Deoxyribonucleic acid is a polymeric molecule that is composed of repeating nucleotide subunits. The order of nucleotide subunits contained in the linear sequence or primary structure of these polymers represents all of the genetic information carried by a cell. Each nucleotide is composed of (1) a phosphate group, (2) a pentose (5 carbon) sugar, and (3) a cyclic nitrogen-containing compound called a base. In DNA, the sugar moiety is 2-deoxyribose. Eukaryotic DNA is composed of four different bases: adenine, guanine, thymine, and cytosine. These bases are classified based on their chemical structure into two groups:
From: Molecular Diagnostics: For the Clinical Laboratorian, Second Edition Edited by: W. B. Coleman and G. J. Tsongalis © Humana Press Inc., Totowa, NJ
Replication f ^ Transcription Translation
Fig. 1. The central dogma of molecular biology. The central dogma defines the paradigm of molecular biology that genetic information is perpetuated as sequences of nucleic acid, but that genes function by being expressed in the form of protein molecules. (From ref. 20.)
Adenine and guanine are double-ring structures termed purines, and thymine and cytosine are single-ring structures termed pyrimidines (Fig. 2). Within the overall composition of DNA, the concentration of thymine is always equal to the concentration of adenine, and the concentration of cytosine is always equal to guanine (22,23). Thus, the total concentration of pyrim-idines always equals the total concentration of purines. These monomeric units are linked together into the polymeric structure by 3', 5'-phosphodiester bonds (Fig. 3). Natural DNAs display widely varying sizes depending on the source. Relative molecular weights range from 1.6 x 106 Daltons for bacteriophage DNA to 1 x 1011 Daltons for a human chromosome.
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