Watson and Crick model of double helix of DNA

Watson and Crick Model of DNA - History & Model - Biology Reader

In 1953

They conducted that:

  • Polynucleotide chain of DNA has a regular helix,
  • Helix has a diameter of about 20 Ao, and
  • Helix makes a full turn at every 34 A0 along its length and contains 10 nucleotides per turn, because per internucleotide distance is 3.4 Ao .
  • To provide thermodynamic stability they thought over the formation of H-bonds with amino (-NH2) or hydroxyl (-OH) and ketone oxygen or amino-nitrogen of two bases.
  • Sequence of bonding is such that for every ATGC on one strand there would be TACG on another.
  • 2 chains are Complementary to eachother.
  • Sequences of nucleotides on one chains are photocopy of sequences of nucleotides on another.
  • 2 strands ran in antiparallel direction i.e., they have opposite polarity.
  • Left hand strand has 5’ to 3’ polarity
  • Right hand strand has 3’ to 5’ polarity
  • Polarity is due to direction of phosphodiester linkage.
  • H-bonds between 2 strands are such that they maintain a distance of 20 A0.
  • Double helix coils in right hand direction. i.e., Clockwise direction.
  • A turning of double helix results in appearance of deep and wide groove: Major groove.
  • Distance between 2 strands form: Minor groove.
  • If changes occur in sequence of base pairs of DNA, mutation takes place.

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