<Tr> <Td> Sugar pucker </Td> <Td> C3' - endo </Td> <Td> C2' - endo </Td> <Td> C: C2' - endo, G: C3' - endo </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> <P> Dehydration of DNA drives it into the A form, and this apparently protects DNA under conditions such as the extreme desiccation of bacteria . Protein binding can also strip solvent off of DNA and convert it to the A form, as revealed by the structure of a rod - shaped virus . </P> <P> It has been proposed that the motors that package double - stranded DNA in bacteriophages exploit the fact that A-DNA is shorter than B - DNA, and that conformational changes in the DNA itself are the source of the large forces generated by these motors . Experimental evidence for A-DNA as an intermediate in viral biomotor packing comes from double dye Förster resonance energy transfer measurements showing that B - DNA is shortened by 24% in a stalled ("crunched") A-form intermediate . In this model, ATP hydrolysis is used to drive protein conformational changes that alternatively dehydrate and rehydrate the DNA, and the DNA shortening / lengthening cycle is coupled to a protein - DNA grip / release cycle to generate the forward motion that moves DNA into the capsid . </P>

Who discovered a and b forms of dna