In this article we will discuss about the Heterochromatin and Euchromatin in the Nucleoplasm.
In the nucleoplasm of interphase nucleus a dark staining network is seen which is formed of chromatin. When chromatin is stained by various procedures such as the feulgen reaction which is specific for DNA and examined under light microscope, some regions are stained darkly whereas the other regions are stained lightly.
The chromatin is feulgen positive material observed during the interphase in the nucleus. It is a complex of DNA and proteins. During cell division, chromatin fibres become condensed to form thick ribbon-like structures, the chromosomes.
Certain segments of chromosomes or the entire chromosomes become more condensed than the rest of the sections of the karyotype during interphase and early prophase. Thus the chromatin occurs in two forms; heterochromatin and euchromatin. The condensed ‘regions of the chromosomes are referred to as heterochromatic and the non-condensed segments as euchromatic (Fig. 9.9).
The heterochromatic regions stain darkly whereas the euchromatic regions stain lightly. During mitosis, the heterochromatic regions are localised at the telomeres, the centromeres and intercalary regions of the chromosomes. The bulk of the chromatin is made up of euchromatin. Within a chromosome there may be small areas of dark staining connected by lightly staining regions.
Recently, staining procedures have been developed that result in patterns of darkly and lightly stained regions or bands. The banding patterns are highly specific to each chromosome pair and have been used in unequivocal identification of 23 pairs of chromosomes in man, as well as in a wide range of other organisms.
Although the genetic difference between the heterochromatin and euchromatin is not clear as both contain DNA, the two represent two different states of the same substance. In electron micrographs the two differ in their physical structures.
Heterochromatin is composed of 250 Å fibrils whereas euchromatin contains 30 to 80 Å thick fibrils. So, heterochromatin is a condensed coiled state of chromatin and it contains two or three times more DNA than euchromatin.
There is good evidence that the heterochromatic regions are genetically inactive as they are not involved in synthesis of RNA and proteins.
Heterochromatic regions may stain more strongly or weakly than the euchromatic regions (heteropycnosis) and this differential staining reaction is governed by the degree of coiling of the strands of chromosomes. Where the strands are highly condensed there is greater density of chromatin material and hence darker staining than in non-condensed regions.
Heteropycnesis may be positive where there is over-condensation or may be negative where there is under- condensation. Chromosomes which remain condensed during interphase are called heterochromosomes, as for example, the sex chromosomes of insects. The chromosomes which remain uncondensed during interphase are called euchromosomes.
The Main differences the between heterochromatin and euchromatin are listed below:
1. Heterochromatin stains deeply while euchromatin stains lightly.
2. Heterochromatin is found in the condensed regions of the chromosomes and represents the densely packed regions of chromatin fibrils while the euchromatin is found in the diffused or loosely coiled regions of chromosome.
3. Heterochromatin replicates at the end of S phase of cell cycle while the euchromatin replicates during early stage of S phase.
4. Heterochromatin does not become acetylated whereas euchromatin contains acetyle group in its histones during interphase.
5. Heterochromatin is more labile than euchromatin and is affected by temperature, sex, age, proximity to the centromere, etc.
6. Heterochromatin is relatively inert metabolically and the heterochromatic segments contain a few genes in relation to their length. In Drosophila melanogaster the Y chromosome is totally heterochromatic in nature. Male flies lacking the Y chromosome are phenotypically normal.
They are, however, sterile owing to the failure of normal sperm formation during spermatogenesis. Supernumerary chromosomes of some plants, for example, in maize, are also heterochromatic.
7. DNA of heterochromatin is genetically inactive as it does not transcribe mRNA for protein synthesis. Euchromatin on the other hand, is genetically active and is partly composed of non- repetitive DNA sequences which are genetically active and transcribe mRNA for protein synthesis during interphase.
But recent findings have suggested that heterochromatin also includes some genetically important regions such as nucleolar organisers, genes for some of the RNAs as well as regions containing highly repeated nucleotides.
8. The cross-over frequency in heterochromatic regions of chromosomes is less than in euchromatic regions. This is apparently because the condensed regions of chromosome fibres do not come close together for frequent crossing over. This situation protects vital genes from the effects of crossing over.
Brown (1966) has recognized the following two main types of heterochromatin:
1. Constitutive Heterochromatin:
It is a most common type of heterochromatin seen consistently in the nuclei of all cells of an organism. It is generally found in blocks, usually around areas such as the centromeres, secondary constrictions, telomeres or as bands in other parts of chromosomes. DNA found in the constitutive heterochromatin is most inactive during protein synthesis.
It consists of identical genes or repetitive DNA segments (i.e., their nucleotide sequences are repeated thousands of times). DNA of constitutive heterochromatin replicates readily but does not transcribe mRNA for protein synthesis.
This DNA is late replicating as it fails to replicate during S phase and does so in a brief period just before G2 period of cell cycle. Constitutive heterochromatin provides strength to the centromere and also acts as spacer between vital genes. It also helps in the attachment of chromosomes with the nuclear membrane and provides sites for recognition and pairing of homologous chromosomes during meiosis.
Recently some important genes have been recognised to be present in the constitutive heterochromatin, for example, polygenes which code for rRNA in nucleolus organizer and those making 5 S rRNA and transfer RNA are localised in heterochromatin regions.
2. Facultative Heterochromatin:
It comprises about 2.5% of the genome and is metabolically inactive. It reflects the existence of a regulatory device to adjust the doses of certain genes in the nucleus. In animal cells this type of heterochromatin is acquired during embryogenesis.
It originates through a process called facultative heterochromatisation during which a chromosome or a set of chromosomes becomes heterochromatic (turned off) in cells of one sex while the remaining sets of chromosomes become euchromatic (turned on) in the cells of opposite, sex.
The best known case is that of X chromosomes in mammalian female, one of which is active and remains euchromatic whereas the other X chromosome becomes inactive and forms, sex chromatin or Barr body at interphase. In human, the sex chromatin appears in the embryo between the sixteenth and eighteenth days after fertilization.
Before that time both X chromosomes are euchromatic. In some insects, one of the two X chromosomes may be heterochromatic and the other one euchromatic. In plants, Britten and Kohne (1968) have denied the occurrence of facultative heterochromatin.
