Chromatin - Exam A2Z

Greek Chroma = colour.
Name given by Flemming (Flemming, 1879)
In the nucleus, DNA combines with a specific type of protein to form a complex structure called Chromatin.
Chromatin contains DNA/RNA/Histone Protein, Alkaline Protein and Non Histone Protein (Acidic Protein).
It is a threadlike structure. Which is called Chromonemata, which intertwine together to form a mesh-like structure.
Receives alkaline arginine (carbol fuchsin orcin).
During cell division, this chromatin material condenses to form a thick thread or ribbon-like structure called a chromosome.
Their number, shape and size are fixed for each species.
Thus, chromatin fibers are found in the nucleus in the diffused type or in the twisted state, indicating chromosomes.

There are two types of chromatin.

This part is lightly stained when dyed with alkaline stains. These are called Euchromatin.
DNA diffuses in the euchromatin part.
The effect of environmental factors, temperature, gender, age etc. is relatively less on this.
In this the frequency of crossing over is high.
Its DNA is genetically and metabolically active.

This is the more condensed part. Which takes deep staining when colored with alkaline stains.
The effect of environmental factors on this is relatively high.
In this the shape of crossing over is relatively less.
Its what. Genetically less active.
Repetitive sequence of DNA is found in this region. Which is genetically non active.
Pearl-like structures are found in this region called Chromomeres. They contain less DNA and more RNA.

Brown has divided heterochromatin into two types.

(a) Facultative heterochromatin :

It represents the temporary state of heterochromatin.
When one chromosome of a pair is fully or partially heterozygous, the other remains normal.
For example, in humans, one of the female 2X chromosomes becomes heterochromatic, which is called the Barr body, it is also called sex chromatin.
Males have only one chromosome which remains euchromatic, hence does not acquire staining when immunized

(b) Constitutive or Organizer heterochromatin):

It is mostly a stable type of heterochromatin which is present in both the chromosomes of the pair.
This region containing the centromere is found on the part of the nuclear component or on the chromosome and is related to the nucleoli.
It is also called satellite DNA.
This DNA plays a structural role in chromosomes. It does not participate in the activities of replication in the S-phase of the cell cycle.

Functions of Chromatin

Stores genetic material.
During reproduction and cell division, it produces chromosomes that essentially carry genetic information.
It controls the growth and differentiation of tissues.
It regulates cellular metabolism.

Chemical composition :

Chromatin is a substance of viscous nature, it contains DNA. RNA histones and non-histone proteins and Ca++ are present.
In terms of ratio, DNA and RNA are 3% and 5%, respectively, histone and non-histone proteins are 36 and 28%.
Histone proteins are basophilic.
DNA is attached to them due to its acidic nature.
Histones are different types of H2A. H2B H3. H4 can be H1.
There is a substantial difference in the organization of the chromatin material for each species.
The ratio of lysine and arginine amino acids varies in the organization of histone proteins. For this reason, differences are found in their properties also.
Non-histone proteins are acidic in nature.
Tryptophan and tyrosine amino acids are mainly found in these.
These proteins that do not attach to DNA are called residual proteins.
They participate in metabolic activities.
Enzymes are also found in the chromatin material, although they are also histone or non-histone proteins like RNA polymerase is non-histone protein.
The main enzymes are nucleo-phosphatase histone acetylase, NAD synthetase.
These are synthesized by genes.

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