Retroviruses follow the same general steps in their replication cycles that are common to other viruses. The steps that differ from other viruses involve the retroviral reverse transcriptase, an enzyme discovered simultaneously by Howard Temin and David Baltimore in 1970. (Temin and Baltimore were awarded the Nobel Prize for this work in 1975.) Reverse transcriptase converts the single-stranded, positive-polarity RNA genome of retrovirus into double-stranded DNA, thereby reversing the typical flow of genetic information (which is from DNA to mRNA). The DNA copy is transported into the nucleus of the host cell, circularized, and integrated into the host chromosome.
This DNA copy of the retrovirus genome is referred to as the provirus or proviral DNA. The genomes of most vertebrates contain abundant numbers of incomplete and complete proviruses (endogenous retroviruses) that appear to represent remnants of past retroviral infections in germline cells. Proviruses contain structures called long terminal repeats (LTR) at each end. The LTRs contain promoter elements and transcriptional start sites that enable the retroviral genes to be expressed. They can also affect the expression of nearby cellular genes.