Retroviruses are a special group of RNA viruses. Their genome is RNA, but inside the host cell it is copied into DNA. This reverses the usual flow of genetic information (DNA → RNA → protein), which is why they are called retroviruses, from the Latin for backwards.The best-known retrovirus is human immunodeficiency virus (HIV), which causes AIDS. Another example is human T-lymphotropic virus (HTLV).A retrovirus particle contains:
• two identical copies of a single-stranded, positive-sense RNA genome
• the enzyme reverse transcriptase, which makes DNA from an RNA template
• integrase, which inserts the viral DNA into a host chromosome
• protease, which cuts viral proteins into their working forms
• a protein capsid around the RNA and enzymes
• a lipid envelope from the host cell membrane, with glycoprotein spikes (in HIV, gp120) used for attachment
The enzymes have to be carried inside the virus particle because host cells do not have them. Human cells cannot make DNA from RNA without reverse transcriptase.Although the genome is positive-sense RNA, it is not used directly as mRNA when the virus first infects a cell. Instead it is reverse transcribed, and the DNA becomes a permanent part of the host genome. This is what makes retroviruses so difficult to remove from the body.Reverse transcriptase has no proofreading ability, so retroviruses mutate very quickly. This makes vaccine development against HIV very difficult.
• two identical copies of a single-stranded, positive-sense RNA genome
• the enzyme reverse transcriptase, which makes DNA from an RNA template
• integrase, which inserts the viral DNA into a host chromosome
• protease, which cuts viral proteins into their working forms
• a protein capsid around the RNA and enzymes
• a lipid envelope from the host cell membrane, with glycoprotein spikes (in HIV, gp120) used for attachment
The enzymes have to be carried inside the virus particle because host cells do not have them. Human cells cannot make DNA from RNA without reverse transcriptase.Although the genome is positive-sense RNA, it is not used directly as mRNA when the virus first infects a cell. Instead it is reverse transcribed, and the DNA becomes a permanent part of the host genome. This is what makes retroviruses so difficult to remove from the body.Reverse transcriptase has no proofreading ability, so retroviruses mutate very quickly. This makes vaccine development against HIV very difficult.