Overview: What Is Gene Expression?
The human genome (see Box 1.1) is composed of some 20,000 protein-coding genes and approximately the same number of genes for non-coding RNAs (ncRNAs), being used as a structural backbone of ribosomes (ribosomal RNA (rRNAs)), adaptor molecules in protein translation (transfer RNA (tRNAs)) and regulators of mRNA stability, protein translation and chromatin density (long non-coding (nc) RNAs and micro RNAs (miRNAs), for more details see Sect. 13.2). A gene is considered as “expressed”, when it is transcribed into RNA. In a given tissue or cell type only approximately half of all genes are transcribed. Moreover, the phenotype of every tissue is defined by its own characteristic pattern of transcribed genes, which changes, when the cells are exposed to a signal, such as a dietary or a stress molecule.
Tissue- and signal-specific gene expression is the central mechanism to control the general properties of a cell and its response to environmental perturbations. The latter applies both to healthy and disease conditions. A detailed insight into gene expression therefore bears the potential not only for an understanding of the dysregulation in disease states but also for their therapeutic treatment.
In this chapter, we will provide an overview of the central features of gene expression. We will describe that gene expression is tightly controlled, in particular on the level of gene transcription. There are at least three levels of control, which are the DNA code, the epigenetic code and the transcription factor program.