Kinking Double-Stranded DNA by Strong Bending

Many years ago Crick and Klug suggested that sharp bends or kinks have to facilitate strong bending of the double-helix, and accurate theoretical analysis confirms this suggestion. It remains, however, to be determined what is the critical curvature of DNA that prompts the appearance of the kinks. Different experimental approaches to the problem will be briefly reviewed. Attention will be paid to influence of the torsional stress in the double helix on the kink formation. The most reliable data suggest that the stable kinks appear only in torsionally relaxed DNA circles smaller than 70 bp. It is possible that the kinks represent openings of isolated base pairs. Although the probability of these openings in long unstressed DNA molecules is close to 10^-5, it increases sharply in small DNA circles reaching 1 open bp per circle of 70 bp.