Discovery Of Viruses

A virus is a nonliving particle made up of nucleic acid and a protein coat or nucleic acid and a lipid-protein (lipoprotein) coat. Even though viruses are not living organisms, they are of interest to biologists because they cause many diseases in living organisms and they are useful tools for genetic research.

Scientists began studying viruses in the late 1800s after they found that a factor smaller than bacteria could cause disease. At that time, scientists did not have the technology to see viruses. But they wanted to know if viruses were very small cells or simply nonliving groups of molecules.

In 1935, Wendell Stanley crystallized the tobacco mosaic virus (TMV). TMV is a virus that infects plants, such as tobacco and tomato plants. The disease causes plants to wither and develop mosaic-like spots on their leaves, as shown in Figure 24-1. Scientists concluded that an infective agent that could be crystallized was unlikely to be made up of cells.

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