- 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.