reverse transcriptase
/ ree-VERS tran-SKRIP-tace /
The usual direction of biological information is DNA to RNA — that is transcription. Reverse transcriptase does the opposite: it reads RNA and writes DNA, running transcription backwards. Its discovery was a genuine surprise that forced biologists to refine one of their core principles.
Reverse transcriptase is an enzyme that synthesizes a DNA strand using an RNA strand as the template — an RNA-dependent DNA polymerase. Feed it a messenger RNA and it builds the complementary DNA (cDNA) base by base, A opposite the RNA's U, and so on, just as a normal polymerase would, but reading an RNA tape instead of a DNA one. It comes from retroviruses such as HIV, which use it to convert their RNA genome into DNA that then integrates into the host's chromosome. Some normal cell processes use related enzymes too — telomerase, which extends chromosome ends, is a specialized reverse transcriptase.
Reverse transcriptase is one of molecular biology's most quietly important tools. It is the enzyme that lets you build a cDNA library, capturing a cell's expressed genes as DNA; it is the 'RT' in RT-PCR, used to measure RNA levels by first copying RNA into DNA. Its existence is also a famous lesson about the central dogma: information CAN flow from RNA back to DNA. Crick's central dogma never forbade this — what it ruled out was information flowing out of protein back into nucleic acid. Reverse transcriptase simply added a legal arrow, RNA to DNA, to the map.
Give reverse transcriptase an mRNA reading 5'-AUG CCU-3' and a short primer. It synthesizes a DNA strand complementary to it: 3'-TAC GGA-5'. That single-stranded DNA is the first half of a cDNA; a second enzyme then fills in the partner strand to give double-stranded, clonable cDNA.
Reverse transcriptase copies an RNA template into DNA.
Reverse transcriptase does not violate the central dogma — Crick only forbade information flowing OUT of protein; an RNA-to-DNA arrow was always allowed and is exactly what this enzyme does.