2.4.2 Secondary Structure of RNA: A-RNA and A-RNA
Two varieties of A-type helices have been observed for fibres of RNA species such as poly(rA)poly(rU).
At low ionic strength, A-RNAhas 11 bp per turn in a right-handed, anti-parallel double-helix. The sugars
adopt a C3-endopucker and the other geometric parameters are all very similar to those for A-DNA (see
Tables 2.3 and 2.4). If the salt concentration is raised above 20%, an A-RNA form is observed which has
DNA and RNA Structure 57
Figure 2.39 Structure and formation of interferon-induced (2→ 5 )(A)n
Figure 2.38 The primary structure of RNA (left) and cleavage patterns with spleen (centre) and snake venom (right)
phosphate diesterases