Histones & DNA Packing: The Core Histones All Exist as Dimers

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Nucleosome Model: Histones (H2A, H2B, H3 and H4 - Core Histones) & DNA Packing
The Linker Histones
  • Histones H2A, H2B, H3 and H4 are known as the core histones, while histones H1/H5 are known as the linker histones.
  • The core histones all exist as dimers, which are similar in that they all possess the histone fold domain: three alpha helices linked by two loops. It is this helical structure that allows for interaction between distinct dimers, particularly in a head-tail fashion (also called the handshake motif)
Human Diploid Cell

Each human diploid cell (containing 23 pairs of chromosomes) has about 1.8 meters of DNA; wound on the histones, the diploid cell has about 90 micrometers (0.09 mm) of chromatin. When the diploid cells are duplicated and condensed during mitosis, the result is about 120 micrometers of chromosomes.

DNA and Protein
  • Chromatin- it is a DNA and protein.
  • Protein we are talking about here is histone. It is DNA binding protein. It made with 4 different subunits.
  • They are repeated twice each sub unit so total have 8 subunits.
  • And hence it is called histone octamer.
  • DNA wrap around it.
  • Chromatin is not fully condensed form.

4 Type of Histone

  • H2A
  • H2B
  • H3
  • H4.

They have C terminal and N terminal tail. Both key components interact with DNA in their own way through a series of weak interactions, including hydrogen bonds and salt bridges. These interactions keep the DNA and histone octamer loosely associated and ultimately allow the two to re-position or separate entirely.

  • These subunits combine together to form histone octamer.
  • One histone alone is called monomer. 2 monomers combine together to form dimer. Then these dimers combine together to form tetramer. Then two such tetramer together form octamer. Now DNA wrap around octamer.
  • When you look at the arrangement the DNA wrap around histone and in this wrapping DNA is entering from one side and exiting from another side.
  • On the entry and exit side there is V like structure
  • At those side there will be H2A and H2B. and at another side there will be H3 and H4 histone.
  • Because all the histone has n terminal tails.
  • And N terminal tails of H3 and H4 are more prone towards the chemical histone modification.
  • They can be easily methylated, acetylation, phosphorylation.
  • This model of wrapping DNA around histone is called nucleosome model.
  • The chromatin further coil to form chromosome.

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