Lecture 2.24 Notes

Lecture 2.24 Notes - DNA STRUCTURE DNA Building Blocks...

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2/24/10 DNA STRUCTURE DNA Building Blocks: Summary and Nomenclature The building blocks of DNA and RNA are called nucleotides, which consist of a phosphate group, a pentose sugar and a purine or pyrimidine base. The bases are attached to the carbonyl carbon of the pentose sugar via an N- glycosidic linkage. The nucleotides used in RNA contain ribose sugars while those used in DNA contain deoxyribose (2’-deoxyribose). This minor substitution has very important consequences for the structure, stability and function of DNA and RNA molecules. A molecule consisting of a base + a sugar is called a nucleos ide while a molecule consisting of base + sugar + phosphate is called a nucleot ide. The nucleotides in polynucleotides are joined together by phosphodiester linkages between the 5’-hydroxyl group of one nucleotide and the 3’-hydroxly of another. Due to this arrangement of linkages, polynucleotides contain a 5’-carbon at one end of the chain and a 3’-carbon at the other; this is what is meant by the directionality or polarity of a polynucleotide. DNA Structure Double helical structure of DNA The base pairing rules: double helix can only be accommodated when base pairing rules obtain - The bases of polynucleotides can interact with each other by forming hydrogen bonds. The most significant of these are the interactions between guanine and cytosine (involving 3 hydrogen bonds) and adenine and thymine (resulting in 2 H-bonds). The chains of DNA are antiparallel ( i.e. each chain has a different directionality 5’ è 3’ This results in a structure in which the bases are stacked on top of each other between the sugar-phosphate backbone of the chains. Moreover, the sugar- phosphate backbones twist around the bases into a right-handed helical arrangement. The resulting structure is a double helix that makes approximately 1 complete turn every 10bp. The geometry of the base stacking and arrangement of the sugar- phosphate backbone results in the formation of major and minor grooves in the
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Lecture 2.24 Notes - DNA STRUCTURE DNA Building Blocks...

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