A physical connection between homologs is required for reductional segregation at the first division of meiosis. This connection is usually provided by one or a few well-spaced crossovers. A speculative overview of processes leading to formation of these crossovers is presented.Meiosis is essential for sexual reproduction. During gamete formation the cellular chromosome complement is reduced by half; union of two gametes to form a zygote thus restores the normal chromosome complement rather than doubling it. Halving of the chromosome complement is accomplished during meiosis because a single round of DNA replication is followed by two successive rounds of chromosome segregation.A diploid cell contains two versions of each chromosome, "homologs." In a mitotic cell cycle, DNA replication and chromosome compaction are followed by equational segregation of sister chromatids, and the starting genetic situation is restored ( Fig. 1). In meiosis, chromosomes replicate and compact, then at the first division, homologs move to opposite poles ("reductional segregation"); and at the second division, sister chromatids move to opposite poles as during mitosis. Meiosis thus yields four haploid products (Fig. 1).During mitosis, connected sister chromatids are aligned between two poles via sets of microtubules that exert opposing poleward forces; when sister chromatid connections lapse, sisters move to opposite poles. Given a physical connection between homologs and suitably modified kinetochores, the same processes are used for alignment and segregation of homologs to opposite poles at meiosis I (1).In most organisms, the requisite physical connection is provided by crossover(s), in combination with connections between sister chromatids along their lengths. In condensed late prophase chromosomes, the interstitial interhomolog connections corresponding to crossovers are seen cytologically as "chiasmata" (refs. 2 and 3; see Fig. 6D).At anaphase of meiosis I, the physical connection between homologs is abrogated by dissolution of connections between sister chromatid arms. Sister connections at the centromere remain; each sister pair is thereby aligned on the meiosis II spindle. When centric connections lapse at anaphase II, individual chromatids segregate.Over the past decade, many laboratories have provided new cytological, genetic and molecular insights into the meiotic process. This article attempts to integrate new and old information in a framework provided by temporal analysis of meiotic prophase chromosome metabolism in yeast (Fig. 2). Events of Meiotic Prophase Intersister connections are established very early, likely during S phase, as in mitotic cells (10)(11)(12)(13)98 (11,14,15). One sliver of this axis is the silver-staining "axial element." Ultimately, the chromatin loops of both sisters come to lie on the same side of this axis (11).A conjoined sister/sister axis and asymmetric positioning of chromatin loops with respect to that axis are both unique to meiosis. In mitotic cells, sister chromatid axes are d...