Chromatin:-

                                     In eukaryotes, the genetic material ie, DNA is complexed with proteins in a specialized structure called as "Chromatin". This chromatin consists of double-stranded DNA to which large amounts of protein and a small amount of RNA are added. This chromatin usually engages with functions like repair, replication, and recombination, etc.  The dynamic structure of this chromatin influences it to work functionally in the genome. 

Nucleosome:                                     

                  The fundamental unit of chromatin is called as 'Nucleosome'. It comprises of DNA, RNA, and some basic proteins such as histones and non-histones (acidic proteins). The amount of RNA and non-histone proteins is variable depending on different chromatin structures whereas there are fixed proportions of DNA and histone proteins in a 1/1 ratio. The histones that are attached to the DNA act as 'anchors' that help in the winding of the components. The non-histones are very heterogenous as they vary in different tissues and include DNA and RNA polymerases among other enzymes. The nucleosome is composed of two main parts:

• the core particle and
• the linker region that adjoins the core particles.

      The core particle is highly conserved and composed of 146 base pairs of DNA wraps around the histone octamer (8 octa core proteins).

       The linker histone links the entry or exit of the DNA strand on the nucleosome.

                  

        Histones:-

                        Histones are the basic proteins present in the DNA of chromatin. There are of five types each one present in large amounts. Histones are small proteins that are basic because of their high content of basic amino acids such as arginine and lysine. Now besides being basic,  they help in binding tightly to DNA, which is an acid. The four main histones are H2A, H2B, H3 and H4. 

             These four histones are present in equimolar units, but H1 is not conserved. It is present only once per 200 basic pairs. It is loosely attached to the chromatin and is not a component of the nucleosome core unit ie, DNA- histone structure. It is bound to the linker segments of DNA that joins the neighbouring nucleosomes. Under the electron microscope, the nucleosome of eukaryotic nuclei it was found that the chromatin had the repeating structure of beads about 10nm of diameter connected by a string. It appeared as "beads-on-string" structure.  

                                                         

Types of Chromatin:-

     There are mainly two types of Chromatin. They are explained below:

1. Heterochromatin:-

•   It is a tightly packed form of chromatin silencing gene transcription ie, the genes or transcription sequences present in them are inactive. 
• Heterochromatin is usually present in the nucleus towards the periphery,
• This heterochromatin is difficult to analyze because of it's condensed state and repetitive DNA sequences.
• It is characterized by intense stains when stained with nuclear stains.
• It is a structure that does not alter in its condensation throughout the cell cycle and it is much more condensed than the euchromatin.
• The tightly packaged DNA in the heterochromatin prevents the chromosomes from various protein factors that lead to the binding of DNA.
• It also helps to prevent the inaccurate destruction of chromosomes by endonucleases.
• Heterochromatin has various functions such as gene regulation, chromosomes integrity etc,
• Telomeres, centromeres, bar bodies, genes 1,9,16 of human beings are some examples of heterochromatin.
• There are mainly two types of heterochromatin. They are as follows:

                     a)Constitutive heterochromatin

                      b)Facultative heterochromatin

Constitutive heterochromatin usually contains and packages the same sequences of DNA in all the same species. It is also repetitive and usually coincides with the structural regions such as telomeres and centromeres. The genes of this constitutive heterochromatin might affect the genes of the tightly-packed ones.                                                                                For eg: In humans, the Y-chromosome in men constitutes this constitutive heterochromatin

Facultative heterochromatin is that region of the chromosome that is heterochromatic in some cells and euchromatin in other cells. It is composed of transcriptionally active genes that adopt the structural and functional characteristics of heterochromatin, but is not as repetitive as the constitutive one.                                                                                               For eg: In humans, one of the X chromosomes in women is inactivated as facultative heterochromatin while the other is expressed as euchromatin. 

2.Euchromatin:-

• It is less condensed and contains the most actively transcribed genes. It is lightly packed DNA that is characterized by less intense staining.
• This is present in the interior of the nucleoplasm and the DNA sequences are transcriptionally active.
• The DNA in euchromatin is unfolded to form a beaded structure, unlike heterochromatin to which histone proteins are folded.
• In euchromatin, the DNA is lightly bound and the genes are active or will be active during the growth.
• Euchromatin forms a more significant part of the genome. It makes 90-92% of the genome in the human being.
• Euchromatin is found in both eukaryotes and prokaryotes. and it exists only in one form ie, constitutive form.
• There are various other chromosomes other than heterochromatin which are examples of  euchromatin 

Significance of Chromatin:

                                Chromatin is meant for efficiently packaging of DNA into small volumes to fit into the nucleus of a cell and protect the DNA structure and its sequence. Packaging DNA into chromatin allows for cell divisions and prevent chromosome breakage besides controlling gene expression and DNA replication.

• courtesy by google images