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A virus has either a DNA or an RNA genome and is called a DNA virus or an RNA virus, respectively. The vast majority of viruses have RNA genomes. Plant viruses tend to have single-stranded RNA genomes and bacteriophages tend to have double-stranded DNA genomes.
All viruses have genes constructed from either deoxyribonucleic acid (DNA) or ribonucleic acid (RNA)âlong helical molecules that carry genetic information. All viruses have a protein coat that protects these genes, and some are wrapped in a viral envelope of fat that surrounds them when they are outside a cell.(Viroids do not have a protein coat and prions contain neither RNA nor DNA). virus is a microscopic organism that can replicate only inside the cells of a host organism. Most viruses are so tiny they are only observable with at least a conventional optical microscope.