For detecting RNA and DNA of microorganisms (bacteria and viruses), polymerase chain reaction (PCR) is used. It consists in amplification (reproducing) of a selected fragment of DNA coming from the microorganism being searched using in vitro conditions. In the case of diagnostics of mutations causing hereditary diseases, the DNA being searched is a genome DNA (present in cells) of the tested person.
The commencement of PCR reaction is enabled by starters; they are short (usually 18-24 nucleotides long), single-stranded fragments of DNA which are combined with an complementary fragment of the DNA being searched. As a result of the reaction, a product (fragment of DNA) with characteristic length identified by electrophoretic distribution (in electrical field) is created and observed after dyeing in ultraviolet light.
The PCR reaction has many variants which found their application in diagnostics of infectious diseases. One of them is RT-PCR (reverse transcription PCR) allowing detection of viruses the genetic material of which is RNA (e.g. HIV or hepatitis C virus).