DNA Folds With Molecular Velcro
Many have heard how the inventor of Velcro got the idea from plant seeds that stick to clothing, but now Carlos Bustamente and team of Howard Hughes Medical Institute have found a velcro-like principle operating at a scale millions of times smaller. Small proteins called condensins are involved in the elaborate folding that DNA undergoes as it is wrapped into chromosomes. The team developed an ingenious method of gently pulling on DNA strands compacted with condensin. Bustamente relates, “when we began to pull it apart carefully, we saw it extend in a sawtooth pattern of force, like the click-click-click of Velcro unzipping.” When they relaxed the force, it collapsed back, then repeated the same pattern when pulled apart again. “That perfect reproducibility strongly suggested to Bustamante and his colleagues that they were seeing a condensed structure with a well defined organization,” the press release explains. Surprisingly, this reversible reaction did not require the expenditure of ATP.
This is just one of the clever design features in the cell that allows over six feet of fragile DNA to be folded and compacted into a nucleus a few millionths of an inch wide. For a simplified view of how this all works, see the PBS “Journey Into DNA,” available in Flash format or as a web page. (The condensin comes into play in the frames where DNA forms tight loops.) Even more amazing is that this tight packing still allows the translation machinery to find the right gene, gain access, and do its work. For a glimpse of this additional complexity, see the 06/13/2002 and 03/08/2002 headlines.