Nucleic Acid Structure

Nucleic acids were first isolated from cell nuclei (hence the name) in 1870. Since then they have been found in other portions of cells as well, especially the ribosomes, which are the sites of proteinA biological polymer of amino acids joined by peptide bonds. synthesisFormation of substances with more complicated sturctures than do their precursors..

Most nucleic acids are extremely long-chain polymers—some forms of DNAAbbreviation for deoxyribonucleic acid; the polymer of nucleotides that constitutes the genetic material of chromosomes. have molecular weights greater than 10⁹. Nucleic acids are made up from three distinct structural units. These are

1 A five-carbon sugarA small carbohydrate that either contains five or six carbon atoms or is a dimer of two units, each containing five or six carbon atoms.. Only two sugars are involved. These are ribose (used in RNAAbbreviation for ribonucleic acid; a biological polymer of nucleotides that is involved in protein synthesis.) and deoxyribose (used in DNA). Their structures are shown in Fig. 1. Note that deoxyribose, as its name implies, has one oxygen less than ribose in the 2 position.

Figure 1 The structure of the monosaccharides (a) ribose and (b) deoxyribose found in nucleic acids.

2 A nitrogenous baseIn Arrhenius theory, a substance that increases the concentration of hydroxide ions in an aqueous solution. In Bronsted-Lowry theory, a hydrogen-ion (proton) acceptor. In Lewis theory, a species that donates a pair of electrons to form a covalent bond.. There are five of these bases. All are shown in Fig. 2. Three of them, adenine, guanine, and cytosine, are common to both DNA and RNA. Thymine occurs only in DNA, and uracil only in RNA.

Firgure 2 Structures of the principal nitrogenous bases obtained by hydrolysis of nu-cleic acids. The hydrogen lost when the base is condensed with a sugar is shown in color. (Thymine occurs only in DNA, while the very similar uracil occurs in its place in RNA. Adenine, guanine, and cytosine occur in both DNA and RNA.)

3 Phosphoric acid. H₃PO₄ provides the unit that holds the various segments of the nucleic acid chain to each other.

The combination of a sugar and a nitrogenous base is called a nucleoside. A typical example of a nucleoside is adenosine, derived by the condensationThe process in which a liquid forms from gas or vapor of the same substance. A chemical reaction in which two molecules combine to form a very small molecule and a larger molecule than either of the two reactants. of ribose and adenine:

Nucleosides in turn can be condensed with phosphoric acid by means of the —OH groupThose elements that comprise a single column of the periodic table. Also called family. at carbon atomThe smallest particle of an element that can be involved in chemical combination with another element; an atom consists of protons and neutrons in a tiny, very dense nucleus, surrounded by electrons, which occupy most of its volume. 5. The result is a structure called a nucleotideAn organic compound consisting of a purine or pyrimidine base bonded to a sugar that is bonded to a phosphate group; a monomer for DNA or RNA.. Thus when adenosine condenses with phosphoric acid, the nucleotide formed is adenosine-5-monophosphoric acid, usually indicated by the acronym AMP:

Finally the presence of —OH groups on both the phosphoric acid and sugar portions allows one nucleotide to link with another. This results in the polymeric structure shown in Fig. 3. Note that directionality is introduced along the chain because a sugar connected to one phosphate through carbon atom 5 always joins to the next phosphate via carbon atom 3.

Figure 3 (a) A portion of a ribonucleic acid chain. (In DNA the oxygens marked in color would not be present and thymine would replace uracil at the two positions where the latter appears along the chain.) (b) Locations of sugar, phosphate, and nitrogenous bases. (c) Representation of each base as a letter of the alphabet.