Ribonucleic acid is a polymer made up of monomeric nucleotide units. RNA has a chemical structure similar to that of DNA, but there are two major differences. First, the sugar in RNA is ribose instead of deoxyribose (Fig. 6.1). Second, although RNA contains the two purine bases adenine and guanine and the pyrimidine cytosine, the fourth base is different. The pyrimidine uracil (U) replaces thymine (Fig. 6.1). The building blocks of RNA are therefore the four ribonucleoside triphosphates adenosine 5'-triphosphate, guanosine 5'-triphosphate, cytidine 5'-triphosphate, and uridine 5'-triphosphate. These four nucleotides are joined together by phosphodiester links (Fig. 6.2). Like DNA the RNA chain has direction. In the first nucleotide in the chain, the 5' carbon of the ribose is phosphorylated and is available for formation of phosphodiester links. This is called the 5' end of the RNA chain. At the
Cell Biology: A Short Course, Second Edition, by Stephen R. Bolsover, Jeremy S. Hyams, Elizabeth A. Shephard, Hugh A. White, Claudia G. Wiedemann ISBN 0-471-26393-1 Copyright © 2004 by John Wiley & Sons, Inc.
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