REORGANIZATION AND CONDENSATION OF CHROMATIN IN MITOTIC PROPHASE NUCLEI OF ALLIUM-CEPA

This paper studies the process and features of chromosome construction in mitotic prophase cells of Allium cepa. The results showed that a p rominent reorganization of chromatin occurred during G(2) - early prop hase. The 250-400 nm thick compact chromatin threads in G(2) nuclei be gan to disorganize into about 30, 100 and 220 nm chromatin fibres whic h constituted the loosely organized chromosome outlines in early proph ase before chromosome condensation. In middle prophase, chromosome con densation was characterized by the formation of many condensed regions (aggregates of chromatin), which increased in size (1-1.5 mu m) when prophase proceeded. Meanwhile, the chromatin threads that constituted and connected the condensed regions became increasingly thicker (120-2 50 nm). In late prophase adjacent condensed regions fused to form cyli nder-shaped chromosomes. Based on these observations, we come to the c onclusion that the construction of prophase chromosomes is a two-step process, that is, the reorganization and condensation of chromatin. In addition, we report the study of silver-stained, DNA- and histone-dep leted prophase chromosomes, describe morphological features of the non -histone protein (NHP) residue in early, middle and late prophase chro mosomes, and discuss the roles of NHPs in chromosome construction.