The central dogma of biology, a term coined by Francis Crick, is that DNA makes RNA and RNA makes protein. It is self-evident, therefore, that the capacity simultaneously to measure DNA, RNA and protein at the individual cell level is of fundamental importance. We have seen in the previous chapter that DNA and RNA can be measured simultaneously. However, those measurements were of total DNA and RNA and ideally we would also wish to be able to measure specific gene copy number, messenger RNA (mRNA) and the protein product at the single cell level. The last of these can be quantitated with ease by flow cytometry using monoclonal antibodies and developments are taking place for the detection and measurement of specific genes with in situ hybridization in whole nuclei (Trask et al., 1985,1988). Furthermore, some recent work has enabled ribosomal RNA to be detected in suspended cells using hybridization (Bauman and Bentvelzen, 1988) and Dunne, Thomas and Lee (1989) have sorted small numbers of tells directly onto nitrocellulose filters then probed for interleukin-1 mRNA. However, specific mRNA and copy number cannot yet be quantitated, but I suspect that this will be possible within the next five years or so with improvements in hybridization techniques, the use of nick-translation with biotinolated nucleotides, streptavidin fluorochrome amplification and improvements in instrumentation.

The simultaneous staining of DNA with propidium iodide and protein with fluorescein, either directly (FITC) or indirectly with antibodies, has been mentioned in sections 3.8.3, 10.3 and 11.1.2 and application of this particular combination will be considered further in sections 12.3, 12.4 and chapter 15.