抄録
It is important to understand how a single circular chromosome in the prokaryotic nucleus evolved into multiple linear chromosomes in the eukaryotic nucleus. In most eukaryotic cells that have ＞∼15 Mbp of genomic DNA, chromosomes remain condensed through all the mitotic phases. Therefore, we observed nuclei of primitive organisms in which linear chromosomes had not been observed previously using conventional methods. Cells of the primitive red alga Cyanidioschyzon merolae, having a genome size of 16.5 Mbp, have been used to study the division of organelles, such as mitochondria, chloroplasts, and peroxisomes. However, morphologically condensed chromosomes have never been observed during mitotic metaphase. Recently, we demonstrated that plastid nuclei are swollen and change from a spherical to a ring shape after being subjected to the glutaraldehyde-fixed-drying method. Using a modified method, we visualized mitotic chromosomes in C. merolae cells. Chromosomal condensation occurred just after the chloroplast division when cells enter metaphase. Thus, chromosomal separation in C. merolae cells likely occurs in a manner similar to that of typical eukaryotic cells. However, mitotic condensed chromosomes were not observed in the primitive green alga Medakamo hakoo, having a genome size of 8 Mbp. Thus, the results support the use of C. merolae as a model for eukaryotic cell analyses.
