[Biology Class Notes] on Difference Between Euchromatin and Heterochromatin Pdf

Around 3 billion base pairs of nucleotides are available in the human genome. There is a linear sequential arrangement of these nucleotides along DNA. This encodes every Protein and genetic trait in the human body. DNA sequencing or genetic sequencing is important for normal cell function and is highlighted when the anomalies go undetected by intrinsic genetic repair mechanisms and thus resulting in dysfunctional Proteins and various disease states.

 

The DNA sequence is maintained through a series of processes and is condensed into 46 Chromosomes in Humans. The number of Chromosomes varies for every species. These Chromosomes undergo further condensation through two ways called mitosis or meiosis. On the other hand, interphase Chromosomes also undergo a series of events like DNA folding, wrapping, and bending which are facilitated by Histones. The combination of DNA and Histone Proteins in the nuclear matter is termed as Chromatin.

 

Chromatin consists of 1147 base pairs of DNA wrapped around the Protein core histone. The histone is made of 2 units of H2A, H2B, H3, and H4 forming an octamer.

The chromatin in the interphase is generally classified into two parts:

• Euchromatin

• Heterochromatin

 

Euchromatin

A region in which DNA is accessible and is present in an open confrontation because of the relaxed state of Nucleosome arrangements is referred to as Euchromatin.

 

Euchromatin is associated with the presence of high levels of proteins in the chromatin. In other words, Euchromatin is made up of histones and protamines. Histones are a group of DNA binding proteins. They play a vital role in regulating the process of gene transcription and thus play an important role in the maintenance of chromosomal organization and cellular function. Histone also maintains the condensation of the chromatin fiber in the nucleus. Protamines are a sub-family of the histone proteins and play an important role in regulating the transition of the chromatin into the Euchromatin.

Euchromatin and Heterochromatin are two structural units that help in maintaining the condensation of the chromatin. There are also other structural units called facultative Heterochromatin and constitutive Heterochromatin.

 

Structure of Euchromatin

Euchromatin majorly has unmethylated first gene exons. They exist in decondensed form and are present in the distal arms of the Chromosome. Euchromatin is spread all around the nucleus and is replicated during the whole S Phase. It is generally known as the transcriptionally active form of chromatin. Euchromatin has less compact structure and is usually referred to as 11 nm fiber with the presence of beads on a string. The beads represent nucleosomes and string refers to DNA.

 

Functions of Euchromatin

The chromatin which is involved in the active transcription of DNA into mRNA is Euchromatin.  As Euchromatin is more open in order to allow the recruitment of RNA polymerase complexes and gene regulatory Proteins, transcription can be initiated.

 

Heterochromatin

A functionally different genomic compartment which has relatively low gene density along with a highly compact chromatin structure is referred to as Heterochromatin.

 

There are two kinds of Heterochromatin: ‘Constitutive Heterochromatin’ is virtually present in all stages of an organism’s life cycle. ‘Facultative Heterochromatin’ occurs in one of a pair of homologs. Heterochromatin can epigenetically administer the expression of nearby genes resulting in varied phenotypes in genetically identical cells.

 

Biochemical and genetic approaches show that the RNAi machinery plays an important role in the formation of Heterochromatin.

 

Heterochromatin is the opposite of Euchromatin. Euchromatin and Heterochromatin are structural units. It is associated with the presence of DNA or histone protein.

Heterochromatin is a part of the chromatin. It is associated with the presence of histone H3 and Heterochromatin proteins. These Heterochromatin proteins consist of proteins of the family of HP1, PH1, and HIRA. The Heterochromatin proteins are associated with the repressive histone marks. These proteins are enriched in the Heterochromatin regions.

The organization of the genetic material into distinct compartments, or domains, within the nucleus is called chromatin structure.

The DNA in a Euchromatin region is loosely packaged and is relatively accessible. In this form, the DNA is in a transcriptionally active state. A key process that contributes to this accessibility is DNA replication. In this process, the genetic material in Euchromatin is replicated and distributed in multiple replication sites, or replication forks, along the length of a Chromosome.

The DNA in Heterochromatin is tightly packaged. These tightly packaged regions have no or very few replication sites. Therefore, the DNA in Heterochromatin is in a transcriptionally repressed state. Transcriptional activity is also repressed in Heterochromatin. This is due in large part to the presence of methyl groups on the DNA in Heterochromatin. Methylation of the DNA prevents transcriptional activity. This process is called transcriptional silencing.

 

Structure of Heterochromatin

The structure of Heterochromatin is tightly packed and condensed. The changes in Heterochromatin occurs due to the modifications to histones and spreading of silencing complexes causing the changes in structure of chromatin. Due to its repressive structure, Heterochromatin does not completely express the genes within it.

 

Heterochromatin usually folds into higher order structures and this induces an increase in negative supercoiling of DNA. The structure of Heterochromatin is stable and is also dynamic and changes with the cell cycle. The formation of chromatin is promoted due to the DNA elements called barriers which promote the formation of active chromatin and remove the nucleosomes. This allows the Heterochromatin to spread.

 

The structure of Heterochromatin is easily explained by analyzing the ‘Constitutive Heterochromatin’ and ‘Facultative Heterochromatin’. Constitutive Heterochromatin is the stable form which consists of repeated sequences of DNA called Satellite DNA. The structural functions are regulated by this form of Heterochromatin and are found in centromeres and telomeres.

 

Facultative Heterochromatin is known to change its structure according to the cell cycle. This consists of repeated DNA sequences termed as ‘LINE Sequences’. This can be seen to change its structure in the inactivated X-Chromosome of females. The structure of Heterochromatin can also be determined by the density gradient data in which the Heterochromatin appears as a regular structure and Euchromatin has an irregular structure.

 

Functions of Heterochromatin

The functional aspects of Heterochromatin are determined by the modifications of chromatin. The Heterochromatin core histones present in yeast are hypoacetylated which makes the lysine residues to become more positively charged, allowing an increase in the interaction between the histone and DNA, making the nucleosome more closed in structure.

 

The closed chromatin structure of Heterochromatin is due to the low acetylation of Histone H4-K16 in Heterochromatin, further promoting the folding of Chromatin to high structure orders. The active transcriptional activity is due to the Hypomethylation of Heterochromatin at H3-K4 and K79.

 

Difference Between Euchromatin and Heterochromatin 

Chromosomes have different segments called Euchromatin and Heterochromatin. Chromatin plays an important role in the structure of a nucleus. It carries DNA. DNA is a part of the genetic material present in every cell of our body. Chromatin also consists of proteins and nucleic acids. It controls the process of cell division. Chromatin is the combination of DNA, histone, and other proteins. They are called epigenetic marks, which play a crucial role in the gene expression process.

In this section, we will be discussing in detail about the difference between Euchromatin and Heterochromatin.

Let us talk about the difference between Euchromatin and Heterochromatin in detail. Here we will discuss the differences between Euchromatin and Heterochromatin with examples.

Constitutive Heterochromatin

The DNA in the Euchromatin is made up of two nucleotides: adednine and thymine. They are called the AT-rich region. The AT-rich regions are associated with histone H1 proteins. Heterochromatin is made up of large regions in the genome, which are AT-rich, and are usually associated with the presence of histone H3. This particular region does not have any Euchromatin or Heterochromatin units. This is called a Heterochromatin block. The Heterochromatin block is usually enriched with genes and does not have any genes.

Heterochromatin is considered as a type of transcriptionally inactive region. Homologous and repetitive sequences are associated with Heterochromatin. Homologous sequences are not repeated more than two times. The repetitive sequences are repeated more than twice. In contrast to homologous sequences, repetitive sequences are associated with Heterochromatin. The repetitive sequences are associated with gene silencing and genomic instability. There are few known genes that are exclusively associated with Heterochromatin.

Repetitive sequences are associated with Heterochromatin. Repetitive sequences have a very important function in the genome. Some examples are listed below:

Insulators. The insulator elements prevent the DNA replication and gene expression from one region to another. Insulators block gene expression. Repetitive sequences form the telomere in Euchromatin. Telomeres are the specialized structures formed at the ends of each Chromosome. It prevents DNA degradation and protects the Chromosome ends. Repetitive sequences are associated with gene silencing. Differentially expressed genes in Heterochromatin and Euchromatin. Repetitive sequences are associated with genomic instability. Tandem repeats or minisatellites are associated with the Heterochromatin. The minisatellites are formed by the sequences that contain tandem repeats. Heterochromatin is associated with the high histone H3 methylation. Euchromatin is associated with low levels of H3 methylation. Repetitive sequences are associated with the repressive histone marks.