Introduction

Deoxyribonucleic acid (DNA) is a polymer composed of three fundamental building blocks: a phosphate, a sugar, and one of four nitrogen containing bases. The cyclic, five-carbon sugars (deoxyribose units) are linked together by the phosphate groups to form the backbone of the polymer. The four heterocyclic nitrogenous bases are bonded to the sugars. The bases are: cytosine, C; guanine, G; adenine, A; and thymine, T.

The cytosine always pairs with guanine, and the adenine always pairs with thymine. The two strands of DNA are joined by hydrogen bonds between base pairs: three hydrogen bonds between cytosine and guanine; two hydrogen bonds between adenine and thymine.

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