Viruses are obligate intracellular parasites of living but non-cellular nature, consisting of DNA or RNA and a protein coat. They range in diameter from 20-300nm. Class I viruses (Baltimore classification) have double-stranded DNA as their genome; Class II have a single-stranded DNA genome; Class III have a double-stranded RNA genome; Class IV have a positive single-stranded RNA genome, the genome itself acting as mRNA; Class V have a negative single-stranded RNA genome used as a template for mRNA synthesis, and Class VI have a positive single-stranded RNA genome but with a DNA intermediate not only in replication but also in mRNA synthesis. The majority of viruses are recognized by the diseases they cause in plants, animals and prokaryotes. Viruses of prokaryotes are known as bacteriophages.
Dictionary of molecular biology. 2004.