MEDICAL BIOLOGY, ANATOMY, HUMAN PHYSIOLOGY AND PATHOLOGY - Ya.I. Fedoniuk 2010

BIOLOGY

SECTION 1. BIOLOGICAL FOUNDATIONS OF HUMAN VITAL ACTIVITY

1.3. MOLECULAR-GENETIC AND CELLULAR LEVELS OF LIFE ORGANIZATION

1.3.2. Structural and chemical functional organization of eukaryotic cells

Chromatin: structural Organization, euchromatin and heterochromatin

Chromatin is a complex of DNA and Proteins—deoxyribonucleoprotein—which represents the interphase form of chromosome existence. Chromosomes themselves are visible only during Cell Division. The chemical composition of chromatin and chromosomes is identical. At the ultrastructural level, chromatin consists of chromosomal fibrils (threads) 20-25 nm thick, which, in turn, are built from 10 nm thick fibrils. The Structural organization of these fibrils is based on nucleosomes (Fig. 1.35).

Interphase nuclear chromatin is subdivided into euchromatin and heterochromatin, which differ in their degree of coiling and genetic activity. Euchromatin comprises decondensed chromosome regions during interphase. This is genetically active chromatin containing structural genes where METABOLISM/31.html">Transcription takes place. Euchromatin does not take up stains and is not visible under a Light Microscope. With the onset of mitosis, all euchromatin condenses within the chromosomes and becomes visible. Heterochromatin represents highly coiled and condensed chromosome regions that remain in a genetically inactive state (non-transcribed). Heterochromatin is divided into structural and facultative types. Structural (constitutive) heterochromatin consists of chromosome regions that remain coiled throughout the entire Cell Cycle (both interphase and mitosis). It consistently occupies identical regions in both homologous chromosomes—near centromeres, secondary constrictions, and at chromosome ends (telomeres). This chromatin intensely takes up stains and is visible under a light microscope. Facultative heterochromatin is localized in one of the two homologous chromosomes. In certain cases, one of them undergoes complete heterochromatinization. An example of facultative chromatin is X-chromatin, or Barr body.



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