A small infectious agent that contains genetic material in the form of DNA or RNA and a protective case, called a capsid. Viruses can infect the cells of plants, animals and bacteria. Each type of virus infects only a specific kind of host cell. Viruses cannot reproduce on their own, but they can hijack the host’s machinery to make copies of themselves. This process is known as lytic replication.
When a virus enters a cell, it attaches to a protein domain on the surface of the cell. The virus then releases its nucleic acid into the cell. This causes the host cell to produce virus proteins, and diverts its energy from producing its own proteins to making new viruses. Eventually, the host cell runs out of cellular resources and bursts, releasing the viruses to infect more cells.
Viruses are quintessential parasites. They depend on the cells they infect for all of their life functions. Unlike cells, they cannot synthesize their own adenosine triphosphate (ATP), the energy that drives all of the cell’s biochemical activities. Instead, they get their energy and all of the amino acids and lipids they need from the cells they infect.
The fact that a virus must invade a living cell to perform its only function has made scientists reluctant to consider it alive. However, some scientists argue that because viruses contain the same building blocks of life as cells—DNA and RNA—they verge on being alive.
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