Assembly of Mitotic Chromosomes in Xenopus Egg Extract

Cell division in eukaryotes follows an extremely complex plan according to which chromosomes are first duplicated and condensed more than 10,000 times to form the mitotic chromosomes, which are finally separated by the cellular machinery into two new nuclei. Although the fascinating process of assembly of mitotic chromosomes has been observed for more than 100 years, the mechanism of assembly as well as the structural organization of chromosomes are poorly understood. Historically, an important step towards understanding both chromosomal assembly and organization was the development of a methodology for the isolation of “pure” mitotic chromosomes and their biochemical characterization (1 ,2 ). The structure of isolated mitotic chromosomes was further studied by microscopic techniques. However, due to the tight compaction of chromatin fibers in chromosomes, their underlying structure could not be viewed by these methods. This problem was overcome by extraction of histone from chromosomes, followed by microscopic visualization of the residual structures (3 –5 ). This led to the suggestion that chromatin fibers were organized into domains, loops attached to a proteinaceous framework called “scaffold.” Thus, the scaffolding model of chromosome organization arose from studies where chromosome mitotic structure was initially destroyed. Such an approach, however, has several limitations and may lead to erroneous conclusions (6 ).