reverse transcription
Meaning of reverse transcription: Genetic information running backward along its usual route, from RNA into DNA, and the step HIV cannot do without.
Definition of reverse transcription
Reverse transcription is the copying of an RNA sequence into DNA, the opposite of the direction in which genetic information normally travels. An enzyme called reverse transcriptase carries it out. Retroviruses use the process to convert their RNA genome into DNA that can be spliced into a host chromosome, and laboratories use it to turn fragile RNA samples into stable DNA copies that can be measured.
Why was it a surprise?
The accepted picture had information moving one way only: DNA is read into RNA, and RNA is read into protein. An enzyme that wrote RNA back into DNA broke that rule and answered a question that had been awkward for years, namely how a virus carrying no DNA at all could become a permanent fixture in a cell's chromosomes. The finding also recast transcription as one direction of traffic rather than the only one.
Where does it occur?
| Where | What it accomplishes there |
|---|---|
| Retroviruses, including HIV | Turns the RNA genome into DNA for insertion into a chromosome |
| Hepatitis B virus | A DNA virus that nonetheless copies itself through an RNA stage |
| Telomerase | Carries its own RNA template and writes DNA onto chromosome ends |
| Mobile genetic elements | Copy themselves through RNA and paste the copy back into the genome |
Describing the enzyme as something found in retroviruses undersells it considerably. Telomerase, which maintains the tips of chromosomes in ordinary human cells, is itself a reverse transcriptase, and a large fraction of human DNA consists of sequence deposited by this process over evolutionary time. Among the viruses, HTLV-1 uses the same route as HIV.
What does the laboratory use it for?
RNA is unstable and cannot be amplified directly, so almost any measurement of it begins by making a DNA copy. That single step underlies the tests that detect viruses whose genetic material is RNA, and the methods that measure which genes a tissue is actively using. The enzyme has one habit that matters everywhere it appears: it does not check its work. Errors accumulate at every round of copying, which is the source of the variability that lets a virus slip past immune recognition and become resistant to medicines, and it is also why the first drugs developed against HIV were aimed squarely at this step.
Also known as
Used in a sentence
Reverse transcription was performed before the sample was amplified.
Good to know
Reverse transcriptase does not proofread, so copying errors accumulate rapidly, and that variability underlies both the escape of HIV from immune recognition and the emergence of resistance to medicines.
Word origin
Where “reverse transcription” comes from
Transcription joins the Latin trans, meaning across, with scribere, to write, so the word describes copying something over; reverse comes from the Latin reversus, turned back.