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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.
Other viruslike particles called prions are composed primarily of a protein tightly integrated with a small nucleic acid molecule. Viruses are generally classified by the organisms they infect, animals, plants, or bacteria.Since viruses cannot penetrate plant cell walls, virtually all plant viruses are transmitted by insects or other organisms that feed on plants.ther viruslike particles called prions are composed primarily of a protein tightly integrated with a small nucleic acid molecule. Viruses are generally classified by the organisms they infect, animals, plants, or bacteria.