ABSTRACT
The greater part of epigenetic control is aimed at repression or activation of specific genes, and there are several mechanisms by which this may be achieved. The cell nucleus is currently considered to be highly complex and organized compartment in which nuclear components tend to occupy nonrandom positions, leading to precise definition of nuclear architecture concept. The chromatin contained within each chromosome is not randomly distributed in the nucleus but occupies a specific location known as the chromosome territory or domain, a feature that constrains the whole spatial organization inside the nuclear compartment. A well-characterized example of multiple chromatin loops-that is, one created by the interaction of gene regulatory elements-is the nucleolus, in which the ribosomal RNA genes are located in clusters. The mechanisms described in this chapter are the basis of the epigenetic control of gene expression; having understood these concepts, we can proceed to examine how epigenetic control is exerted over diverse cellular functions such as mitosis.
