Packaging of DNA

Length of DNA double helix molecule, in a typical mammalian cell is approximately 2.2 meters.

Approximate size of a typical nucleus is I0"6m.

DNA is therefore must be condensed and coiled and super coiled to fit in the nucleus.

In Eukaryotes. this packaging or organization of DNA is complex.

  • Histones are required for the packaging of DNA.
  • Histones are proteins that are rich in the basic amino acid residues lysines and arginines which carry positive charge in their side chains.
  • Eight molecules of histones (two each of H2A, H2B, H3 and H4) get organized to form histone octamer.
  • DNA is negatively charged and it is wrapped around the positively charged histone octamer to form nucleosome.

nuceosome.jpg

 

Under the electron microscope,

  • nucleus shows chromatin network.
  • The nucleosomes in chromatin are seen as *beads-on- string'.
  • Around the octamer, DNA molecule is wrapped as 1 and 3/4th turn.
  • This DNA is called core DNA and it consists of about 146 bp (base pairs).
  • Adjacent nucleosomes are linked with small segments of DNA called linker DNA
    • It has about 54 bp.
  • Thus the string is DNA- 2nm or 20A in diameter.
  • This 'beads- on- string' structure gets condensed into nucleosome fiber, which is 10 nm (100A) in diameter.

beads.jpg

  • H1 histone is present in the linker region.
  • DNA makes two complete turns wrapping the octamer and leaves it.
  • Each nucleosome contains 200 bp of DNA helix.
  • The thin and long nucleosome fiber is coiled like a telephone wire to make solenoid fiber with diameter 30 nm or 300A.
  • Nucleosome and solenoid fibers are characteristic of nucleus at interphase.
  • During prophase of cell division,
  • there is formation of chromatin fibers of 200 nm
  • and then a chromatid of 700nm.
  • At metaphase of cell division
  • a chromosome with two chromatids joined by common centromere can be seen under the compound microscope.
  • The packaging of chromatin at higher levels need additional set of proteins that are collectively called Non-Histone Chromosomal (NHC) proteins.
  • Loosely packed region of chromatin that stains light is called Euchromatin
  • Densely packed region that stains dark is called Heterochromatin.
  • Euchromatin is considered as transcriptionally active chromatin, while heterochromatin is inactive.