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Chromatin is a complex of DNA and proteins that forms chromosomes within the nucleus of eukaryotic cells. Nuclear DNA does not appear in free linear strands; it is highly condensed and wrapped around nuclear proteins in order to fit inside the nucleus. Chromatin exists in two forms. One form, called euchromatin, is less condensed and can be transcribed. The second form, called heterochromatin, is highly condensed and is typically not transcribed. Under the microscope in its extended form, chromatin looks like beads on a string. The beads are called nucleosomes.
The proteins that bind to the DNA to form eucaryotic chromosomes are traditionally divided into two general classes: the histones and the nonhistone chromosomal proteins. The complex of both classes of protein with the nuclear DNA of eucaryotic cells is known as chromatin.lthough long strings of nucleosomes form on most chromosomal DNA, chromatin in a living cell probably rarely adopts the extended âbeads on a stringâ form. Instead, the nucleosomes are packed on top of one another, generating regular arrays in which the DNA is even more highly condensed.