Another application of the Polymerase Chain Reaction (PCR) is to screen samples for the presence of viruses. For example, do you remember the Ebola outbreak? A blood sample can be sent for PCR based analysis. In this instance, many copies of viral specific DNA sequences are made to cross a detection threshold.Each Ebola virus is composed of a single strand of RNA approximately 18,950 nucleotides long enclosed in a cylindrical viral envelope.In some laboratories, the first step might be to incubate the samples with the enzyme reverse transcriptase. Why might this be?