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In 1970, two  investigators Temin and Baltimore, independently discovered reverse transcriptase, an enzyme used by retroviruses.  Retroviruses store their genetic information in RNA, but reproduce by  reverse transcribing their RNA into DNA which is then inserted in the chromosome of the infected cell.  When the infected cell replicates, so does the retrovirus.  Temin and Baltimore shared a Nobel prize in 1975 for discovering the enzyme that controls reverse transcription.

Although some new technologies can measure RNA molecules directly, most RNA measurements are taken by extracting RNA from the tissue sample and doing a cycle of reverse transcription PCR (RT-PCR) to synthesize complementary DNA (cDNA). Then ordinary PCR can be applied to the sample.  This is handy for a number of reasons - cDNA is more stable than RNA; primers can be used to select the RNAs of interest (often mRNA with poly-A tails or precursors of ncRNAs, or ribosomal RNA for metagenomics).  Most importantly, any chemical process or measurement device that can be used with DNA can be used with cDNA and so with RNA.