Chromatin

Chromatin refers to the DNA (deoxyribonucleic acid) and associated proteins packaged together in the nucleus of a eukaryotic cell in a nondividing state. During cell division (mitosis or meiosis) chromatin masses 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 was named by Walther Flemming in 1882 who examined cells using a light microscope. Flemming 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 condenses during cell division, the resulting structures are called chromosomes, which means “colored bodies.” Chromatin granules that form the chromosomes are known as chromomeres.

Chemically, chromatin fibers consist of DNA and several types of proteins found in the cell nucleus (nucleoproteins): core histones, histone H1 and scaffold proteins. The core nucleosome is comprised of a segment of DNA wrapped around an octomer of histones forming ball-like strucutres. The nucleosomes are found in a “beads on a string” formation, with 20 to 60 base pairs of “linker DNA” between nucleosomes. This formation optimises access to DNA for transcription.

The beads on a string formation can be further coiled into a filament known as a 30nm chromatin fiber which is more compact and is found in DNA that is less actively being transcribed. However, the 30nm fiber can easily be unwound to the beads on a string structure for transcription. The compact structure of chromomeres is an efficient means of storing long stretches of DNA. During meiosis, chromatin further coils onto scaffold proteins and forms the classic chromosomes known from karyotypes. These structures are optimised to withstand the physical forces imposed on the DNA during replication and division.

Chromatin is thought to have a very well organized layout in the nucleus. Certain strands of chromatin are always found in specific locations. Regions of DNA that are not transcribed clump together in the nucleolus. In 1949 a condensed X chromosome (termed the Barr body), which is visible at interphase (nondividing periods) in the body cells of female mammals was first observed. Its dense, compact form led researcher M. F. Lyon to hypothesize in 1962 that it was inactive. It has since been verified that the Barr body is inactive, preventing females from transcribing double the genes found on X chromosomes. The influence of the inactive X chromosome expression in off-spring is known as “lyonization.”

See also Eukaryotae; Nucleus, cellular.

Chromatin

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 .

See also Eukaryotae; Nucleus, cellular.

Chromatin

Chromatin are ropelike fibers containing deoxyribonucleic acid (DNA) and proteins that are found in the cell nucleus and that contract into a chromosome just before cell division. In its unraveled state, chromatin look like beads on a string. In its condensed state, they fold into tight loops that coil up and form x-shaped chromosomes.

When scientists first became able to examine the cell under a microscope using stains to distinguish among its many parts, they noticed that a particular granular material inside the nucleus became more brightly colored by the stain than did other structures. These colored granular structures were named chromatin as derived from the Greek word khroma meaning "color." At much higher magnification however, it was discovered that chromatin was not granular but was much more threadlike, with proteins attached to it like beads on a chain. It was soon discovered that in every cell that is not about to actually divide, the cell's genetic material floats about the nucleus as unwound, extremely fine threads or strings called chromatin. In human cells, there are forty-six strands of chromatin forming a tangled mass that has been described as "a bowl of microscopic spaghetti." When a single "noodle" or strand of this mass is examined more closely, it is seen as a coil made up of another compactly folded strand of material which itself is made up a series of loops that are coiled around protein molecules called "histones." It is within these loops that the "twisted ladder" or double helix structure of DNA is found.

Just before a cell is about to divide, this apparently tangled mass of forty-six strands of chromatin begins to condense or gather together to form forty-six easily recognizable, x-shaped packages of genetic information called chromosomes. One of the main purposes of a chromosome is to package the DNA into tight coils so that it all fits into the nucleus. Each chromosome then makes a copy of itself and splits apart, dividing into two identical new cells. In the new cell, the condensed chromosome unravels into its earlier state, containing all the instructions needed to make the cell work and ready to pass on genetic material to the next generation.

[See alsoCell; Cell Division; Chromosome; DNA; Nucleus; Mitosis; Protein ]

chromatin The substance of which eukaryotic chromosomes are composed. It consists of proteins (principally histones), DNA, and small amounts of RNA. The DNA molecule is wrapped around the histones to form a series of linked globular nucleosomes, resembling beads on a string. This is itself coiled to form a highly condensed solenoid arrangement, constituting a form of chromatin called heterochromatin, which stains densely with basic stains. The genes in the solenoid can only be transcribed if the solenoid unfolds to some extent, forming an expanded chromatin (euchromatin), which is lighter staining. The degree of condensation in any particular region is regulated by reversible acetylation of the histones: the greater the degree of acetylation, the less condensed the chromatin, and hence the greater the availability of genes for transcription.

chromatin The substance of chromosomes, which includes DNA, chromosomal proteins, and chromosomal RNA. Chromatin stains strongly with basic dyes. It is thought that the chromatin is most deeply stained when it is most condensed and inactive.