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Vol. 8, Issue 11, 2217-2231, November 1997
*Department of Physics, The University of Illinois at Chicago,
Chicago, Illinois 60607-7059; and
Polymers tied together by constraints exhibit an internal pressure;
this idea is used to analyze physical properties of the bottle-brush-like chromosomes of meiotic prophase that consist of
polymer-like flexible chromatin loops, attached to a central axis.
Using a minimal number of experimental parameters, semiquantitative predictions are made for the bending rigidity, radius, and axial tension of such brushes, and the repulsion acting between brushes whose
bristles are forced to overlap. The retraction of lampbrush loops when
the nascent transcripts are stripped away, the oval shape of diplotene
bivalents between chiasmata, and the rigidity of pachytene chromosomes
are all manifestations of chromatin pressure. This two-phase (chromatin
plus buffer) picture that suffices for meiotic chromosomes has to be
supplemented by a third constituent, a chromatin glue to understand
mitotic chromosomes, and explain how condensation can drive the
resolution of entanglements. This process resembles a thermal annealing
in that a parameter (the affinity of the glue for chromatin and/or the
affinity of the chromatin for buffer) has to be tuned to achieve
optimal results. Mechanical measurements to characterize this
protein-chromatin matrix are proposed. Finally, the propensity for
even slightly chemically dissimilar polymers to phase separate (cluster
like with like) can explain the apparent segregation of the chromatin into A+T- and G+C-rich regions revealed by chromosome banding.
Center for Studies in
Physics and Biology, The Rockefeller University, New York, New York
10021-6399
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