Chromatin Structure and Gene Expression in Eukaryotes

Abstract

Control of gene expression in eukaryotes appears to be exerted, at least in part, through selective transcription. Cytological and biochemical evidence indicates that the synthesis of RNA occurs at selected loci in interphase chromosomes. Thus RNA synthesis occurs in extended euchromatic fibrils and not in condensed, heterochromatic masses. Similar cytological observations tend to correlate RNA synthesis with loops of amphibian lamp-brush chromosomes and with puffs in insect polytene chromosomes. The concept that active and inactive portions of the genome may differ in chemical and physical terms has provided an impetus for the development of biochemical procedures for fractionating interphase upon two overriding considerations. First, it is probable that in many ukaryotic cells, the genome is repressed at any one time. Therefore, biochemical or physical studies of total chromatin refer primarily to the genome in its inactive state so that features characteristic of active chromatin may be obscured. Furthermore, the quite distinct appearance of condensed and extended chromatin on microscopic examination must have a chemical basis. Thus a separation of active from inactive chromatin should allow determination of the chemical elements responsible for both transcriptional activity and distinctive morphological features.