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lecture 4_notes - Lecture 4 Nucleic acids(continued...

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Lecture 4 Nucleic acids (continued) Pyrimidines: Cytosine, Thymine, Uracil. Single ring. Purines: Adenine, Guanine. Double ring. Polymer can be of two types: DNA, deoxyribonucleic acid, chemically more stable; RNA, ri- bonucleic acid, chemically less stable. DNA: A,G, C, T. RNA: A, G, C, U One polymer (5' to 3') is called a strand or chain DNA: typically double stranded and double chained (ds). RNA: typically single chained with double-stranded portions due to intrastrand base-pairing dsDNA: the two strands associate through H-bonds formed between bases The H-bonds are formed between a pyrimidine and a purine: two in A=T pair, three in C= G pair The H-bonds involve N---H and O---H interactions provided by atoms that are positioned on the distal edge of each base (see book drawing) The paired strands are in antiparallel: one 5' to 3', the other 3' to 5' The strands wind to form a right-winded helix: the double helix Origin of life Consider the following: organisms are formed of the molecules that we described
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