SLIPPED LOOP STRUCTURE - A NEW-TYPE OF POLYNUCLEOTIDE CHAIN FOLDING

Studies carried out in the last decade showed that DNA can acquire dif ferent secondary and tertiary non canonical conformations. It was also established that in nature DNA superhelicity and interaction with dif ferent proteins may lead to the formation of alternative noncanonical structures that are not found in relaxed DNA under physiological condi tions. At present they are called ''unusual structures,'' and their li st has grown significantly in the recent years. The role they play in the biological functioning of DNA is of great interest. To these unusu al structures belong: left-handed Z-DNA, parallel helices, triple and quadruple helices, cruciforms, and H forms. Each of these noncanonical structures (unlike the A form) can be formed only in a region with a specific nucleotide sequence. Further studies showed that these unusua l structures exist also in vivo, and probably carry out important biol ogical functions.