<prompt> True about DNA structure: PGI Nov 2010Purines are adenine and guanine ; pyrimidines are uracil and cyTosineWatson and Crick discovered structure in 1973Doxyribose-phosphate backbone with bases stacked insideMainly consists of left handed helix  <response>: Ans; C Deoxy.. In the DNA helix, hydrophilic deoxyribose-phosphate buckboneQ of each chain is on the outside of the molecule, whereas the hydrophobic bases are stacked insideQ- Lippincott Biochemistry 4th397DNA is covalently linked by 3-5 phosphodiester bondQ- Lippincott Biochemistry 4th397There are 3 form of DNA- B, AZ. B-DNA A-DNA forms right handed helix whereas Z-DNA forms left handed helix. Chromosomal DNA is thought to be predominantly B-DNA- Lippincott Biochemistry 4th 397So consists primarily of right handed helix Lippincott Biochemistry 4th 397DNA StructureThe structure of DNA is illustrated by a right bonded double helix, with about 10 nucleotide pairs per helical turn. Each spiral strand, composed of a sugar phosphate backbone and attached bases, is connected to a complementary strand by hydrogen bonding non- covalent between paired bases, adenine A with thymine T and guanine G with cytosine CQThe structure of DNA of all species comprises two helical chains each coiled round the same axis, and each with a pitch of 34 Angstroms 3.4 nanometres and a radius of 10 Angstroms 1.0 nanometres.Adenine and thymine are connected by two hydrogen bonds non-covalent while guanine and cytosine are connected by three.This structure was first described by James Watson and Francis Crick in 1953QAlthough the B-DNA form is most common under the conditions found in cells, it is not a well-defined conformation but a family of related DNA conformations that occur at the high hydration levels present in living cells.Compared to B-DNA, the A-DNA form is a wider right- handed spiral, with a shallow, wide minor groove and a narrower, deeper major groove. The A form occurs under non-physiological conditions in partially dehydrated samples of DNA 