Chromatin Fiber Matrix Supports Mitotic Chromosomes

A recent study conducted on large chromosomes extracted from the newt Notophthalmus viridescens has revealed that during cell division the tightly packed mitotic chromosomes are supported by chromatin fibers and not by proteins. The study was published November 18, 2002, in the online edition of the Proceedings of the National Academy of Sciences.

Investigators from the University of Illinois at Chicago (USA) studied isolated newt chromosomes by using micromechanical force measurement during nuclease digestion. Micrococcal nuclease and short-recognition-sequence blunt-cutting restriction enzymes first removed the native elastic response of the metaphase chromosomes and then continued to disintegrate them completely. This finding ruled out the possibility that the stability of the mitotic chromosome was based on a protein "scaffold.” Instead, the mechanical integrity of the metaphase chromosome was due to chromatin itself. Blunt-cutting restriction enzymes with longer recognition sequences only partially disassembled mitotic chromosomes and indicated that chromatin in metaphase chromosomes was constrained by isolated chromatin cross-linking elements spaced by approximately 15 kb.

"What we have shown is that the mitotic form of chromosomes, when they are folded up for transmission during cell division--the long, chromatin fibers made of DNA complexed with a lot of protein--are linked at cross-link points, kind of like playground monkey bars,” said senior author Dr. John Marko, associate professor of physics at the University of Illinois. "And the bars are just chromatin. Our study can provide very severe constraints on how other researchers should think about the state of the chromosome during cell division. We say there is no internal protein structure perhaps made by all these proteins sticking together. Instead, these proteins are distributed through a network of chromatin.”




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