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A retrovirus is a virus in the family Retroviridae. Unlike other viruses that use DNA to store genetic information, a retrovirus uses RNA. Because the infected cell will only replicate the retrovirus' genes if they are also DNA, the virus must rely on reverse transcriptase to transcribe its RNA into DNA and then insert it into the host genome. The retrovirus has significant clinical importance since HIV, which causes AIDS, is a retrovirus.

The retrovirus consists of a protein capsid, base, RNA genome, and the reverse transcriptase. Since RNA is never coded back into DNA in an uninfected cell there is no endogenous reverse transcriptase in the cytoplasm. The retrovirus must bring it with it. The discovery of reverse transcriptase in retroviruses has been a major boon for the biomedical industry where conversion of RNA to DNA is used in many genetic treatments and research.

Reverse transcriptase is prone to making errors in transcription. Sometimes these errors will inactivate the retrovirus' genes and the host cell will not produce new viruses. These mutated strands of retrovirus DNA are still integrated in the host genome. In some cases in a multicellular organism this will be in a germ line cell. This means that the any offspring created from that germ line cell will have the inactivated retrovirus gene in its genome. These are referred to as endogenous retroviral insertions, it has been estimated that up to 10 percent of the human genome consists of these types of inserts. These insertions have recently become important in constructing phylogenetic trees.