The cloning of expressed genes and the polymerase chain reaction (PCR), two biotechnological breakthroughs of the 1970s and 1980s, continue to play significant roles in science today. Both technologies give researchers the means to make more DNA, but they do so in different ways. In particular, cloning involves the synthesis of DNA from mRNA using an enzyme called reverse transcriptase. Although this method reverses the flow of genetic information as described by the central dogma, it effectively mimics the process by which RNA viruses “flip” the direction of transcription in their host cells, thereby causing these cells to manufacture viral DNA even though the viruses themselves contain only RNA. In contrast, the polymerase chain reaction does not involve the use of an initial mRNA template to manufacture DNA. Rather, PCR involves the synthesis of multiple copies of specific DNA fragments using an enzyme known as DNA polymerase. This method allows for the creation of literally billions of DNA molecules within a matter of hours, making it much more efficient than the cloning of expressed genes. However, cloning remains the go-to method for researchers when only the mRNA template (and not the DNA template) of a sequence of interest is available.
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