PACKAGING OF DNA

Packaging of DNA Helix

·         Distance between two conjugative base pairs is 0.34nm, the length of the DNA in a typical mammalian cell will be 6.6 X10bp X 0.34 X10-9 /bp, it comes about 2.2 meters.

·         The length of DNA is more than the dimension of a typical nucleus (10-6m), how is such a long polymer packaged in a cell.

Packaging in prokaryotes:

·         They do not have definite nucleus.

·         The DNA is not scattered throughout the cell.

·         DNA is held together with some proteins in a region is called ‘nucleoid’.

·         The DNA in nucleoid is organized in large loops held be proteins.

 

 

Packaging in Eukaryotes:

  • In eukaryotes the packaging is more complex.
  • There is a set of positively charged, basic protein called Histones.
  • Histones are positively charged due to rich in basic amino acids like Lysines and arginines.
  • Histones are organized to form a unit of eight molecules called histone octamere.
  • Negatively charged DNA wrapped around positively charged histone octamere to form a structure called nucleosome.
  • A typical nucleosome contains 200 bp of DNA helix.
  • Nucleosome constitutes the repeating unit of a structure in nucleus called chromatin, thread like stained bodies seen in the nucleus.
  • The nucleosomes are seen as ‘beads-on-string’ structure when viewed under electron microscope.
  • The chromatin is packaged to form chromatin fibers that are further coiled and condensed at metaphase stage to form chromosome.
  • Packaging at higher level required additional set of proteins called Non-histone Chromosomal (NHC) proteins.

          

  • In a typical nucleus some loosely coiled regions of chromatin (light stained) is called euchromatin.
  • The chromatin that more densely packed and stains dark are called Heterochromatin.
  • Euchromatin is transcriptionally active chromatin and heterochromatin is inactive.

          

                         

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