Heterochromatin: Evolutionary aspects

Abstract

This chapter discusses the evolutionary aspects of heterochromatin. Heterochromatin was first defined as chromatin that remains condensed during interphase and suggested that it is genetically inactive. A criteria used to identify the two subsets of heterochromatin are based on condensation properties. Later interest in heterochromatin extended from cytological to biochemical. Heterochromatin that is located at the same position of homologous chromosomes in all cells as a permanent structural entity is considered “constitutive heterochromatin,” whereas heterochromatin that varies in its state of condensation in different cell types and developmental stages is regarded as “facultative heterochromatin.” The structural organization of heterochromatin has been subject of scrutiny and debate owing to its obscurity. The pericentromeric heterochromatin is associated with DNA sequences that are highly repeated in long tandem arrays and are generally known as “satellite DNA.” Centromeres are composed of very large repeated sequences whose structure is highly complex. The tandemly repeated satellite DNA sequences have been subject of great investigation. The centromeric regions contain highly specific code domains that are highly conserved. The amplification, insertion, and deletion of different sequences of repetitive code domain create a variety of variations, which are highly conserved.