reverse-transcription PCR
/ RT-PCR /
PCR copies DNA, but a lot of what we want to measure is RNA — the messenger molecules a cell makes when a gene is switched on, or the genome of an RNA virus. You cannot put RNA straight into a normal PCR. The trick is to first turn the RNA into DNA, then amplify that DNA. That two-stage method is reverse-transcription PCR.
The first stage uses an enzyme called reverse transcriptase, which reads an RNA template and writes a complementary DNA strand (called cDNA) — the reverse of the usual DNA-to-RNA direction, which is where the name comes from. Once the RNA has been copied into cDNA, ordinary PCR takes over and amplifies the cDNA with primers and Taq just as it would any DNA. So RT-PCR is really two enzymes working in sequence: reverse transcriptase to make the DNA copy, then a DNA polymerase to multiply it.
RT-PCR is everywhere in modern biology and medicine. It is how you detect an RNA virus — the 'PCR test' for SARS-CoV-2 is really an RT step followed by (quantitative) PCR. It is how you check whether a particular gene is being expressed, since active genes produce mRNA. One important point of confusion: 'RT-PCR' (reverse-transcription PCR, for RNA) and 'real-time PCR' (quantitative PCR, which watches product form as it goes) are different ideas that are often combined — and the abbreviations look alike, which trips people up constantly.
A clinical 'PCR test' for an RNA virus first uses reverse transcriptase to copy the viral RNA from a swab into cDNA, then amplifies that cDNA. No viral RNA means no cDNA, no amplification, and a negative result.
RNA is first copied to cDNA, then amplified by ordinary PCR.
Reverse transcription does not violate the central dogma — Crick's rule never forbade RNA-to-DNA; it forbade information flowing out of protein back into nucleic acid. Also beware: 'RT-PCR' (reverse transcription) and 'real-time PCR' (quantitative) are different things often used together.