Chromosomal Proteins: Their Role in the Regulation of Gene Expression

Abstract

It has been known for some time that the genetic information of eucaryotic cells is contained within the nucleus and encoded in the nucleotide sequences of the DNA double helix. Genetic information is made functionally available via the synthesis of complementary messenger RNA molecules, which are transferred to the cytoplasm where they serve as templates for the synthesis of proteins. Our concept of gene regulation in eucaryotic cells is largely predicated upon three important observations. First, the similarity of DNA in all nuclei of an organism is quantitative as well as qualitative. Thus, each cell possesses a complete and identical set of genetic information. (The haploid gametes of plants and animals constitute an exception to this rule, with half the amount of DNA found in diploid nuclei.) Second, the availability of genetic information for transcription is restricted. In differentiated eucaryotic cells only 5 to 20% of the genome is transcribable at any time, and the specific genetic sequences expressed are different in each cell type, reflecting the metabolic requirements of the cell. Third, cells are able to modulate gene expression in response to specific demands. Such modifications in gene readout occur during development and differentiation, during the cell cycle, and in response to hormones; in general they provide a cell with the flexibility required to deal with changes in the intracellular and extracellular environment.