Structural Organization of Heterochromatin in the Human Genome

Abstract

This chapter discusses the structural organization of heterochromatin in the human genome. Heterochromatin represents specific portions of the sex chromosomes as well as autosomes. Heterochromatin can be defined as darkly stained genetic material that is late replicating and genetically inactive and remains in a condensed state or heteropycnotic throughout the interphase. Heterochromatin is subclassified into two types: constitutive heterochromatin and facultative heterochromatin. The constitutive heterochromatin is present in all cells at identical positions on both homologous chromosomes and forms a permanent structural characteristics of a given chromosome pair. The facultative heterochromatin varies in its state in different cell types, at different developmental stages, or even from one homologous chromosome to another, that is, it is heteropycnotic in special cell types or at special stages. In 1949, Barr and Bertram noted a heteropycnotic element in the nuclei of the nerve cells of female cats that was absent in male cats. This has been referred to as sex-chromatin or a Barr body. In the somatic cells of mammals, only one X chromosome is active, while the other one is condensed and appears as sex chromatin in interphase. The inactive X chromosome is a classic example of facultative heterochromatinization.