Watson and Crick model of DNA

Watson and Crick model of DNA

This model of DNA proposed by Watson and Crick and supported by Wilkins et. al., by x-ray crystallography of DNA.

According to this model.

  • DNA consists of two, polynucleotide chains which are twisted around each other and  forming a right-handed double helix around the same central axis.
  • These chains are antiparallel, means one chain (3! 5) appears as ascending and another one (5, 3′) as descending. 
  • In both the chains orientation of the sugar molecule is in the opposite direction.
  • In this the phosphate groups of lies on the outside of the deoxyribose and the nitrogenous bases are stacked inside the helix..
  • The nitrogenous bases of the two chains are linked through hydrogen bonds formed between oxygen and nitrogen atoms of the adjacent bases.
  • Size of one turn of helix measures about 34 Å. 
  • One helix contains 10 base paired nucleotides.
  • The distance between two nucleotide is 3.4 Å. 
  • Each successive base pair in the stack turns 36° in a clock-wise direction. 
  • The double helix, therefore, makes a complete turn´of 360° every ten base pairs like circular staircase.
  • The adenine (A) base always pairs with thymine (T) by two H-bonds.
  • (C) pairs with guanine (G) by three H-bonds . 
  • Due to this type of pairing, the two chains or strands are complementary to each other.
  •  for example :
  • I strand 5′ A-T-G-A-C-T-A-G 3
  • II strand 3 T-А-С-Т-G-A-T-C 51
  • Due to this property, during DNA replication the two dissociated chains serve as templates for the synthesis of new complementary chains.
  • The diameter of the DNA molecule is about 20 A or 2 nanometers (1 nanometer = 1 billionth of a meter).
  • The twisting of the strands results in the formation of narrow (minor) and wide (major) grooves.

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