Chemistry of Nucleic Acids

Once DNA (and, in certain viruses, RNA) had been identified as the genetic material, the next task was to work out its chemical make-up. Both nucleic acids are long chain polymers built from repeating subunits called nucleotides, and each individual nucleotide subunit is itself built from three separate chemical parts joined together: a nitrogenous base, a five-carbon (pentose) sugar, and a phosphate group. The identity of the sugar is what distinguishes the two nucleic acids from each other: molecules built around deoxyribose sugar are called deoxyribonucleic acid (DNA), while molecules built around ribose sugar are called ribonucleic acid (RNA); the only chemical difference between the two sugars is that deoxyribose is missing one oxygen atom that ribose retains at its 2' carbon. DNA is found in the nucleus of eukaryotic cells and in the nucleoid region of prokaryotic cells. The nitrogenous bases themselves come in two chemical families: adenine (A) and guanine (G) each have a double-ring (fused six-membered and five-membered ring) structure and are called purines, while thymine (T), cytosine (C) and uracil (U) each have a single-ring structure and are called pyrimidines; of these, thymine is found only in DNA and uracil only in RNA. The phosphate group is derived from phosphoric acid and, once incorporated into a nucleotide chain, carries a negative charge that gives nucleic acids their overall acidic character (hence the name 'nucleic acid') and helps retain the molecule within the cell or nucleus. Structurally, when a nitrogenous base attaches to the 1' carbon of the sugar, the resulting two-part molecule is called a nucleoside; once a phosphate group is further attached to the 5' carbon of that same sugar, the molecule becomes a full nucleotide. Successive nucleotides are then joined end to end by a condensation reaction in which the 3'-hydroxyl group of one sugar forms an es …