DNA has two distinct functions in the life of the individual and of the species. As Watson and Crick had immediately realised, its double helical structure means that, when unwound, each strand of the helix serves as a template on which the other can be built – resulting in two identical molecules of DNA. This makes possible the transfer of genetic information, coded for in the strands of DNA, from one generation to the next during reproduction. The second function is its central role in the cellular economy. The code written in unique sequences of As, Cs, Gs and Ts is read by the cell via an intermediate, RNA (a molecule similar to DNA), to direct the synthesis of proteins. At the time of the Watson-Crick paper it was assumed that one gene coded for one specific protein. Crick called this the Central Dogma of molecular genetics: ‘DNA makes RNA makes protein makes us’. He described DNA as an informational macromolecule, and saw the flow from DNA to protein as a one-way transfer of information. Again, it was Crick who put it most succinctly: ‘Once information has got into the protein it can’t get out again.’
In the decades following his formulation of the Central Dogma, the elegant clarity of Crick’s slogans became distinctly fuzzy. Not merely were the information flows less unidirectional than he had imagined, but it had become clear that genes coding for proteins only constitute a tiny proportion – less than 2 per cent- of the DNA in the human genome. The remaining 98 per cent of the DNA in the genome was disparagingly referred to by molecular biologists as ‘junk’, as it was thought to have little or no biological function.
So why sequence the entire genome, as the HGP proposed, if much of the sequence would be ‘junk’? The term, however, proved to be seriously misleading, with painful consequences for the hopes of the HGP as “the book of life’, as we explain in the next chapter. But at the time when the HGP was being contemplated, it was the genes, not the junk, which offered clinical hope and raised the possibility of future patents.
(an extract of "Genes, Cells and Brains" by Hilary & Steven Rose)