THE CHROMOSOME PERIPHERY DURING MITOSIS

A complex structure, visible by electron microscopy, surrounds each ch romosome during mitosis. The organization of this structure is distinc t from that of the chromosomes and the cytoplasm. It forms a perichrom osomal layer that can be isolated together with the chromosomes. This layer covers the chromosomes except in centromeric regions. The perich romosomal layer includes nuclear and nucleolar proteins as well as rib onucleoproteins (RNPs). The list of proteins and RNAs identified inclu des nuclear matrix proteins (perichromin, peripherin), nucleolar prote ins (perichro-monucleolin, Ki-67 antigen, B23 protein, fibrillarin, p1 03, p52), ribosomal proteins (S1) and snRNAs (U3 RNAs). Only limited i nformation is available about how and when the perichromosomal layer i s formed. During early prophase, the proteins extend from the nucleoli towards the periphery of the nucleus. Thin cordon-like structures rea ch the nuclear envelope delimiting areas in which chromosomes condense . At telophase, the proteins are associated with the part of the chrom osomes remaining condensed and accumulate in newly formed nucleoli in regions where chromatin is already decondensed. The perichromosomal la yer contains several different classes of proteins and RNPs and it has been attributed various roles: (1) in chromosome organization, (2) as a barrier around the chromosomes, (3) involvement in compartmentation of the cells in prophase and telophase and (4) a binding site for chr omosomal passenger proteins necessary to the early process of nuclear assembly.