1. Viruses are obligate intracellular parasites of living but noncellular 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.
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 recognised by the diseases they cause in plants, animals and prokaryotes. Viruses of prokaryotes are known as bacteriophages.
(13 Oct 1997)
