Reverse transcription (which occurs in both prokaryotes and eukaryotes) is the synthesis of DNA from an RNA template. A class of RNA viruses, called retroviruses, are characterized by the presence of an RNA‐dependent DNA polymerase (reverse transcriptase). The virus that causes AIDS, Human Immunodeficiency Virus (HIV), is a retrovirus. Because nuclear cell division doesn't use reverse transcriptase, the most effective anti‐HIV drugs target reverse transcriptase, either its synthesis or its activity. Telomerase, discussed in the previous section, is a specialized reverse transcriptase enzyme. See Figure 1 .
Like other DNA polymerases, reverse transcriptases are primer‐ and template‐dependent. They also possess an RNase H activity (H stands for hybrid) that can degrade the RNA template after it is used for synthesis of the first DNA strand. The enzyme then can copy the first strand of DNA to make a double‐stranded molecule.
Reverse transcription is error prone relative to DNA replication because reverse transcriptases don't have an editing (3′‐5′) exonucleolytic activity. This has one important consequence for HIV treatment and epidemiology. HIV mutates very rapidly. In advanced AIDS patients, the virus that is isolated from the bloodstream often bears very little resemblance to the original infecting strain. This rapid variation means that drug‐resistant mutant strains of the virus arise frequently, and drug treatment doesn't work well. Secondly, the rapid mutation rate complicates vaccine development—new strains that are not neutralized by the vaccine can appear and infect individuals that were vaccinated against the original strain.