The retroviruses are a specialized subset of the RNA viruses, with two important distinguishing features. First, all retroviruses have a lipid envelope surrounding their capsid. (This makes them particularly vulnerable to substances which dissolve oils, like detergents or alcohol - HIV, for example, can be easily killed by washing your hands.) Secondly, packaged with the genetic material in the viral core are several unique key enzymes - among them reverse transcriptase, integrase, and protease - that are essential to the virus's reproduction. Retroviruses (literally, "reverse viruses") - have earned their name from the unusual nature of the reproduction of their genetic material, a process which works in reverse of the normal cycle.

After releasing their RNA and their core enzymes into the host cell, the viral RNA is copied into double-stranded DNA by the reverse transcriptase enzyme (the opposite of transcriptase, which converts DNA to RNA). Another enzyme, integrase, then transports the DNA copy of the virus to the cell nucleus - where the viral DNA is spliced directly into the cell's DNA. The choice of location for the splice is more or less a random choice, although there is evidence for a number of preferred sites. When complete, the new viral DNA is termed a provirus - integrated directly into the DNA of its host. The

The structure of a typical retrovirus

result of this is that the virus is now inherited by all the offspring of the cell - passed down directly in their DNA.

Once the virus has been copied into the cell's DNA, it uses the cell's own processes to copy itself. The provirus is copied into messenger RNA, just like the DNA of the cell - and these mRNA copies are of course exact copies of the original viral RNA, and become the genetic cores of new viruses. Passing down orders from the cell nucleus, the provirus builds the components for the retrovirus protein coat and the necessary enzymes, providing the packaging for new viruses. Migrating to the plasma membrane, the new viruses bud off from the membrane, encapsulating themselves in their lipid coats as they leave.

HIV viruses at the surface of a cell

Some retroviruses never kill their host cells, using the cell to reproduce themselves indefinitely within their hosts. Others permanently alter their host cells, causing cancers or other disorders. However, most of the retroviruses do eventually kill their host cell through processes which are not well understood.