<Dd> <Ul> <Li> Spumavirus </Li> </Ul> </Dd> <Ul> <Li> Spumavirus </Li> </Ul> <P> A retrovirus is a single - stranded positive - sense RNA virus with a DNA intermediate and, as an obligate parasite, targets a host cell . Once inside the host cell cytoplasm, the virus uses its own reverse transcriptase enzyme to produce DNA from its RNA genome, the reverse of the usual pattern, thus retro (backwards). The new DNA is then incorporated into the host cell genome by an integrase enzyme, at which point the retroviral DNA is referred to as a provirus . The host cell then treats the viral DNA as part of its own genome, transcribing and translating the viral genes along with the cell's own genes, producing the proteins required to assemble new copies of the virus . It is difficult to detect the virus until it has infected the host . At that point, the infection will persist indefinitely . </P> <P> In most viruses, DNA is transcribed into RNA, and then RNA is translated into protein . However, retroviruses function differently, as their RNA is reverse - transcribed into DNA, which is integrated into the host cell's genome (when it becomes a provirus), and then undergoes the usual transcription and translational processes to express the genes carried by the virus . The information contained in a retroviral gene is thus used to generate the corresponding protein via the sequence: RNA → DNA → RNA → polypeptide . This extends the fundamental process identified by Francis Crick (one gene - one peptide) in which the sequence is DNA → RNA → peptide (proteins are made of one or more polypeptide chains; for example, haemoglobin is a four - chain peptide). </P>

Viral dna that is incorporated into the genome of a host cell is called a