Abstract
In this chapter, principles of gene expression are discussed. An essential condition that a gene can be expressed, i.e., transcribed into RNA, is that its regulatory regions, such as transcription start sites (TSSs) and enhancers, are located within euchromatin. Chromatin is the physical representation of epigenetics . In a tissue- and cell type-specific fashion the majority of the genome is in heterochromatin and only approximately half of all genes are expressed, i.e., transcribed. The wrapping of genomic DNA around nucleosomes and the posttranslational modification of histones determine the density of chromatin packing. Tissue- and signal-specific gene expression is the central mechanism to control the general properties of a cell and its response to environmental perturbations. Large protein complexes that are formed by transcription factors, polymerases and other nuclear non-histone proteins organize the 3-dimensional (3D) architecture of the chromatin into functional units being used for most efficiently coordinated gene expression.