<P> B - DNA is the most common form of DNA in vivo and is a more narrow, elongated helix than A-DNA . Its wide major groove makes it more accessible to proteins . On the other hand, it has a narrow minor groove . B - DNA's favored conformations occur at high water concentrations; the hydration of the minor groove appears to favor B - DNA . B - DNA base pairs are nearly perpendicular to the helix axis . The sugar pucker which determines the shape of the a-helix, whether the helix will exist in the A-form or in the B - form, occurs at the C2' - endo . </P> <P> A-DNA, is a form of the DNA duplex observed under dehydrating conditions . It is shorter and wider than B - DNA . RNA adopts this double helical form, and RNA - DNA duplexes are mostly A-form, but B - form RNA - DNA duplexes have been observed . In localized single strand dinucleotide contexts, RNA can also adopt the B - form without pairing to DNA . A-DNA has a deep, narrow major groove which does not make it easily accessible to proteins . On the other hand, its wide, shallow minor groove makes it accessible to proteins but with lower information content than the major groove . Its favored conformation is at low water concentrations . A-DNAs base pairs are tilted relative to the helix axis, and are displaced from the axis . The sugar pucker occurs at the C3' - endo and in RNA 2' - OH inhibits C2' - endo conformation . Long considered little more than a laboratory artifice, A-DNA is now known to have several biological functions . </P> <P> Z - DNA is a relatively rare left - handed double - helix . Given the proper sequence and superhelical tension, it can be formed in vivo but its function is unclear . It has a more narrow, more elongated helix than A or B. Z - DNA's major groove is not really a groove, and it has a narrow minor groove . The most favored conformation occurs when there are high salt concentrations . There are some base substitutions but they require an alternating purine - pyrimidine sequence . The N2 - amino of G H - bonds to 5' PO, which explains the slow exchange of protons and the need for the G purine . Z - DNA base pairs are nearly perpendicular to the helix axis . Z - DNA does not contain single base - pairs but rather a GpC repeat with P-P distances varying for GpC and CpG . On the GpC stack there is good base overlap, whereas on the CpG stack there is less overlap . Z - DNA's zigzag backbone is due to the C sugar conformation compensating for G glycosidic bond conformation . The conformation of G is syn, C2' - endo and for C it is anti, C3' - endo . </P> <P> A linear DNA molecule having free ends can rotate, to adjust to changes of various dynamic processes in the cell, by changing how many times the two chains of its double helix twist around each other . Some DNA molecules are circular and are topologically constrained . More recently circular RNA was descirbed as well to be a natural pervasive class of nucleic acids, expressed in many organisms (see circRNA). </P>

What are the structural components of dna and rna