Provirus
A provirus is viral DNA that has become integrated into a host cell genome and can be copied with that genome.
What a provirus is
A provirus is viral genetic material that has been integrated into the DNA of a host cell. In retroviruses, the viral RNA genome is first copied into DNA by reverse transcriptase. That DNA is then inserted into a host chromosome, where it becomes a stable part of the cell's genetic material.
From retrovirus to provirus
The provirus stage follows several steps. A retrovirus enters a cell, releases its RNA and enzymes, makes a DNA copy, and moves that DNA into the nucleus. Integrase then joins viral DNA to host DNA. After integration, the cell contains proviral DNA rather than just free viral genetic material.
What integration changes
Integration changes the infection from a temporary viral molecule into a genome-embedded sequence. The provirus can be copied when the host cell divides, and it can be transcribed by host RNA polymerase. This makes retroviral infection unusually durable compared with viruses that never integrate into host chromosomes.
Active and latent states
A provirus may be transcriptionally active, producing viral RNA and proteins, or it may be latent, remaining integrated with little or no viral production. Latency depends on the integration site, chromatin state, host-cell type, immune pressure, viral regulatory proteins, and cellular activation signals.
Provirus and HIV
HIV persistence is closely tied to proviral DNA. Antiretroviral therapy can strongly suppress new rounds of viral replication, but integrated proviruses can remain in long-lived cells. Some proviruses are defective, while others can be replication-competent and contribute to viral rebound if therapy stops.
Endogenous proviruses
If a provirus integrates into a germline cell and is inherited, it can become an endogenous retroviral sequence. Over evolutionary time, such sequences can accumulate in host genomes. Many are mutated or inactive, but some influence gene regulation, genome structure, or host biology.
Provirus versus prophage
The word provirus is usually used for viral DNA integrated into eukaryotic host genomes, especially retroviral DNA. A prophage is the comparable integrated state of a bacteriophage inside a bacterial host. Both describe viral genetic material carried inside a host genome, but they belong to different biological contexts.
Why it matters
Proviruses matter because integration can make viral genetic information persistent, heritable through cell division, and sensitive to host genome regulation. The concept helps explain HIV latency, retroviral vector design, endogenous retroviruses, insertional mutagenesis, and why some viral infections are difficult to eliminate completely.