PowerPoint PresentationSlide 2Slide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14Slide 15Slide 16Slide 17Slide 18Slide 19Slide 20Slide 21Slide 22Slide 23Slide 24Slide 25Slide 26Slide 27Slide 28Slide 29Slide 30Slide 31Slide 32Slide 33Slide 34Slide 35Slide 36Slide 37Slide 38Slide 39Pyrimidines (Py, Y)Major pyrimidinesConformation of nucleic acid componentsRotation around the N-C glycosidic bondSummaryThe building blocks of nu cleic acids are nucleotides, which are the phosphate esters ofnucleosides. These are formed by condensation of a base and a pentose. In RNA, the pentose isD-ribose and is linked in its furanose form from C’-1 to N-9 of a purine, adenine, or guanine, or N-1 of a pyrimidine, cytosine or uracyl. In DNA, 2-deoxy-D-robose is joined in the same way to thefour bases, among which thymine takes place of uracyl. The phosphate esters are strong acidsand exist as anions at neutral pH. The “bases” are, in reality, only very weakly basic and A, C,and G b ecome protonated only below pH 4. The amide NHs in G, T, and U a re deprotonated atpHs above 9.Hydrogen bonds can be formed between the major amino-keto tautomers of the bases to link A with Tand C with G in Watson-Crick base-pairing. Such hydrogen bonds are largely electrostatic in character.“Wobble” and Hoogsteen base-pairs offer minor variations to Watson-Crick pairing, and are seen in tRNAstructures.Nucleotides have defined shapes with a g eneral preference for the anti-conformers of th e glycosylicbond χ, for the C4’-C5’ bonds γ, and for the two C-O(P) bonds β and α. The furanose ring is puckered torelieve strain and can adopt either the C2’-endo or the C3’-endo conformation, which are in rapidequilibrium at room temperature.Hydrolysis on nucleic acids gives 3’ and 5’ monophosphonucleotides (implies 3’-5’ phosphodiester bonds)A-T (T-A) and G-C (C-G) base pairs have the same geometrySUMMARYThe primary structure of DNA has a string of nucleosides, each joined to it s neighbors throughphosphodiester bonds. Each 5’-hydroxyl group is linked through a phosphate to a 3’-hydroxyl group. Thus,the uniqueness of any primary structure depends on the sequence of the bases only.A-DNA and B-DNA are the major standard DNA se condary structures with right-handed double helicesand Watson-Crick base-pairing.A-DNA has 11 residues per turn, the bases are tilted 20o to enhance stacking, and they lie 4.5A away fromthe axis. As a result, A-DNA helix is s tiff and shows little sequence-dependent variation in structure. Themajor groove is deep and narrow, the minor groove broad and shallow.B-DNA has 10 bases per turn with no tilting of the bases. The wide major groove and narrow minor grooveare of the similar depth, and both grooves are solvated. The structure is flexible to permit conformationalchanges in the backbone in response to different local sequences.Z-DNA is left-handed helix stabilized by high concentrations of salt. It is most favored for alternating GCsequences. Watson-Crickpairing remains, but the purines adopt the syn-glycoside and the C3’-endo sugarpucker. The phosphate backbone has a zig-zag appearance. The minor groove is very narrow and deep; themajor groove is very