Читать книгу Snyder and Champness Molecular Genetics of Bacteria - Tina M. Henkin - Страница 109

It is possible to estimate the extent of supercoiling of natural DNAs. According to the Watson-Crick structure, the two strands are wrapped around each other about once every 10.5 bp to form the double helix. Therefore, in a DNA of 2,100 bp, the two strands should be wrapped around each other about 2,100/10.5, or 200, times. In a supercoiled DNA of this size, however, the two strands are wrapped around each other either more or less than 200 times. If they are wrapped around each other more than once every 10.5 bp, the DNA is said to be positively supercoiled; if less than once every 10.5 bp, it is negatively supercoiled.

Most DNA in bacteria is negatively supercoiled, with an average of one negative supercoil for every 300 bp, although there are localized regions of higher or lower negative supercoiling. Also, in some regions, such as ahead of a transcribing RNA polymerase, the DNA may be positively supercoiled (see above).