Several levels of chromatin organization exist. Cytologically, chromatin classes may be distinguished by appearance when stained for light microscopy. Heterochromatin remains condensed throughout the cell cycle; microscopically, it looks “clumped” in the nucleus. Heterochromatin sequences are not transcribed. Euchromatin refers to the chromatin that appears less condensed in the microscope. Transcribed genes are found in euchromatin. Two types of heterochromatin exist. Facultative heterochromatin is present in the nucleus of some cells, but not in all. For example, in animals, the genes encoding β‐globin are condensed in cells that are not precursors to blood cells. The term constitutive heterochromatin refers to DNA sequences that are condensed in all cells of an organism. Constitutive heterochromatin is associated with highly reiterated DNA. The DNA in centromeres and telomeres (the parts of the chromosome that attach to the mitotic spindle and the ends, respectively) is found in constitutive heterochromatin.
Chromatin structure is organized at several levels. The basic structure of chromatin—either heterochromatin or euchromatin—is called the nucleosome. The nucleosome is a complex of 146 base pairs of DNA, wound in two turns around the outside of a disk‐like complex of eight proteins (called histones). The histone core contains two copies each of four histones, H2A, H2B, H3, and H4. The histone octamer is wrapped by very close to two turns of DNA. Linker DNA and another histone (H1) join together the nucleosomes (about 65 base pairs' worth). H1 binds cooperatively to nucleosomes, so that a gene can be “zipped up” all at once by the binding of many H1 molecules successively. See Figure 1.
Nucleosomes are packed into more compact structures. First, the nucleosome fiber is condensed in a helix‐like array, called the 30 nm fiber (nm refers to a nanometer, which is 10 ‐9 meters). Scaffolding proteins hold the chromatin domains together. For active genes, these domains form loops, with the active sequences on the brushy part of the loops, and the regions between genes attached to the scaffold. Nonhistone proteins regulate the condensation and decondensation of chromatin. Again, active genes (those that are being transcribed) are present in the less‐condensed portions of the chromatin.