Chapter 26 - Regulation of Gene Expression

Abstract

This chapter focuses on various mechanisms by which differences in gene expression are achieved. Regulation of gene expression is achieved by regulation of the number of mRNA molecules produced per unit time, regulation of the translation of mRNA, regulation of the number of copies of a gene, and the posttranslational modification. In prokaryotes, mRNA synthesis can be controlled simply by regulating initiation of transcription. Regulation of gene expression in eukaryotes proceeds primarily by control of transcription as in prokaryotes. Some systems are also regulated at the translational level. Synthesis of mRNA in eukaryotes includes several steps in which the primary transcript is converted to mRNA. Control of these processing steps is also used to regulate gene expression in eukaryotes. Various regulatory mechanisms have evolved to ensure a constant supply of housekeeping genes. In some instances, the amount of each protein may be regulated by the strength of the promoter and ribosomal binding site. However, in housekeeping genes with strong promoters, the gene product functions as a repressor and binds to a site adjacent to the gene, thus regulating the level of transcription. If the gene product is in short supply, transcription is activated, and as the concentration of the product increases, the level of transcription is reduced. Gene expression in human cells is regulated primarily at the level of transcription, as it is in prokaryotic cells. However, it can also be regulated at various stages such as initiation of transcription, processing of the primary transcript, and transport of the processed mRNA from the nucleus to the cytoplasm.