Retroviruses write themselves into our DNA, and some never left
Most genetic information flows from DNA to RNA. Retroviruses run the process backwards, turning their RNA into DNA and stitching it into the genome of the cells they infect. Ancient infections that reached eggs and sperm were inherited, and those viral leftovers now make up 5–8% of every human genome.
Once inside a cell, a retrovirus uses its own enzyme, reverse transcriptase, to build DNA from its RNA template; that backward step is what 'retro' refers to. Another enzyme, integrase, splices the new DNA into the host's chromosomes, where it is called a provirus. From then on the cell reads the viral genes as if they were its own, manufacturing the parts for fresh virus particles. The virus must carry reverse transcriptase with it, because an ordinary cell has no machinery for making DNA from RNA in this way.
The particles themselves are enveloped spheres about 100 nm across, each holding two identical RNA strands joined at a so-called kissing stem-loop. Their genes follow a standard order: gag builds the capsid, pro makes a protease that trims proteins into working form, pol handles copying and insertion, and env lets the virus latch onto and fuse with a target cell. A working env gene is what separates a true retrovirus from simpler retroelements.
The family spans very different outcomes. Oncoretroviruses such as HTLV can cause a form of leukemia, lentiviruses include HIV-1 and HIV-2, which cause AIDS, and foamy viruses are not linked to any disease at all. Because reverse transcription skips the proofreading used in normal DNA copying, retroviruses mutate constantly, which lets them dodge drugs aimed at their protease or reverse transcriptase and hampers vaccine development.
Endogenous retroviruses suggest these infections have hit vertebrates for at least 450 million years. Most inherited copies seem to do nothing, but some help regulate genes, resist new infections, or drive the cell fusion needed to form the placenta. Their knack for inserting DNA has also made retroviruses useful tools for delivering genes in research.
Source: Retrovirus
