Retrovirus

A retrovirus is an enveloped virus whose genome is single-stranded RNA but which multiplies by way of a DNA stage. The name points to this backward step: in cells, genetic information usually flows from DNA to RNA, but a retrovirus carries an enzyme, reverse transcriptase, that makes DNA from an RNA template. Human immunodeficiency virus (HIV), the cause of AIDS, is the best-known example.

After entering a cell, the virus uses reverse transcriptase to build a double-stranded DNA copy of its genome. A second viral enzyme, integrase, splices this DNA into one of the host's chromosomes, where it is called a provirus. Unlike the prophage of a temperate bacteriophage, the provirus does not cut itself back out; it stays put and is copied whenever the cell divides. The cell then reads the viral genes and makes new virions, which bud from its surface. Because a provirus can lie quiet for long periods, retroviral infections are lifelong and hard to cure.

Some retroviruses can cause cancer, and traces of ancient retroviral infections are scattered through the human genome. Drugs that block reverse transcriptase are central to HIV treatment, and modified retroviruses are used as vectors to deliver genes in research and gene therapy.