The term virus is used to describe submicroscopic infectious agents that replicate only inside living cells of a host organism. Viruses are found in every ecosystem on Earth and are the most abundant type of biological entity. The study of viruses is called virology.
A virus is a particle of genetic information enclosed in a protein shell. The simplest virus encodes only four proteins and is encased in a lipid membrane, while the most complex contains over 200 genes. The nucleocapsid is the nucleus of a virus and contains the viral genome. The outer surface of a virus is responsible for recognition and attachment to a host cell. This is accomplished by a receptor-binding protein that interacts with a specific molecule in the host cell to initiate replication.
Viruses derive energy, like all living organisms, from the synthesis of adenosine triphosphate (ATP) in the host cell. Unlike cells, they cannot produce or store their own ATP. Viruses therefore depend on their host cell for the production of all metabolic functions.
Viruses are unique in that they do not contain mitochondria, ribosomes or other cell-like organelles and therefore cannot make their own energy or proteins. They are therefore considered genetic parasites, and must infect a host cell to replicate. Once a virus has infected a cell, the viral genome is transcribed into RNA, which is then translated into proteins by the host cell’s enzymes. The proteins then form a capsid around the DNA or RNA molecule. During the replication cycle, thousands of new viruses can be produced in a short time and are released into the medium or blood.
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