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.