Visualization of G1 chromosomes: a folded, twisted, supercoiled chromonema model of interphase chromatid structure.
We have used light microscopy and serial thin-section electron microscopy to visualize intermediates of chromosome decondensation during G1 progression in synchronized CHO cells. In early G1, tightly coiled 100-130-nm "chromonema" fibers are visualized within partially decondensed chromatin masses. Progression from early to middle G1 is accompanied by a progressive uncoiling and straightening of these chromonema fibers. Further decondensation in later G1 and early S phase results in predominantly 60-80-nm chromonema fibers that can be traced up to 2-3 microns in length as discrete fibers. Abrupt transitions in diameter from 100-130 to 60-80 nm along individual fibers are suggestive of coiling of the 60-80-nm chromonema fibers to form the thicker 100-130-nm chromonema fiber. Local unfolding of these chromonema fibers, corresponding to DNA regions tens to hundreds of kilobases in length, reveal more loosely folded and extended 30-nm chromatin fibers. Kinks and supercoils appear as prominent features at all observed levels of folding. These results are inconsistent with prevailing models of chromosome structure and, instead, suggest a folded chromonema model of chromosome structure.
Belmont
A S
AS
Department of Cell and Structural Biology, University of Illinois, Champaign-Urbana 61801.
Bruce
K
K
D000818:Animals; D016466:CHO Cells; D002467:Cell Nucleus / Q000648:ultrastructure*; D002842:Chromatids / Q000737:chemistry / Q000648:ultrastructure*; D002843:Chromatin / Q000737:chemistry / Q000648:ultrastructure*; D006224:Cricetinae; D004247:DNA / Q000032:analysis / Q000737:chemistry; D016193:G1 Phase; D008853:Microscopy; D008854:Microscopy, Electron; D008954:Models, Biological; D011487:Protein Conformation
D002843:Chromatin; D004247:DNA