The Crystallization and Selection of Dynamical Order in the Evolution of Metazoan Gene Regulation

Abstract

A higher metazoan such as man contains sufficient DNA per nucleus to encode on the order of 2,000,000 average size proteins [1–3]. A very large fraction of that total may not be codonic, nevertheless, the complexity of RNA transcript sequences now detectable in the nuclei of higher plant and animal cells is on the order of 20,000 to 100,000 distinct sequences. A central dogma of developmental biology is that, with unusual exceptions, the genetic constitution of each cell in a higher plant or animal is essentially identical, and therefore that differentiation of the zygote into an array of distinct cell types is associated with alternative, coordinated patterns of gene expression; each cell type is thought of as associated with a delimited set of expressed genes. Increasingly refined data are coming available both with respect to the regulation of gene expression and the overlap in expression in different cell types of one organism. Expression appears to be modulated at a variety of levels: transcription itself, processing the primary transcript by splicing, capping, and polyadenylation, transporting the mature message from the nucleus, translation to protein, subsequent secondary modifications of the protein [1]. With respect to the overlap of gene expression patterns in different cell types the observations depend somewhat upon whether consideration is restricted to cytoplasmic RNA, or to total nuclear plus cytoplasmic transcripts. For total transcript complexity, the central observations are that a very large fraction of sequences appear to be ubiquitously transcribed in all cell types of the organism.