosis I while centromeric cohesion is protected until meiosis II, when sister chromatids finally separate. The two divisions thus distribute one chromatid of each homolog into each of four haploid products. Errors in this choreography — nondisjunction — produce gametes with the wrong chromosome number; Hassold and Hunt review how such meiotic errors are the leading source of human aneuploidy.

Clinical relevance

Meiosis is the cellular basis of gamete formation, and meiotic nondisjunction is described as the origin of aneuploidies whose frequency rises with maternal age. This entry is a reference description of germ-cell division and the cellular genesis of aneuploidy; it does not provide diagnostic or reproductive medical advice.

History

Meiosis was recognised in the late nineteenth century as the chromosome-halving division required for sexual reproduction, complementing Weismann's ideas on the germ line. Twentieth-century cytogenetics linked meiotic recombination and nondisjunction to inheritance and to chromosomal disorders, and molecular work in model organisms — synthesised by Petronczki and colleagues — later explained how cohesion and kinetochore orientation enforce the two-step reductional programme.

Key figures

• Kim Nasmyth
• Terry Hassold
• Patricia Hunt

Seminal works

• petronczki-2003
• hassold-hunt-2001

Frequently asked questions

How does meiosis differ from mitosis?

Mitosis is a single division producing two diploid cells genetically identical to the parent, whereas meiosis comprises two divisions after one replication, producing four haploid cells that differ genetically because of recombination and independent assortment.

Why does meiosis involve two divisions?

The first division (meiosis I) separates paired homologous chromosomes to halve the chromosome number, and the second (meiosis II) separates sister chromatids much as mitosis does; together they convert one diploid cell into four haploid gametes.