reverse transcription

Meaning: Reverse transcription is the synthesis of DNA from an RNA template, catalyzed by reverse transcriptase (an RNA-dependent DNA polymerase). It runs opposite to the usual DNA → RNA direction of transcription. Retroviruses such as HIV carry an RNA genome. After entering a host cell they make a DNA copy, which integrase inserts into the host chromosome as a provirus. Like other DNA polymerases, the enzyme adds dNTPs to a primer in the 5′ → 3′ direction. It lacks proofreading, so retroviruses mutate quickly.

On the MCAT: Passages on HIV ask for the order of events: viral RNA → DNA → integration → host transcription and translation of viral genes. In the lab, RT-PCR uses reverse transcriptase to make cDNA from mRNA. Because mRNA has already been spliced, cDNA has no introns, which makes it useful for expressing eukaryotic genes in bacteria. Telomerase also works as a reverse transcriptase, using its own built-in RNA template to extend telomeres.

Common confusion: Students call it a violation of the central dogma or mix it up with transcription. The template is RNA, and the substrates are dNTPs, not NTPs. Also, not every RNA virus uses it: many use RNA-dependent RNA polymerase to copy RNA into RNA.