Systematic Veterinary Virology

REPLICATION

• Replication occurs inside the cytoplasm and it is the only RNA virus that depends on host cell transcriptionary mechanisms. The virus enters the cell by attaching it with a host cell receptor. The SU and TM proteins help in this stage. The virus enters by fusion. Only partial uncoating occurs inside the cytoplasm. Though the RNA is positive sense they are not translated immediately inside the cytoplasm.
• A copy of the DNA is synthesized inside the cell with a virus coded enzyme RNA dependant RNA polymerase (reverse transcriptase). The d/s DNA product formed by this reaction is known as the provirus. Three forms of provirus DNA are found in all infected cells.  This DNA later migrates to the nucleus and integrates with host cell DNA with the help of the enzyme integrase. Retroviruses use the cellular transcriptional machinery for expression. Therefore they are expressed like cellular genes.
•  Assembly occurs at the cytoplasm and the mature virions later bud out through the cell membrane, acquiring the envelope during this process. As a result of complex replication steps, the replication processes of retroviruses are error prone. Further changes in the retro viral genome occurs due to high rate of mutation, recombination and exchange of genes with host genome.