SUNY at Albany
June 19-23, 2001
Relationship between Interphase Chromosome Structure and Nuclear Geometry
Eukaryotic interphase chromosomes have several structural levels. The rules that determine the chromosome configurations (homologous chromosome pairing, centromere or telomere clustering, linear vs. folded vs. loop configurations, etc.) are not well understood. This report is an attempt to show (using a simplified model) that, for budding and fission yeast and Drosophila embryo and polytene nuclei, the allowed chromosome configurations are defined by the nuclear volume and shape.
Our quantitative model is based on the following observations: the random coil behavior of the chromatin fiber, non-overlapping of chromosomal domains, and their tight packing in the nucleus. Three constraints Ð'density', 'linear' and 'cross-section'- are considered in the model. For the 'density' constraint, the average (per nucleus) number of nucleosomes per unit of fiber contour length is calculated from the nuclear volume. The 'linear' constraint states that the chromosome length should not exceed the nuclear maximal dimension. The 'cross-section' constraint yields the maximal possible number of chromosomes, or chromosome arms, per cluster located at the nuclear envelope. The 'linear' and 'cross-section' constraints depend only on the nuclear shape, not on the size.
The results of calculations agree with the numerous observations published in the literature. Among those of interest are: 1) the diameters of confinement spheres for chromatin diffusional motion measured in live cells of budding yeast S. cerevisiae and of Drosophila by Sedat and co-workers (Marshall et al., Curr. Biol. 7, 930-939, 1997) are close to the estimates of maximal size of corresponding chromosomal domains; 2) the cone angle of pear-shaped nuclei in fission yeast S. pombe correlates with association of chromosomes in one domain and with a loss of the fiber flexibility during the meiotic cycle; 3) the profound ellipticity of the nucleus correlates with two chromocenters observed in some Drosophila polytene tissues.
The persistence length of a G2 phase fiber, consisting of two cohesion fibers, is in the range of 75-135 nm as estimated from the yeast and Drosophila nuclear volumes, or ~ 2-4 times that of a single G1 phase fiber. This is experimentally testable.
Dept of Radiation Oncology, SUNY Downstate Medical Center, Brooklyn, NY 11203, USA