Tobacco mosaic viruses. Coloured transmission electron micrograph of a cluster of rod-shaped red virus particles of tobacco mosaic virus (TMV). Each virus consists of a protein coat with internal RNA genetic material. Some viruses are attached end-to-end. The virus enters a host cell in a tobacco plant and commandeers its DNA, instructing it to reproduce more TMV's. The effect of TMV infection varies from outright death of the host plant to severe lesions or mottling of the leaves. Plant viruses like TMV are transmitted by insects, pests and nematode worms. Negatively stained. Magnification: unknown.