Territorial polymers

Eukaryotic cells store, manage, and use up to a meter of DNA in a micronsized nucleus. How that is achieved is not completely understood and, indeed, is not easy to imagine: Consider that in polymer physics, chains packed at a similar density tangle and knot heavily, becoming as unmanageable as a fishing line crumpled without proper care. By contrast, different chromosomes in a cell nucleus remain fairly well separated throughout the cell-division cycle and occupy distinct regions termed territories. Topological enzymes and other sophisticated tools are surely involved in DNA management. However, an interesting question is what simple physics a cell could employ to reduce the use of such energy-thirsty and expensive machinery.

The likely attachment of the ends of chromosomal DNA to the nuclear envelope may prevent chains from intermixing, because the unavailability of chain ends suppresses reptation—the snakelike wriggling of a polymer past its neighbors. A new study by a team of researchers from Germany and the US has tested that idea through Monte Carlo simulations of dense systems of closed, non-interlaced polymers, represented by as many as 142 rings of up to 10 082 connected beads each. Results such as the one shown here indicate that topological constraints do have a significant geometric effect in keeping the polymers of high molecular weight largely segregated into territories and only moderately penetrating each other. (T. Vettorel, A. Y. Grosberg, K. Kremer, Phys. Biol. 6 , 025013, 2009 http://dx.doi.org/10.1088/1478-3975/6/2/025013 .)

To submit candidate images for Back Scatter, visit http://www.physicstoday.org/backscatter.html .