Class 12 Biology Molecular Basis of Inheritance Detailed structure of DNA

Detailed structure of DNA

 DNA polynucleotide chain has two free ends:

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


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
  • The two polynucleotide chains have anti-parallel polarity
  • Two chains are coiled in a right handed fashion forming a right-handed helix
  • Uniform distance is maintained between the two strands of helix


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