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Chromatin is the masses of fine fibers comprising the chromosomes in the nucleus of a eukaryotic cell in a nondividing state. During cell division (mitosis or meiosis) the chromatin fibers pull together into thick shortened bodies which are then called chromosomes. Chromatin is present only in cells with a nuclear membrane; it is not found in prokaryotic cells (e.g., bacteria) that lack a nucleus.

Chromatin earned its name from early biologists who examined cells using light microscopes. These scientists found that in cells stained with a basic dye, the granular material in the nucleus turned a bright color. They named this material "chromatin," using the Greek word chroma, which means color. When the chromatin condensed during cell division, the researchers called the resulting structures chromosomes, which means "colored bodies." Chromatin granules which form the chromosomes are known as chromomeres, and these may correspond to genes.

Chemically, chromatin fibers consist of DNA (deoxyribonucleic acid) and two types of proteins found in the cell nucleus (nucleoproteins): histones and nonhistones. The histones are simple proteins found in chromosomes bound to the nucleic acids. Histones may be important in switching off gene action. While all cells of the body contain the same DNA instructions for every type of body cell, the specialized cells do not use all of these instructions. The unneeded DNA is put into storage, by binding with proteins, forming a complex called a nucleosome. Histones link the nucleosomes, forming large pieces of chromatin. DNA contains the genetic material that determines heredity. That chromatin contains DNA is to be expected, since chromosomes are made of chromatin. The compact structure of chromatin chromomeres, where DNA is wrapped around protein balls, is an efficient means of storing long stretches of DNA.

The discovery in 1949 of a condensed X chromosome of sex (termed the Barr body) which was visible at interphase (nondividing) in body cells of female mammals, provided physicians with a new means of determining the genetic sex of hermaphrodites. The sex chromatin test has now given way to direct M chromosomal analysis.

The Barr (X chromosome) is the inactive partner of the two X sex chromosomes in female mammals, including humans. Its dense, compact form led researcher M. F. Lyon to hypothesize in 1962 that it was inactive. In effect, then, both sexes have only one active X chromosome. Males (XY) have the sex chromosomes X and Y, while females (XX) have one functional almost inactive X chromosome. The influence of the inactive X chromosome expression in offspring is known as "lyonization," and is responsible for female tortoiseshell cats.

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