Watanabe Y, Yokobayashi S, Yamamoto M, Nurse P
Department of Biophysics and Biochemistry, Graduate School of Science, University of Tokyo, Hongo, Japan.
Nature. 2001 Jan 18;409(6818):359-63. doi: 10.1038/35053103.
Meiosis is initiated from G1 of the cell cycle and is characterized by a pre-meiotic S phase followed by two successive nuclear divisions. The first of these, meiosis I, differs from mitosis in having a reductional pattern of chromosome segregation. Here we show that meiosis can be initiated from G2 in fission yeast cells by ectopically activating the meiosis-inducing network. The subsequent meiosis I occurs without a pre-meiotic S phase and with decreased recombination, and exhibits a mitotic pattern of equational chromosome segregation. The subsequent meiosis II results in random chromosome segregation. This behaviour is similar to that observed in cells lacking the meiotic cohesin Rec8 (refs 3, 4), which becomes associated with chromosomes at G1/S phase, including the inner centromere, a region that is probably critical for sister-centromere orientation. If the expression of Rec8 is delayed to S phase/G2, then the centromeres behave equationally. We propose that the presence of Rec8 in chromatin is required at the pre-meiotic S phase to construct centromeres that behave reductionally and chromosome arms capable of a high level of recombination, and that this explains why meiosis is initiated from G1 of the cell cycle.
减数分裂从细胞周期的G1期开始，其特征是减数分裂前的S期，随后是两次连续的核分裂。其中第一次，减数分裂I，与有丝分裂的不同之处在于其染色体分离模式为减数型。我们在此表明，在裂殖酵母细胞中，通过异位激活减数分裂诱导网络，减数分裂可以从G2期开始。随后的减数分裂I在没有减数分裂前S期的情况下发生，且重组减少，并表现出染色体均等分离的有丝分裂模式。随后的减数分裂II导致染色体随机分离。这种行为类似于在缺乏减数分裂黏连蛋白Rec8（参考文献3、4）的细胞中观察到的情况，Rec8在G1/S期与染色体结合，包括内着丝粒，这一区域可能对姐妹着丝粒定向至关重要。如果Rec8的表达延迟到S期/G2期，那么着丝粒就会表现出均等行为。我们提出，在减数分裂前的S期，染色质中需要有Rec8来构建表现为减数型的着丝粒和能够进行高水平重组的染色体臂，这就解释了为什么减数分裂从细胞周期的G1期开始。
