reverse transcriptase
The usual flow of genetic information goes from DNA to RNA. Reverse transcriptase runs that arrow backward: it is an enzyme that reads an RNA strand and builds a matching DNA strand from it. The DNA copy of an RNA is called complementary DNA, or cDNA.
The enzyme was discovered in retroviruses, such as HIV, which carry their genes as RNA and must convert them into DNA to integrate into a host's genome — reverse transcriptase is how they do it. Its existence showed that the flow of genetic information is not strictly one-way, an important refinement to the central dogma rather than a contradiction of it.
As a laboratory tool, reverse transcriptase is invaluable because it lets researchers convert RNA into stable, workable DNA. Turning a cell's messenger RNA into cDNA captures exactly which genes were active and yields sequences free of introns, ready for cloning or for measuring gene expression. Pairing reverse transcription with PCR (RT-PCR) is a standard way to detect and quantify specific RNAs, used everywhere from research to diagnostics.
To detect an RNA virus, a test first uses reverse transcriptase to copy the viral RNA into cDNA, which PCR can then amplify and reveal.
Reverse transcription turns fragile RNA into DNA that PCR can amplify.
cDNA made by reverse transcriptase lacks introns because it is copied from already-spliced messenger RNA, which is one reason cDNA, not genomic DNA, is used to express eukaryotic genes in bacteria.