Virus
A virus is an infectious agent that can only replicate within the cells of living organisms. It is so small that it can only be seen with a microscope and has a simple structure consisting of a nucleic acid, or genetic material, enclosed in a protein shell called a capsid. Viruses are important to the study of molecular and cell biology because they provide simple systems that can be used to test various aspects of cell biology.
Unlike other living organisms, viruses do not make the energy-producing compound adenosine triphosphate (ATP), nor do they have ribosomes to make proteins. Instead, they rely on the complex metabolic machinery of prokaryotic or eukaryotic host cells to support their replication process.
To reproduce, a virus attaches to host cells by binding to specific receptors on the surface of the host cell’s membrane or cell wall. Virions then engulf the host cell by penetrating its outer layer, and its genetic material is released into the host cell’s cytoplasm, where it can be copied by viral RNA polymerases. The proteins encoded by a virus’s DNA or RNA genome are then translated into functional proteins, and the newly made proteins are assembled into new viral particles.
Viruses may be classified by their chemical composition and mode of replication, and by the structural characteristics of the capsid, including its icosahedral or helical shape, and the presence or absence of an envelope. Some viruses are also named according to their pathogenicity and the organs or tissues they target.
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