Euchromatin
Encyclopedia
Chromatin
Chromatin is the combination of DNA and proteins that make up the contents of the nucleus of a cell. The primary functions of chromatin are; to package DNA into a smaller volume to fit in the cell, to strengthen the DNA to allow mitosis and meiosis and prevent DNA damage, and to control gene...
(DNA, RNA and protein) that is rich in gene
Gene
A gene is a molecular unit of heredity of a living organism. It is a name given to some stretches of DNA and RNA that code for a type of protein or for an RNA chain that has a function in the organism. Living beings depend on genes, as they specify all proteins and functional RNA chains...
concentration, and is often (but not always) under active transcription
Transcription (genetics)
Transcription is the process of creating a complementary RNA copy of a sequence of DNA. Both RNA and DNA are nucleic acids, which use base pairs of nucleotides as a complementary language that can be converted back and forth from DNA to RNA by the action of the correct enzymes...
. Unlike heterochromatin
Heterochromatin
Heterochromatin is a tightly packed form of DNA, which comes in different varieties. These varieties lie on a continuum between the two extremes of constitutive and facultative heterochromatin...
, it is found in both cells with nuclei (eukaryotes) and cells without nuclei (prokaryotes). Euchromatin comprises the most active portion of the genome within the cell nucleus.
Structure
The structure of euchromatin is reminiscent of an unfolded set of beads along a string, wherein those beads represent nucleosomes. Nucleosomes consist of eight proteins known as histones, with approximately 147 base pairs of DNADNA
Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...
wound around them; in euchromatin, this wrapping is loose so that the raw DNA may be accessed. Each core histone possesses a `tail' structure, which can vary in several ways; it is thought that these variations act as "master control switches," which determine the overall arrangement of the chromatin. In particular, it is believed that the presence of methylated lysine 4 on the histone tails acts as a general marker for euchromatin.
Appearance
In general, euchromatin appears as light-colored bands when stained in G bandingG banding
G-banding is a technique used in cytogenetics to produce a visible karyotype by staining condensed chromosomes.It is useful for identifying various genetic diseases through the photographic representation of the entire chromosome complement. The metaphase chromosomes are treated with trypsin and...
and observed under an optical microscope
Microscope
A microscope is an instrument used to see objects that are too small for the naked eye. The science of investigating small objects using such an instrument is called microscopy...
, in contrast to heterochromatin
Heterochromatin
Heterochromatin is a tightly packed form of DNA, which comes in different varieties. These varieties lie on a continuum between the two extremes of constitutive and facultative heterochromatin...
, which stains darkly. This lighter staining is due to the less compact structure of euchromatin. The basic structure of euchromatin is an elongated, open, 10nm microfibril, as noted by electron microscopy. In prokaryotes, euchromatin is the only form of chromatin present; this indicates that the heterochromatin structure evolved later along with the nucleus
Cell nucleus
In cell biology, the nucleus is a membrane-enclosed organelle found in eukaryotic cells. It contains most of the cell's genetic material, organized as multiple long linear DNA molecules in complex with a large variety of proteins, such as histones, to form chromosomes. The genes within these...
, possibly as a mechanism to handle increasing genome size.
Function
Euchromatin participates in the active transcription of DNA to mRNA products. The unfolded structure allows gene regulatory proteins and RNA polymeraseRNA polymerase
RNA polymerase is an enzyme that produces RNA. In cells, RNAP is needed for constructing RNA chains from DNA genes as templates, a process called transcription. RNA polymerase enzymes are essential to life and are found in all organisms and many viruses...
complexes to bind to the DNA sequence, which can then initiate the transcription process. Not all euchromatin is necessarily transcribed, but in general that which is not is transformed into heterochromatin
Heterochromatin
Heterochromatin is a tightly packed form of DNA, which comes in different varieties. These varieties lie on a continuum between the two extremes of constitutive and facultative heterochromatin...
to protect the genes while they are not in use. There is therefore a direct link to how actively productive a cell is and the amount of euchromatin that can be found in its nucleus. It is thought that the cell uses transformation from euchromatin into heterochromatin as a method of controlling gene expression and replication, since such processes behave differently on densely compacted chromatin, known as the `accessibility hypothesis'. One example of constitutive euchromatin that is 'always turned on' is housekeeping genes, which code for the proteins needed for basic functions of cell survival.