The terms provirus, virus, and virion are all related to the study of viruses, but they refer to different aspects of viral existence and behavior.
Virus: A virus is a microscopic infectious agent that can only replicate inside the living cells of an organism. Viruses are composed of genetic material (either DNA or RNA) encased in a protein coat, and they can infect various types of cells in animals, plants, and even bacteria. Viruses lack the cellular machinery necessary for metabolism and reproduction, which is why they need to invade host cells to multiply.
Virion: A virion is the complete, infective form of a virus that exists outside the host cell. It consists of the viral genome (DNA or RNA), the protein coat (capsid), and sometimes an additional lipid envelope. When a virion infects a host cell, it releases its genetic material, which then hijacks the cell’s machinery to produce new viral particles. In essence, while all virions are viruses, not all viruses are in the virion form at all times since they also exist in an active or non-infectious state once inside a host.
Provirus: A provirus is the form that viral DNA takes when it integrates into the host cell’s genome. This happens with certain types of viruses, especially retroviruses, during their replication process. Once a provirus is formed, it can remain dormant for an extended period, not producing new virions until it is activated. When triggered, the provirus can start producing new viral particles, thus continuing the infection.
In summary, a virus is the infectious agent itself, a virion refers specifically to the viral particle that can infect cells, and a provirus is the integrated viral DNA within a host genome. Each term highlights a different stage or form that viruses can take in their life cycle.