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Describe about thedoublestrandhelix structure ofDNA . |
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Answer» Solution :(i) Waston and Crick postulateda 3-dimensionalmodel of RNAstructurewhichconsistedof twoantiparalleld helical DNA chains wound aroundthe sameaxis to FROMA righthanded doublehelix. (II) Thehydrophilicbackbones of alternatingdeoxyribose and phosphate groupare on the outsideof thedoublehelix, facingthe surrodingwater . The purineand pyrimidinebases of both stands are stackedinsidethe doublehelix,withtheirhydrophobicand ringstrcutures very close together and perpendicular to thelongaxis ,therebyreducingthe replusions betweenthe CHARGED phosphategroups. The offsetpairing ofthe twostrandscreates a majorgrooveand minorgrooveon the surface of the duplex. (iii) The modalrevealedthat there are 10.5 pairs`(36A^(@))`per turn of thehelix and `3.4A^(@)`between the stackedbases. They alsofound thethat each base is hydrogenbonded to a base in oppositestrandto forma planar base pair . (iv)Two hydrogen bonds are formedbetweenadenineand thymineand threehydrogenbonds are fomed between guanine and cytosineotherpairingtendstodestablizes the double helicalstrcutre . Thisspecifcassocation of thetwo chains of thedoublehelix is knowas complementarybasepairing . (a) Hydrogen bodingbetweencomplementarybase pairs. (B) Base - stackinginteractions. The complementarybetweenthe DNAstrands is attributableto thehydrogenbondingbetweenbase pairs butbasestackinginteractionare largely non - specific,makethe majorcontrribution to thestabilityofthe doublehelix. ,  `)
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