RETROVIRUSES

RETROVIRUSES
Retroviruses have single-stranded RNA genomes, which are replicated through a double-stranded DNA intermediate. The life cycle of these viruses involves an obligatory stage in which the double stranded DNA inserted into the host genome by transposition-like event, which produces short direct repeats of 4, 5 or 6 bp in the target DNA. Each retrovirus particle contains two copies of the RNA genome, and becomes surrounded by a segment of the membrane of its host cell during its release. During infection, the membrane surrounding the virus particle fuses with plasma lemma of the new host cell and the virus particle is thereby released into the cytoplasm.
The virus particle carries along with its genome the enzyme reverse transcriptase, which converts the RNA genome into linear DNA duplex. The DNA duplex migrates into the cell nucleus, becomes inserted into the host genome in one or more copies, and is transcribed by the host machinery to yield RNA copies to itself, this RNA may function either as mRNA or as viral genome. The integration reaction is catalyzed by a single enzyme called integrase that is carried within the virus particle. The virus particle also carries an uncharged host tRNA molecule that serves as primer during reverse transcription of the viral genome.