Breaking point: not unlike a slab of cooling rock, DNA "cracks" under pressure in roughly predictable patterns

But what generates stresses in a chromosome analogous to the stresses in cooling lava? Kleckner proposes several factors that could be acting in concert. First, during the meiotic cycle, the chromosomes condense into more tightly looped skeins of DNA. That compression could set up stresses simply by confining the DNA to a smaller space--possibly causing the DNA to stiffen as well. Second, because DNA is also replicating during meiosis, there is simply more DNA trying to fit into the constant volume of the cell nucleus. Chromosomes might be pushing against the walls of the nucleus--or against each other--so hard that they buckle under the stress.

Kleckner and her colleagues tested a theoretical model, which predicted where such mechanical stresses would cause "cracks" along a chromosome, against maps of observed crossover points--and found the model to hold true. Furthermore, micrographs of chromosomes show clear evidence of stress buildup. Some are twisted like phone cords; others have sharp flexures from buckling.

If there were not enough stress built up in a chromosome to ensure at least one crossover event, chromosome pairs would not separate properly. That possibility alone is a powerful selective force for a stressful environment. In this case it is important to crack, as they say, under pressure.

ADAM SUMMERS (asummers@uci.edu) is an assistant professor of ecology and evolutionary biology and bioengineering at the University of California, Irvine.

Illustrations by Tom Moore

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