organize classification along lines of genetic relatedness. The key constituents of current classification systems are the type and structure of the viral nucleic acid and the strategy used in its replication; the type of symmetry of the virus capsid (helical versus icosahedral); and the presence or absence of an envelope (10:1124).
The genetic information of viruses is encoded in nucleic acids, which occurs in individual viral families in a wide variety of forms and sizes. The nucleic acid may be composed of either RNA or DNA. The genomes of the smallest viruses probably encode only three or four unique proteins, whereas those of the largest viruses may encode several hundred proteins. The DNA and RNA can occur in either single- or double-stranded forms and can be either circular (closed) or linear (opened ended) in shape. Viruses with linear RNA may have either a single piece of nucleic acid or a variable number of segments. The number of RNA segments can vary in number from two (Arenaviridae) to 12 (some Reoviridae) (10:1125).
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