![]() During meiotic prophase I and metaphase I, sister chromatid cohesion along the chromosomal arms is required to maintain bivalents. ![]() Rec8p, a meiosis-specific Scc1p homolog, from both fission and budding yeasts, is essential for normal meiosis ( Molnar et al., 1995 Orr-Weaver, 1999 StoopMyer and Amon, 1999 Watanabe and Nurse, 1999). In budding yeast, sister chromatid cohesion depends on the cohesin complex, which contains at least four proteins: Smc1p, Smc3p, Scc1p and Scd3p ( Michaelis et al., 1997 Nasmyth, 1999). In addition, sister chromatids are closely associated due to sister chromatid cohesion during prophase I ( Klein et al., 1999 Hirano, 2000). In budding yeast, the recA homologs Dmc1p and Rad51p are required for recombination and SC formation ( Bishop et al., 1992 Rockmill et al., 1995 Shinohara et al., 1997). In C.elegans, DSBs also initiate homologous recombination, which is not required for SC formation ( Dernburg et al., 1998). In budding yeast, double-stranded DNA breaks (DSBs) are generated by the Spo11p protein and are required for both recombination and SC formation. Meiotic recombination is thought to begin in late leptotene or zygotene stages after homolog pairing has initiated, and continues in the pachytene stage while the homologs are synapsed. Such interactions include pairing, synapsis and recombination, which are intimately associated with each other. Molecular genetic and biochemical studies in yeasts, Caenorhabditis elegans and Drosophila have provided many of the molecular insights into homolog interactions during prophase I ( Roeder, 1997 Dernburg et al., 1998 Orr-Weaver, 1999 Zickler and Kleckner, 1999). Although the mechanisms may differ, it is universal that homologs are attached until the onset of anaphase I. Therefore, an important outcome of prophase I is the formation of bivalents. Finally, during the diakinesis stage, chromosomes contract lengthwise to produce highly condensed bivalents. During the diplotene stage, homologs become desynapsed along much of their length, but remain attached through chiasmata. The synaptonemal complex (SC) forms at this stage and can be observed in detail using electron microscopy. In the pachytene stage, fully synapsed homologs are observed as thick thread-like structures under a light microscope. Homolog pairing initiates in leptotene and continues extensively in the zygotene stage. The prophase I continuum is divided into five substages based on chromosomal characteristics: first, in the leptotene stage, chromosomes begin to condense and can be seen as thin thread-like structures. During prophase I, chromosomes condense and homologous chromosomes (homologs) pair. Cytological studies indicate that the meiotic prophase I is a long and complex stage that differs from the mitotic prophase (see reviews by Ashley and Plug, 1998 Dawe, 1998 Zickler and Kleckner, 1999). Meiosis is required for eukaryotic sexual reproduction and provides an important mechanism for generating genetic diversity among individuals of a species. These results argue strongly that homolog interactions during meiotic prophase I require a novel meiosis-specific cyclin in Arabidopsis. We further demonstrate that the mutation affects a meiosis-specific gene encoding a novel protein of 578 amino acid residues with up to 31% amino acid sequence identity to known cyclins in the C-terminal portion. We describe here the isolation and characterization of a new Arabidopsis mutant called solo dancers that exhibits a severe defect in homolog synapsis, recombination and bivalent formation in meiotic prophase I, subsequently resulting in seemingly random chromosome distribution and formation of abnormal meiotic products. ![]() However, few regulatory genes have been identified in particular, it is not clear what roles the proteins similar to the mitotic cell cycle regulators might play during meiotic prophase I. Interactions between homologs in meiotic prophase I, such as recombination and synapsis, are critical for proper homolog segregation and involve the coordination of several parallel events.
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