Chromosome Dynamics in Tobacco BY-2 Cultured Cells

Abstract

During mitosis in eukaryotes, the nuclear genome is organized into highly condensed structures referred to as chromosomes. The dynamic processes of chromosome condensation and segregation play crucial roles in the equal separation of genetic information to both daughter cells. Since disruption of these processes is harmful to cells, causing, for example, chromosome aneuploidy, appropriate regulation is indispensable. Chromosome dynamics involves modification of the molecules that regulate these processes during mitosis. The usefulness of tobacco BY-2 (Nicotiana tabacum cv. Bright Yellow-2) cultured cells in analyses of chromosome dynamics has been recently demonstrated. Tobacco BY-2 cells possess the following advantages. Their mitotic chromosomes are not small and monocentric, which is advantageous because analyses of chromosome dynamics are generally performed under a microscope. Thehaploid genome size of the amphidiploid speciesNicotiana tabacum (2n = 4x = 48) is about 4,500Mb (Arumuganathan and Earle 1991) and the total nuclear genome is divided into 48 mitotic chromosomes. The length of thesemitotic chromosomes varies from 2 to 6 μm (Kenton et al. 1993;Moscone et al. 1996), which is larger than that in Arabidopsis thaliana (c.a. 2 μm). Such chromosome size makes it possible to analyze the detailed morphology and structure of chromosomes. Although BY-2 cultured cells are derived from N. tabacum, it is probable that a certain level of ploidy change and chromosome abnormality occurs during cell culture. However, any problem encountered in studies of chromosome dynamics has not been reported to date. Another advantage of tobaccoBY-2 cells is their ability for high synchronization. Because mitotic chromosomes only emerge in the short mitotic phase, synchronous cell systems are very useful for studying chromosome dynamics. A highly synchronous method has been established in tobacco BY-2 cells (Nagata et al. 1992) and the usefulness of synchronized BY-2 cells has been widely demonstrated inplant cell cycle studies (Ito 2000). SynchronizedBY-2 cells provide the most efficient mitotic chromosomes because a mitotic index of more