It seems almost ridiculous to start this chapter by stating that there are two nucleic acids, ribo- and deoxyribo-nucleic acids, RNA and DNA respectively, as everyone knows this. However, I had to start somewhere. DNA is the custodian of all the fundamental data (but not all the information, it's that distinction again) required to construct most biological entities be these E. coli, amoebae, daffodils, elephants or humans. It is the ‘Encyclopaedia Biologica‘ which has the inherent capacity within its structure to be replicated exactly (Watson and Crick, 1953). The retroviruses are the exceptions in which the genetic material is RNA, and these organisms use the DNA of a cell in their replicative process.

Nucleic acid stains

A large number of fluorescent ligands bind to the nucleic acids. Each is fluorescent in its own right but the fluorescence is modulated or enhanced very considerably after binding to RNA or DNA. These fluorescent stains fall into four categories namely, DNA specific, nucleic acid specific, non-specific poly-anion and RNA ‘part-specific’ stains.

DNA specific

The fluorochromes in this category which are most useful in flow cytometry include the antibiotics chromomycin A3, olivomycin and mithramycin, DAPI (4',6-diamidino-2-phenylindole), its analogue DIPI (4',6-bis(2'imidazolinyl- 4H,5H)-2-phenylindole) and the bisbenzimidazole group of dyes which are identified by their Hoechst ‘telephone numbers’.

Chromomycin, olivomycin and mithramycin are similar tricyclic agents. They have the common structure shown in figure 11.1 and they differ in sugar-containing side chains (Ward, Reich and Goldberg, 1965; Kersten, Kersten and Szybalski, 1966).