Euchromatin vs. Heterochromatin
Euchromatin and Heterochromatin Definitions
What is Euchromatin?
The human body is composed of billions of cells. A typical cell consists of a nucleus and nucleus has chromatin. According to biochemist scientist, the operational definition of chromatin is the DNA, RNA, and protein extracted from eukaryotic lysed interphase nuclei. The loosely packed form of chromatin is called euchromatin. DNA exists in the chromatin form after cell division and becomes loosely packed. Condensation of DNA with histone proteins makes chromatin which exhibits beads on a string like structure. Euchromatin has transcriptionally active sites of the genetic material. Genomic parts which have active genes are loosely packed and allows the transcription of these genes occurs. The frequency of chromosomal crossing over is more in euchromatin and allowing the chromatin DNA to be genetically active. Euchromatin parts in the genome can be observed under the microscope as loops which seem to have 40 to 100 kb regions of DNA in it. Under microscope staining, euchromatin shows light colored bands. The diameter of the chromatin fiber is about 30 nm in euchromatin. Matrix associated regions which have AT-rich DNA are attached to euchromatin loops into the matrix of the nucleus. Euchromatin presence can be seen both in eukaryotic and prokaryotic cells.
What is Heterochromatin?
The tightly packed form of DNA in the nucleus is called heterochromatin. But it is less compact than metaphase DNA. Staining and observation under a light microscope of nondividing cells in the nucleus show two distinct regions which depend on the intensity of the staining. The lightly stained region is called as euchromatin while the dark area is known as heterochromatin. Organization of heterochromatin is more compact in such a way that their DNA is inaccessible to the proteins which are involved in the gene expression. Chromosomal crossing over is avoided by the compact nature of heterochromatin. So, it is considered as transcriptionally and genetically inactive. Functions of the heterochromatin are gene expression and protection of chromosomal integrity. These functions are possible because of dense DNA packing. It is inherited when two daughter cells are divided from a single parent cell, which means newly cloned heterochromatin has same DNA regions which result in epigenetic inheritance. There may be the occurrence of expression of transcribable materials due to the boundary domains. This may lead to the development of different levels of gene expression. Two types of heterochromatin can be identified in the nucleus matric. One is known as constitutive heterochromatin while other is facultative heterochromatin. Constitutive heterochromatin consists of no genes in the genome so it can be retained in its compact structure also during the interphase of the cell. It is a permanent structure of the nucleus of the cell. DNA in the telemetric and centromeric regions belong to the constitutive heterochromatin. Some parts of the chromosomes belong to the constitutive heterochromatin. For example, most of the parts of Y chromosome is a case of constitutional heterochromatin. Facultative heterochromatin has inactive genes in the genome, so it is not a permanent feature of the cell’s nucleus. It can be seen in the nucleus some of the time. Inactive genes of this part may be inactive in some cells or during some periods. When those genes are inactive, they make facultative heterochromatin. Chromatin structures is in the form of beads on a string, 30 nm fiber, active chromosomes in the interphase.