Nucleic Acids

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University of Massachusetts Amherst
Mark Huyler

Nucleic Acids  A nucleic acid is a polymer of nucleotide monomers  Nucleotides are each composed of a phosphate group, a sugar, and a nitrogenous base o The sugar is ribose in ribonucleotides and deoxyribose in deoxyribonucleotides  There are two groups of nitrogenous bases: o Purines (adenine/guanine) o Pyrimidines (cytosine/uracil/thymine)  Uracil is found only in ribonucleotides, and thymine is found only in deoxyrionucleotides  Nucleic acids form when nucleotides polymerize  A condensation reaction forms a phosphodiester linkage between the phosphate group on the 5’ carbon of one nucleotide and the –OH group on the 3’ carbon another  Types of nucleotides involved: o Ribonucleotides which contain the sugar ribose and form RNA o Deoxyribonucleotides which contain the sugar deoxyribose and form DNA  The sugar phosphate backbone of a nucleic acid is directional – one end has an unlinked 5’ carbon, and the other end has an unlinked 3’ carbon  The nucleotide sequence is written in the 5’-3’ direction. This reflects the sequence in which nucleotides are added to a growing molecule  This nucleotide sequence comprises the nucleic acid’s primary structure The polymerization of nucleic acids is endergonic  Polymerization of nucleic acids is an endergonic process catalyzed by enzymes…requires an input of energy  Energy for polymerization comes from the activation (phosphorylation) of the nucleotides o Phosphorylation is the transfer of one or more phosphate groups to a substrate molecule. This raises the potential energy of the substrate and enables endergonic reactions  In nucleic acid polymerization, two phosphates transferred, creating nucleoside triphosphate ^RNA What is the nature of DNA’s secondary system?  Erwin Chargraff established two empirical rules for DNA (Chargraff’s Rule) o The total number of purines and pyrimidine’s is the same o The numbers of A’s and T’s are equal and the number of C’s and G’s are equal Watson and Crick’s Model of DNA’s secondary structure  James Watson and Francis Crick determined: o DNA strands run in an antiparallel configuration o DNA strands form a double helix  The hydrophilic sugar-phosphate backbone faces the exterior  Nitrogenous base pairs face the interior o Purines always pair with pyrimidine’s  Specifically strands from complement
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