<P> A-DNA and Z - DNA differ significantly in their geometry and dimensions to B - DNA, although still form helical structures . It was long thought that the A form only occurs in dehydrated samples of DNA in the laboratory, such as those used in crystallographic experiments, and in hybrid pairings of DNA and RNA strands, but DNA dehydration does occur in vivo, and A-DNA is now known to have biological functions . Segments of DNA that cells have been methylated for regulatory purposes may adopt the Z geometry, in which the strands turn about the helical axis the opposite way to A-DNA and B - DNA . There is also evidence of protein - DNA complexes forming Z - DNA structures . </P> <P> Other conformations are possible; A-DNA, B - DNA, C - DNA, E-DNA, L - DNA (the enantiomeric form of D - DNA), P - DNA, S - DNA, Z - DNA, etc. have been described so far . In fact, only the letters F, Q, U, V, and Y are now available to describe any new DNA structure that may appear in the future . However, most of these forms have been created synthetically and have not been observed in naturally occurring biological systems . There are also triple - stranded DNA forms and quadruplex forms such as the G - quadruplex . </P> <Table> Structural features of the three major forms of DNA <Tr> <Th> Geometry attribute </Th> <Th> A-DNA </Th> <Th> B - DNA </Th> <Th> Z - DNA </Th> </Tr> <Tr> <Td> Helix sense </Td> <Td> right - handed </Td> <Td> right - handed </Td> <Td> left - handed </Td> </Tr> <Tr> <Td> Repeating unit </Td> <Td> 1 bp </Td> <Td> 1 bp </Td> <Td> 2 bp </Td> </Tr> <Tr> <Td> Rotation / bp </Td> <Td> 32.7 ° </Td> <Td> 34.3 ° </Td> <Td> 60 ° / 2 </Td> </Tr> <Tr> <Td> bp / turn </Td> <Td> 11 </Td> <Td> 10.5 </Td> <Td> 12 </Td> </Tr> <Tr> <Td> Inclination of bp to axis </Td> <Td> + 19 ° </Td> <Td> − 1.2 ° </Td> <Td> − 9 ° </Td> </Tr> <Tr> <Td> Rise / bp along axis </Td> <Td> 2.3 Å (0.23 nm) </Td> <Td> 3.32 Å (0.332 nm) </Td> <Td> 3.8 Å (0.38 nm) </Td> </Tr> <Tr> <Td> Pitch / turn of helix </Td> <Td> 28.2 Å (2.82 nm) </Td> <Td> 33.2 Å (3.32 nm) </Td> <Td> 45.6 Å (4.56 nm) </Td> </Tr> <Tr> <Td> Mean propeller twist </Td> <Td> + 18 ° </Td> <Td> + 16 ° </Td> <Td> 0 ° </Td> </Tr> <Tr> <Td> Glycosyl angle </Td> <Td> anti </Td> <Td> anti </Td> <Td> C: anti, G: syn </Td> </Tr> <Tr> <Td> Sugar pucker </Td> <Td> C3' - endo </Td> <Td> C2' - endo </Td> <Td> C: C2' - endo, G: C2' - exo </Td> </Tr> <Tr> <Td> Diameter </Td> <Td> 23 Å (2.3 nm) </Td> <Td> 20 Å (2.0 nm) </Td> <Td> 18 Å (1.8 nm) </Td> </Tr> </Table> <Tr> <Th> Geometry attribute </Th> <Th> A-DNA </Th> <Th> B - DNA </Th> <Th> Z - DNA </Th> </Tr>

How many residues are present in each turn of dna