MOLECULAR GENETICS 2

Detailed structure of DNA

 DNA polynucleotide chain has two free ends:

• 5’ end
  • Free phosphate moiety at 5'-end of deoxyribose sugar
• 3’ end
• Free 3'-OH group of deoxyribose sugar
• 
  

  

  Add caption
  

Watson & Crick were the first to propose the Double helix structure of DNA, based on X-ray diffraction technique.

DNA Double-helix model

• DNA is composed of two polynucleotide chains
• Sugar-phosphate forms the backbone
• Nitrogenous bases form the interior, paired through H- bonds

• Complementary base pairing is an important feature of DNA structure (C-- G, A--T, means, purine with pyrimidine, why not purine and purine OR pyrimidine and pyrimidine?  To maintain equal distance between 2 chains, purine 2ring structure, while pyrimidine 1 ring structure)

• The two polynucleotide chains have anti-parallel polarity (one chain, 5'--3' ,other chain will be 3'--5' )

• Two chains are coiled in a right handed fashion forming a right-handed helix
• Uniform distance is maintained between the two strands of helix
• Base pair distance = 3.4 Angstrom
• Each helix = 10 base pairs = 34 Angstrom

Add caption

Packaging of DNA helix

• Length of DNA is found to be far greater than dimension of a typical nucleus

Total number of base pairs in a typical mammalian cell= 6.6 * 109

Distance between two base pairs= 0.34nm

Therefore, Length of DNA= 0.34 *10-9 *6.6 *109 = 2.2m

Size of a nucleus is of the order of 10-6m

So, Length of DNA is greater than the size of nucleus.

DNA is packaged very strategically to fit inside the nucleus.

• In Prokaryotes, DNA is organized into loops held by proteins. The region where DNA is present is termed as ‘Nucleoid’

• In Eukaryotes, there exist positively charged basic proteins called Histones.
• DNA wrap around the histone octamer (group of 8 histone proteins) to form a Nucleosome.

•  Each nucleosome contains 200 base pairs of DNA helix.
•  Nucleosomes in chromatin are seen as ‘beads-on-string’ under Electron microscope.

Based on different types of DNA packaging, there are two forms of Chromatin.

• Euchromatin
  • Less condensed structure with looser DNA packaging
  • Lightly stained when observed under microscope
  • Contains less DNA
  • Transcriptionally active
  • Found in eukaryotes & prokaryotes
• Heterochromatin
  • Highly condensed structure with tighter DNA packaging
  • Dark stained when observed under microscope
  • Contains more DNA
  • Transcriptionally inactive, as those regions of the genes which need to interact with proteins for transcription is inaccessible.
  • Found in eukaryotes