Epigenetics of heterochromatin

摘要

The term "heterochromatin" was coined by Heitz (1928) to describe certain regions along the longitudinal axis of mitotic chromosomes in a variety of moss species, which displayed differential staining (heteropycnosis) after aceto-carmine application and boiling following his specially devised squashing method. The heterochromatic regions of specific chromosomes or the heterochromatic chromosomes, did not get reorganized at telophase and thus were distinctly identifiable even in the non-dividing nucleus. By contrast, euchromatin ('chromatin proper′) regions were those that got structurally altered at telophase so that their individuality was no more visible in the non-dividing (inter-phase) nucleus. To be sure that the differential staining or heteropycnosis displayed by the heterochromatic chromosomes or their parts on using his special staining method was not an artifact, Heitz (1928) demonstrated the presence of heterochromatin in live cells as well. He concluded, "The cause of heteropycnosis can only lie in the concerned chromosomes themselves". Subsequent identification of comparable heterochromatic regions or chromosomes in Drosophila by Heitz and correlation of genetic maps with specific segments of polytene chromosomes provided the first evidence of heterochromatin being devoid of, or poor in, "genes" (see Zacharias 1995).