• 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.

  1. Euchromatin
  • 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.


  1. Heterochromatin.
  • 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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