Euchromatin and Heterochromatin

• When chromosomes are stained with stains like acetocarmine or feulgen (basic fuchsin) at prophase/interphse, a linear differentiation into regions having dark stain and those having light stain becomes conspicuous.

• In 1928 Emil Heitz and other cytologists studied this aspect. The darkly stained regions were called heterochromatic and light regions were called euchromatic

Euchromatin-is lightly stained due to the less compact structure.

- is loosely-packed in loops of 30-nm fibers

- early-replicating and GC rich region.

- it should be noted that in prokaryotes, euchromatin is the only form of chromatin present.

- euchromatin participates in the active transcription of DNA to mRNA products.

Euchromatin

- One example of constitutive euchromatin that is 'always turned on' is housekeeping genes, which codes for the proteins needed for basic functions of cell survival.

Heterochromatin- condensed chromatin and genetically inactive (limited transcription) during interphase.

- it consists of repetitive DNA sequences which are relatively rich in AT base pairs and is late replicating in the cell cycle.

- heterochromatic regions of chromosomes are generally referred to as positively heteropycnosis (pycnosis = dense) by cytologist.

- is darkly stained.

-Heterochromatin is believed to serve several functions, from gene regulation to the protection of the integrity of chromosomes.

Heterochromatin- heterochromatin is usually localized to the periphery of the nucleus.

HeterochromatinThose genes present in heterochromatin are generally inactive; that is, not transcribed and show :

• increased methylation of the cytosines in CpG islands of the DNA

• decreased acetylation of histones and • increased methylation of lysine-9 in histone H3, which

now provides a binding site for heterochromatin protein 1 (HP1), which blocks access by the transcription factors needed for gene transcription.

• increased methylation of lysine-27 in histone H3 (H3K27).

- there are two types of heterochromatin:

Constitutive heterochromatin and

Facultative heterochromatin.

Constitutive Heterochromatin

- constitutive heterochromatin is present at identical positions on all chromosomes in all cell types of an organism.

- any genes contained within the constitutive heterochromatin will be poorly expressed.

- all human chromosomes 1, 9, 16, and the Y chromosome contain large regions of constitutive heterochromatin.

- in most organisms, constitutive heterochromatin occurs around the chromosome centromere and near telomeres.

Facultative Heterochromatin

- facultative heterochromatin is variable in its expression. It varies with the cell type and may be manifested as condensed, or heavily stained, chromatin in only certain differentiated somatic cells in the same organism.

- an example of facultative heterochromatin is X-chromosome inactivation in female mammals: one X chromosome is packaged in facultative heterochromatin and silenced, while the other X chromosome is packaged in euchromatin and expressed.

Heterochromatin

• Heterochromatic regions are constituted into three structures namely chromomeres, chromocenters and knobs.

• Chromomeres are regular features of all prophase chromosomes, large enough to reveal them, but their number, size, distribution and arrangement are specific for a particular species at a particular stage of development.

• Chromomere : beadlike granules of chromatin that constitutes a chromosome during cell division.

• Chromomeres are visible during meiotic prophase (pachytene) and invisible in mitotic metaphase chromosomes

• Chromomeres are regions of tightly folded DNA.

• Chromomeres of single chromosome show considerable variation in size.

• They may differ in size as in the case of maize or they may be

of uniform size as in the case of rye.

• Chromocenters are heterochromatic regions of varying size which occur near the centromeres in proximal regions of chromosome arms.

• At mid-prophase, many chromocentres can be resolved into strings of chromomeres, which are larger than chromomeres found in distal regions.

• In some dipteran salivary glands, the chromocentres of different chromosomes fuse to form a large chromocentre.

• Knobs are spherical heterochromatin bodies which may have a diameter equal to the chromosome width but may reach a size having a diameter which is several times the width of the chromosome.

• Very distinct chromosome knobs can be observed in maize at pachytene stage.

• Knobs are valuable chromosome markers for distinguishing chromosomes of related species and races.