retrovirus and reverse transcriptase
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In normal cells, information flows one way through a step called transcription: DNA is copied into RNA, which is then used to build proteins. For decades this seemed like a one-way street. Then a peculiar group of viruses turned up that drive traffic backwards — copying RNA back into DNA. Because they run transcription in reverse, they are called retroviruses (retro means 'backward'), and the enzyme that does it is reverse transcriptase.
A retrovirus carries its genes as RNA, not DNA. When it infects a cell, it brings along its own reverse transcriptase, an enzyme that reads the viral RNA and writes a matching DNA copy. Another viral enzyme then splices that DNA copy permanently into one of the host cell's own chromosomes. From then on, the host cell treats the viral genes as part of itself, faithfully copying and reading them — which means the infection can become a permanent part of the cell's genetic material, passed on every time that cell divides.
Retroviruses matter both as a threat and as a tool. HIV, the virus that causes AIDS, is a retrovirus, and several important anti-HIV drugs work by jamming its reverse transcriptase so it cannot make that crucial DNA copy. On the constructive side, scientists harness reverse transcriptase as a workhorse laboratory enzyme to turn fragile RNA into stable DNA for study, and use gutted retroviruses as delivery vehicles to insert helpful genes in gene therapy. The key honest point: a retrovirus does not merely visit a cell — it can write itself permanently into the cell's own genome.
The drug AZT, one of the first treatments for HIV, is a decoy building block: when the virus's reverse transcriptase tries to copy its RNA into DNA, it grabs AZT by mistake and the growing DNA chain dead-ends. The copy is never finished, so the virus cannot integrate.
Many HIV drugs work by sabotaging reverse transcriptase, the enzyme that makes retroviruses possible.
Reverse transcriptase (RNA to DNA) is the reverse of normal transcription (DNA to RNA); it is not the same as DNA replication. Not all RNA viruses are retroviruses — only those that copy their RNA into DNA and integrate it.