Genetic Molecules

Nucleic acids encode genetic information but also participate in additional physiological processes ranging from metabolism to energy transfer. Nucleotides constitute the monomeric units of nucleic acids (Figure 1-1). Nucleosides consist of two components (ribose or deoxyribose in RNA and DNA, respectively, and either a purine or pyrimidine base). A nucleotide is produced from a nucleoside by the addition of one to three phosphate groups through a cova-lent bond with the hydroxyl group of the 5' carbon of the nucleoside's sugar ring.

Nucleic acids are formed by chains of nucleotides linked by phosphodiester bonds between the 3' carbon of the first nucleotide's sugar ring and the 5' carbon of the adjacent nucleotide's sugar ring. The phosphodiester linkages cause nucleic acids to have a 5' to 3' directionality. The alternating sugar-phosphate chain forms a continuous molecule with bases extending from the 1' carbon of each sugar. For this reason, the sugar-phosphate chain is referred to as the backbone of nucleic acids (Figure 1-2). The phosphate groups give nucleic acids a negative charge that imparts important physiochemical properties to nucleic acids. The negative charge of DNA facilitates the binding of mammalian DNA to various proteins and allows separation of nucleic acid molecules by charge and size during gel or capillary electrophoresis.

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