|Genome: RNA Virus, Retrovirus||Structure: Enveloped Virus, Icosahedral Virus|
- Retroviruses are a family of viruses related by their unique replicative cycle which requires reverse transcription of their RNA genome into DNA.
- All retroviruses possess an envelope and an icosahedral capsid. Their genome consists of two identical ssRNA molecules. Because the molecules are identical, Retroviruses are not considered "Segmented Viruses".
- The ssRNA genomes of retroviruses are complexed with an enzyme "Reverse Transcriptase" which uses the RNA genome as a template to synthesize a DNA copy following viral uncoating. The DNA copy then enters the nucleus and integrates within the host genome through the action of accessory enzymes brought in by the infecting virus. The integrated DNA is then used as a template to synthesize new viral structural proteins and ssRNA genomes which are then packaged and shed from the host cell. Because infected host cells carry an integrated DNA copy of the virus, they can never eliminate the virus.
- Of historical interest, the unique enzymatic activity of Reverse Transcriptase was quite shocking upon its discovery as it falsified the "Central Dogma" of molecular biology that RNA can only be synthesized from a DNA template. Naturally, its discovery and characterization was rewarded with a Nobel Prize. Today, Reverse Transcriptase is a workhorse of modern molecular biology research.