Packing It All In

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Science  02 Oct 2009:
Vol. 326, Issue 5949, pp. 19-21
DOI: 10.1126/science.326_19d
CREDIT: COOK AND MARENDUZZO, EVOLUTION 63, 10.1111/J.1558-5646.2009.00823.X (2009)

Though the double helix structure of DNA has been known for half a century, it is only the starting point for arranging genetic material. Exactly how the 3 billion base pairs of human DNA, which are distributed unevenly across 23 chromosomes, are organized within a micron-sized nucleus, while retaining the flexibility to open and close individual regions so that specific genes can be transcribed (which requires an unwinding of the double helix), is not entirely clear. The chromosomes are known to be spatially organized within the nucleus, in a function-dependent fashion; for instance, they each occupy distinct territories, and the general consensus is that in higher eukaryotes gene-poor chromosomes are found preferentially at the periphery, with gene-rich chromosomes more centrally located.

Cook and Marenduzzo suggest that entropy may influence the positioning of chromosomes within nuclei. Using Monte Carlo simulation methods, they analyzed the distribution of stiff (blue) and flexible (red) self-avoiding polymers composed of strings of beads within a confined sphere, representing compact heterochromatin (blue) and gene-rich chromatin (red) in the nucleus. The stiff polymers tended to localize to the periphery of the sphere, whereas the flexible ones moved to the center, analogous to the situation in vivo. Looping the polymers, which would represent the joining of distal regions of chromatin, promoted the formation of distinct territories, and flexible polymers were less likely to form contacts with other polymers, consistent with gene-rich regions being less frequently involved in chromosomal translocations.

J. Cell Biol. 186, 825 (2009).

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