Chromatin: Definition, Types, Structure

Introduction 

Chromatin refers to a complex of DNA and proteins in which the formation of chromosomes takes place.  This formation of chromosomes happens within the nucleus of eukaryotic cells. There is no appearance of nuclear DNA in free linear strands. The nuclear DNA is highly condensed, and its wrapping takes place around nuclear proteins to make it fit inside the nucleus. There are two types of chromatin– euchromatin and heterochromatin. Heterochromatin is an area of highly coiled DNA and stains darkly whereas euchromatin is an area of less coiling and stains lightly. As a result of this chromosomes show transverse bands of lightly and darkly stained regions. The appearance of this complex is like beads on a string; when observed under a microscope. Experts call these beads nucleosomes. Keep on reading to learn about the basic structure, functions and types of this complex. 

Chromatin Structure

Chromatin is a complex of macromolecules that comprises three main parts- protein, DNA, and RNA. One can find this complex inside the nucleus of eukaryotic cells. Two types of chromatin exist in nature in two forms. These two forms are- heterochromatin (which is the condensed form) and euchromatin (which is the extended form). 

Histones are the primary protein components of this complex.  These primary protein components organise the DNA into “bead-like” structures. Experts call these structures nucleosomes. Histones provide a base on which the wrapping around of the DNA can take place.

 A nucleosome involves 147 DNA base pairs that are wrapped around an octamer. An octamer is simply a set of 8 histones. Further folding of nucleosomes can result in the production of this complex’s fibre. 

The coiling and condensation of these fibres happen to produce chromosomes. Some cell processes are made possible by this complex to occur. Such cell processes are as follows: 

• DNA replication
• Transcription
• DNA repair
• Genetic recombination
• Cell division

Relationship between epigenetics & chromatin:

Epigenetics refers to the study of changes in patterns of gene expression patterns. The reason for such changes is the mechanisms in the underlying DNA sequence. Daughter cells inherit epigenetic modifications during the process of cell division. 

Chromatin structure alterations govern the changes in gene expression, whose association is with epigenetics. This shows the relationship between epigenetics & chromatin structure.

Chromatin Functions

Below are the various functions of chromatin in a cell:

DNA Packaging:

Perhaps the critical function of chromatin in a cell is the compactification of long DNA strands. In the nucleus, DNA length is far more significant in comparison to the size of the compartment in which it is stored. There is a need to condense the DNA to fit it into this compartment. 

Experts use the packing ratio to describe the degree of condensation of DNA. The DNA packaging does not take place directly in the complex structure. This is necessary to achieve the overall packing ratio. So, instead of direct placement, there exist several levels of packing. 

The winding of DNA takes place around the nucleosome to achieve the first level of packing.  This structure is basic in both types of chromatin.  The second level of packing involves the wrapping of beads that can be found in a 30 nm fibre. In the final packaging, the organisation of the fiber takes place in domains, scaffolds, and loops.

Transcription regulation:

Transcription is a three-step process which is as follows:

• Reading of the DNA stored genetic information occurs by proteins
• Transcribing takes place into RNA
• Finally, the translation of the RNA takes place into functional proteins. It will not happen if the complex restricts access to the read proteins

In euchromatin, the extended type, the transcription process is conducted. In contrast, in heterochromatin, the condensed type, the tight packing prevents the DNA reading by proteins.

Chromatin and DNA Repair:

The packaging of DNA into the complex poses a barrier to every process related to DNA. This complex readily changes its structure because of the high dynamic arrangement of proteins and DNA. Relaxation happens at a rapid rate at the DNA damage site. This leads to the repairing of proteins, which binds to DNA to repair it.

Chromatin Types

Two types of chromatin are found in nature. These two types are heterochromatin and euchromatin. These two types of chromatin are responsible for giving light and dark bands to chromosomes.

Heterochromatin:

Heterochromatin is characterised by very tight packing and condensing. When heterochromatin forms, histones are modified as well as the recruitment and spreading of silencing complexes happens. The result of all this is bringing about changes to its structure. These changes impact gene transcription and other DNA processes.

Heterochromatin is inhibitory to  gene expression because of its repressive structure. It can fold into structures that are of higher order. The formation of heterochromatin causes an increase in the DNA negative supercoiling.

Euchromatin:

Euchromatin involves a loosely wrapped complex, thereby making DNA more accessible. Histone modifications allow the complex to be more open; as we have read previously, modifications lead to the closing off of heterochromatin to DNA replication enzymes.  Such modifications, in euchromatin, are reversed.

Euchromatin is more open compared to heterochromatin when it comes to recruiting gene regulatory proteins as well as RNA polymerase complexes. This allows the initiation of the transcription. There is a direct relationship between the amount of euchromatin in a cell’s nucleus and the level of the cell’s operational productivity.

Conclusion

Chromatin is a complex of proteins and DNA in which the formation of chromosomes happens.  This happens within the eukaryotic cell’s nucleus. The nuclear DNA is highly condensed to make it fit inside the nucleus. This complex looks like beads on a string under a microscope. Its basic structure is essentially a macromolecules complex consisting of protein, DNA, and RNA. There exists a relationship between epigenetics & chromatin structure. This complex has three functions- DNA Packaging, transcription regulation, and chromatin and DNA Repair. There are two types of chromatin in existence which are euchromatin and heterochromatin.