However, if either of these cyclins is expressed prematurely, the spindle microtubules are assembled too early and, as a result, sister chromatids are segregated rather than chromosomes in a significant fraction (∼40%) of the cells (see Figure 2). Two of the cyclins that have a role in driving the cells through meiosis, Clb1 and Clb3, are transcribed at the end of prophase ( Dahmann and Futcher, 1995 Chu et al., 1998 Carlile and Amon, 2008). When the enzyme cyclin-dependent kinase (Cdk) is bound to a cyclin, it drives cell cycle events by phosphorylating substrates ( Enserink and Kolodner, 2010). The work of Miller and Ünal, who are joint first authors on the paper, and their co-workers reveals another level of regulation of meiosis I that involves proteins called M phase cyclins. Together these three mechanisms ensure that homologous chromosomes are segregated in meiosis I, while sister chromatids remain together. When the two homologous chromosomes are attached to opposite spindle poles during metaphase I (a stage after prophase), the spindle forces are resisted by the physical linkages and the cohesion between sister chromatids. Third, protein rings made up of cohesins are thought to encircle the two sister chromatids: this creates cohesion between the chromatids and prevents them from separating prematurely during meiosis I. Second, the sister kinetochores offer only one site for microtubules to bind to: in budding yeast, for example, a protein complex called monopolin clamps the sister kinetochores together just before microtubule attachment begins. This process creates physical links that hold the homologs together, and ensures their attachment to opposite spindle poles ( Brar and Amon, 2008). First, during prophase, which is the first stage of meiosis I, pairs of homologous chromosomes undergo recombination. Three mechanisms help ensure the proper attachment of chromosomes to spindle microtubules in meiosis I. The MIT-UCSF team used budding yeast as a model to study these processes and interactions in greater detail. In meiosis II essentially the same cast of players (that is, spindle microtubules and kinetochores), segregate the sister chromatids to produce a total of four cells. In meiosis I, the paired chromosomes segregate to opposite ends (or poles) of the spindle. In addition to attaching the microtubules to the chromosomes, the kinetochores also correct improper attachments and move the chromosomes along microtubules. During metaphase II, sister chromatids attach to opposite spindle poles and separate in anaphase II, creating meiotic products with half the set of chromosomes.ĭuring the first round of segregation, called meiosis I, spindle microtubules attach themselves to the chromosomes with the help of large protein complexes called kinetochores that are found on each chromatid (see Figure 1). Homologous chromosomes then segregate during anaphase I. Next, during metaphase I, the sister kinetochores (black circles) are clamped together by a protein complex called monopolin, and the spindle microtubules (purple) attach homologous chromosomes to spindle poles (also purple) at opposite ends of the cell. During the next stage of meiosis, called Prophase I, chromosomes with similar sequences form pairs and undergo recombination, creating physical links that hold the homologs together. ![]() These chromosomes are replicated to produce sister chromatids that are held together by cohesins (grey circles around the sister chromatids). ![]() In this illustration the cell has two chromosomes (shown here in yellow and blue in the leftmost cell) before meiosis starts.
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