These viruses are lipid containing viruses whose genomes can act as mRNA. The most notorious of these is HIV the viruses resulting in AIDS. Aside from HIV retroviruses are rather uncommon in humans, but prevalent in other mammals and birds. The genomes of retroviruses are similar in structure and size to picornaviruses like polio virus and one might suppose that the replicative strategy of a retrovirus resembles that of poliovirus. This is not the case. The life cycle of a retrovirus is unique and unusual
After entering the cell the (+) strand of RNA is not associated with ribosomes, even though it has all the attributes of mRNA. Instead, the virion RNA is used as a template to make a DNA copy of the viral genome. This copying of RNA into DNA is foreign for the host cell and must be carried out by a viral enzyme that is packaged in virion
The viral enzyme called reverse transcriptase carries out this process. The terms "reverse" and "retro" imply a mechanism that is the opposite of that normally operating in all cells. The usual flow of information in a cell is from DNA to RNA not from RNA to DNA. The initial product of reverse transcription is an RNA DNA hybrid double helix.
The RNA portion of this hybrid is degraded and reverse transcriptase copies the remaining DNA strand into dsDNA. These processes take place in the cytoplasm. Once the viral dsDNA is synthesized it is transported into the nucleus where it is inserted and covalently linked to the host chromosomal DNA. The viral DNA that is integrated into the host genome is called a provirus and it is indistinguishable from the host cell genes. In effect the retrovirus has created a version of the viral genome that has all the attributes of a cellular gene found in the host. The integrated provirus can now be transcribed by the host cell into (+) RNA that is transported to the cytoplasm and used either as mRNA in viral protein synthesis or as the genome for new progeny viruses
After entering the cell the (+) strand of RNA is not associated with ribosomes, even though it has all the attributes of mRNA. Instead, the virion RNA is used as a template to make a DNA copy of the viral genome. This copying of RNA into DNA is foreign for the host cell and must be carried out by a viral enzyme that is packaged in virion
The viral enzyme called reverse transcriptase carries out this process. The terms "reverse" and "retro" imply a mechanism that is the opposite of that normally operating in all cells. The usual flow of information in a cell is from DNA to RNA not from RNA to DNA. The initial product of reverse transcription is an RNA DNA hybrid double helix.
The RNA portion of this hybrid is degraded and reverse transcriptase copies the remaining DNA strand into dsDNA. These processes take place in the cytoplasm. Once the viral dsDNA is synthesized it is transported into the nucleus where it is inserted and covalently linked to the host chromosomal DNA. The viral DNA that is integrated into the host genome is called a provirus and it is indistinguishable from the host cell genes. In effect the retrovirus has created a version of the viral genome that has all the attributes of a cellular gene found in the host. The integrated provirus can now be transcribed by the host cell into (+) RNA that is transported to the cytoplasm and used either as mRNA in viral protein synthesis or as the genome for new progeny viruses