In this article we will discuss about the heterochromatin and euchromatin in eukaryotic chromosomes.
The terms “heterochromatin” and “euchromatin” were given by Heitz in 1928-29, although they had been discovered much earlier. Heterochromatic blocks observed during interphase were earlier termed as pro-chromosomes. The substance of which eukaryotic chromosomes are composed is known as chromatin; it contains DNA, protein and a small amount of RNA.
During interphase most of the chromatin is in diffuse (de-coiled) state, but some segments are visible because of their condensed or coiled state.
The condensed segments stain deeply during interphase; this phenomenon is called positive heteropycnosis in contrast, the phenomenon of negative heteropycnosis denotes the absence of condensation, hence lack of or poor staining, in certain chromosome parts during cell division (especially during prophase and metaphase), when the rest of the chromosome is highly condensed.
Thus the chromatin that follows the normal coiling and de-coiling cycle is called euchromatin whereas the chromatin that deviates from the normal is called heterochromatin. The heterochromatic regions take more stain (dark stained) than euchromatic regions. Pachytene is the most suitable stage for locating the heterochromatic regions.
There are three kinds of heterochromatic regions in the chromosomes observed during interphase and prophase stages:
(i) Chromomeres,
(ii) Chromocentres and
(iii) Knobs.
Chromocentres are the heterochromatic regions which occur near the centromeres. Dipteran salivary gland cells contain one large chromo-centre formed by the fusion of the chromocentres of all the chromosomes present in the cell. Knobs are spherical and heterochromatic structures and are observed more clearly in some species, such as, maize, during pachytene.
Heterochromatin is classified into the following two types:
(1) Constitutive heterochromatin:
It forms a permanent structural characteristic of a particular chromosome and it does not revert to euchromatin. Examples of this type of heterochromatin occur in the centromeric and telomeric regions. Constitutive heterochromatin contains repetitive DNA, and non-repetitive A-T rich main band DNA.
Satellite DNA is also localized in the centromeric heterochromatin. Repetitive DNA contains many to a million copies of base sequences each of which is few to hundreds of base pairs in length.
(2) Facultative heterochromatin:
It represents the inactivated and condensed segments of euchromatin; it is expressed under certain conditions. Heterochromatinization of one of the two X chromosomes of human females is a common example of facultative heterochromatin. This type of heterochromatin can revert back to euchromatin and thus it is an important means of genetic regulation.
Euchromatin is known to contain genes which are active, whereas, the genes located in heterochromatic regions are repressed. This inactivity of the genes is chiefly due to the highly condensed state of the chromatin.
Replication of heterochromatin occurs late in the S-phase. Certain genes have been located in the heterochromatic regions of Drosophila and tomato. The Y chromosome of Drosophila is heterochromatic but it carries the gene for bobbed bristles (bb).
The Y chromosome, which is heterochromatic, is also necessary for male fertility in the fly. Cytological observations have revealed that a part of the Y chromosome becomes euchromatic in the spermatocytes. Lima-de-Faria in 1969 reported the occurrence of gene amplification for ribosomal cistrons in the heterochromatic DNA body of Acheta domesticus (house cricket).
The heterochromatic regions contain more DNA as compared to the euchromatin and, therefore, they must contain more genes than euchromatic regions of the same size. In the intact interphase lymphocyte nuclei, Frenster and coworkers in 1963 found that DNA content was 74% in heterochromatin and 13% in euchromatin.
Specific template activity of heterochromatin fraction was 26 and 28% for DNA and RNA syntheses, respectively, while that of euchromatin fraction was 400 and 470%, for DNA and RNA syntheses, respectively.
Thus most of the newly synthesized RNA and DNA were localized with in the euchromatin fraction. It has been found that gross differences do not exist between DNA from euchromatin and heterochromatin with respect to the base composition.