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Chromatin (from Greek chromatos, color) is a set of strands, each formed by a long DNA molecule associated with histone molecules, a special type of protein. These wires are the chromosomes.
When nuclei stained under the light microscope are observed, certain regions of chromatin stain more intensely than others. The ancient cytologists had already observed this fact and rightly imagined that the most colored regions corresponded to portions of the more coiled or more condensed chromosomes than others.
To point out differences between the types of chromatin, the term heterochromatin (from the Greek straight, different), which refers to the most densely rolled chromatin. The rest of the looser chromosomal material was called eucromatin (from the Greek I, true).
Different levels of DNA condensation. (1) Simple strand of DNA. (2) Chromatin filament (DNA with histones). (3) Interphase condensed chromatin with centromeres. (4) Prophase condensed chromatin. (There are now two copies of the DNA molecule.) (5) Metaphase chromosome