POLYMER MODELS OF MEIOTIC AND MITOTIC CHROMOSOMES

Polymers tied together by constraints exhibit an internal pressure; th is idea is used to analyze physical properties of the bottle-brush-lik e chromosomes of meiotic prophase that consist of polymer-like flexibl e 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 t he repulsion acting between brushes whose bristles are forced to overl ap. The retraction of lampbrush loops when the nascent transcripts are stripped away, the oval shape of diplotene bivalents between chiasmat a, and the rigidity of pachytene chromosomes are all manifestations of chromatin pressure. This two-phase (chromatin plus buffer) picture th at suffices for meiotic chromosomes has to be supplemented by a third constituent, a chromatin glue to understand mitotic chromosomes, and e xplain how condensation can drive the resolution of entanglements. Thi s process resembles a thermal annealing in that a parameter (the affin ity of the glue for chromatin and/or the affinity of the chromatin for buffer) has to be tuned to achieve optimal results. Mechanical measur ements to characterize this protein-chromatin matrix are proposed. Fin ally, the propensity for even slightly chemically dissimilar polymers to phase separate (cluster like with like) can explain the apparent se gregation of the chromatin into A+T- and G+C-rich regions revealed by chromosome banding.