ABSTRACT
Having established that the RNA and protein content of different cell types shows considerable variation, the fundamental dogma of molecular biology, in which DNA makes RNA which in turn makes protein, directs us to consider whether such variation is caused by differences in the DNA present in each tissue. Thus, in theory, DNA which corresponded to an RNA required in one particular tissue only might be discarded in all other tissues. Alternatively, it might be activated in the tissue where RNA was required, by a selective increase in its copy number, in the genome via an amplification event or by some rearrangement of the DNA necessary for its activation. The possible use of each of these mechanisms (DNA loss, amplification, or rearrangement) will be discussed in turn (for a review see Brown, 1981).
