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?