Carbon atom NH2 (amino) group Hydrogen atom COOH (carboxyl) group Variable group carry the information for one or a few genes. Moreover, RNA is usually much shorter-lived than DNA, which is used for long-term storage of the genome. Some classes of RNA molecules, especially tRNA, contain unusual, chemically modified bases that are never found in DNA (see Ch. 8).

The above differences in function between RNA and DNA apply to living cells. However, certain viruses carry their genomes as either single or double-stranded RNA. In such cases, multiple genes will obviously be present on these RNA genomes. Furthermore, double-stranded viral RNA can form a double helix, similar though not identical in structure to that of DNA. The properties of viruses and the novel aspects of their genomes are discussed more fully in Chapter 17.

Typically, about 60% of the organic matter in a cell is protein. Most of the cell's activities and many of its structures depend on its proteins.

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