euchromatin and heterochromatin

Euchromatin and Heterochromatin

The material of the chromosome is the chromatin. Depending on their staining properties, the two types of chromatin may be found in the prophase nucleus.

When the chromosomes are stained with stains like acetocarmine or feulgen at prophase difference in staining is observed i,e some regions with dark stain called heterochromatin and some regions with light stain called euchromatin.

Euchromatin: 

Portions of chromosomes that stain lightly are only partially condensed. It represents most of the chromatin that disperse after mitosis has completed. Euchromatin contains structural genes which replicate and transcribe during G1 AND S-phage of interphase. The euchromatin is considered genetically active chromatin, since it has a role in the phenotype expression of the genes. In euchromatin DNA is found packed in three to eight nm fibre.

Heterochromatin:

The portion of chromosomes that stain dark are remain in the condensed state. Heterochromatic region are constituted into three structures namely

1. Chromomeres
2. Chromocenters and
3. Knobs

1)Chromomeres:These are regular feautures of all prophase chromosomes. They are large enough to reveal them, but their number, size, distribution and arrangement are specific for a particular species at a particular stage of development.

2) Chromocentres: These are also heterochromatic regions of varying size which occur in the centromeres in proximal regions of chromosome arms. Many chromocentres can be resolved into strings of chromomeres which are larger than chromeres found in distal regions. In some Dipteran salivary glands, the chromocentres of different chromosomes fuse to form a large chromocentre. The relative distribution of chromocentres are some times considered as evolutionary values.

3) Knob: These are spherical heterochromatin bodies which may have a diameter equal to the chromosomes width but may reach the size several times larger than width of chromosome markers for distinguishing chromosomes of related species and races.

Types of Heterochromatin

1. Constitutive Heterochromatin: In such as heterochromatin the DNA is permanently inactive and remains in the condensed state throught the cell cycle. This type occurs around the centromere, in telomeres and c-bands of chromosome. In Drosophila it appears fourty percent of chromosome around the centromere hence called pericentromeric heterochromatin. In many species, entire chromosome become heterochromatic and are called B-Chromosome or Satellite Chromosome or Accessory Chromosome and are very minor biological roles. This heterochromatin has short repeated sequences of DNA called satellite DNA.
2. Facultative Heterochromatin: Such type of heterochromatin is not permanently maintained in the condensed state. It undergoes periodic dispersal and active in transcription. Frequently, one of the period chromosome becomes either totally or partially heterochromatic. The best example is X-chromosome of mammalian female i,e one X-chromosome remains heterochromation called Barr Body and other X- chromosome remains euchromatin.