Meiotic chromosomes: it takes two to tango.

Meiosis is a special type of cell division that produces haploid gametes from diploid parental cells. Chromosome number is reduced during meiosis because a single round of DNA replication is followed by two rounds of chromosome segregation (Fig. 1). Fusion of two gametes during sexual reproduction restores the diploid chromosome complement. The second division of meiosis (the equational division) resembles mitosis: Sister chromatids separate and segregate. The first division, however, is unique. Reductional chromosome segregation at meiosis I differs from mitosis and meiosis II in a number of respects. First, sister chromatids remain associated with each other. Second, the two copies of the same chromosome (called homologous chromosomes or homologs) behave in a coordinate fashion, such that one chromosome moves to one pole of the spindle apparatus and its homolog moves to the opposite pole. This coordination in chromosome behavior depends on complex processes and elaborate structures that bring homologs together during meiotic prophase and hold them together until the transition between metaphase I and anaphase I. In most organisms, the relevant processes include alignment of homologs, assembly of the synaptonemal complex (SC, described below), genetic recombination, and the formation of chiasmata (stable connections between homologs formed at the sites of crossovers). These events occur during a very lengthy prophase that is divided into a series of substages based on changes in chromosome morphology (Table 1). This article reviews our current knowledge of the processes and structures that promote and maintain interactions between homologs and thereby ensure proper reductional chromosome segregation. Emphasis is placed on observations made in recent years, particularly those that have enhanced our understanding of the molecular mechanisms underlying meiotic chromosome behavior.