The nuclear envelope virtually breaks down by increasing its permeability during both mitosis and meiosis in S. pombe as well (Asakawa et al., 2010, 2011). These studies indicate that an intermediate type of cell division takes place in fission yeasts. The microtubule organizing centres (MTOCs) remain outside the nuclear envelope throughout the cell cycle in metazoans. This allows KT–MT interaction to take place only during mitosis. A metazoan KT is typically associated with multiple MTs (20–30 in humans; McDonald et al., 1992).

MTOCs, known as the spindle pole bodies (SPBs) in yeast, remain attached to the nuclear envelope throughout the cell cycle in Saccharomyces PLX3397 concentration cerevisiae (Byers & Goetsch, 1975). In addition, KTs remain attached to the MTs throughout the cell cycle in this organism. However, a temporary detachment of chromosomes from MTs occurs (for 1–2 min) at the time of CEN DNA replication in S phase in S. cerevisiae (Kitamura et al., 2007). After completion of CEN replication, a KT reassembles and reestablishes its

attachment with MTs. Subsequently, sister CENs precociously separate from each other (Goshima & Yanagida, 2000; Jaspersen & Winey, 2004; Sazer, 2005; Kitamura et al., 2007; Tanaka & Tanaka, 2009). This transient detachment between KT and MT may Olaparib be specific in organisms (such as S. cerevisiae and C. albicans) in which only one MT interacts with a KT (Ding et al., 1993; Joglekar et al., 2008; Thakur & Sanyal, 2011). Interestingly, SPBs in S. pombe remain outside the nuclear envelope during interphase. Following mitotic initiation, the duplicated SPBs penetrate the nuclear membrane (Ding et al., 1997). The

KT is associated with 2–3 MTs in fission yeast (Winey et al., 1995). Therefore, budding yeasts, fission yeasts and metazoans exhibit obvious divergence in timing of commencement 4-Aminobutyrate aminotransferase of KT–MT interaction, the number of MTs associated with a KT and the fate of the nuclear membrane during the cell cycle. Early microscopy of mitotic chromosomes in human cells revealed that the human KT is a tri-layered structure (Brinkley & Stubblefield, 1966; McEwen et al., 2007): inner and outer layers that are bridged by a middle layer. Proteins that form the inner layer interact directly with the CEN DNA, while the outer layer proteins form the chromosomal attachment sites of the MT plus ends. Proteins in the middle layer act as linkers between the inner and outer KT (Cheeseman & Desai, 2008). However, in unicellular organisms like yeasts, the structure of a KT cannot be ascertained due to the small-sized cells. Immunostaining of a KT protein in these organisms appears as a single conspicuous focus of clustered KTs at nuclear peripheral regions and close to spindle pole bodies (Meluh et al., 1998; Takahashi et al., 2000; Sanyal & Carbon, 2002). All KTs remain clustered together throughout the cell cycle in S. cerevisiae (Anderson et al., 2009; Duan et al., 2010) and C.