Human Chromosomal Q-heterochromatin Regions as a System

The eukaryotic genome consists of the two forms of chromatin: euchromatin and heterochromatin. The DNA of euchromatin contains the gene portion of the genome, while heterochromatin is represented predominantly from non-coding repetitive nucleotide sequences that do not encode proteins or enzymes. In higher eukaryotes, some part of the non-coding, highly repetitive nucleotide sequences were transformed into complex forms of DNA organization as chromosomal constitutive heterochromatin regions. There are two types of constitutive heterochromatin: C- and Q-heterochromatin. C-heterochromatin is found in the chromosomes of all eukaryotic cells, while Q-heterochromatin is found in the karyotype of only three higher primates (Homo sapiens, Gorilla gorilla and Pan troglodytes). Since the discovery of the position effect variegation phenomenon C-heterochromatin has been attributed to gene silencing effects. Dosage compensation of genes is another epigenetic gene silencing mechanism that makes it possible to equalize the level of expression of sex-linked genes in males and females. In mammals, this is done by inactivating one X chromosome in the cells of females using facultative heterochromatin, which is a heterochromatinized euchromatin. However, no epigenetic gene silencing was found in chromosomal Q-heterochromatin regions (Q-HRs). The question is discussed why human chromosomal Q-HRs does not exhibit gene silencing or other epigenetic effects and what their biological role might be.