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DNA Structure

                               Watson and Crick Model of DNA




     In 1953 Watson and Crick devised DNA model . DNA is the genetic material found in all living organisms like plants , animals , prokaryote and many virus . In eukaryotes DNA is found in the form of chromosome inside the nucleus but in mitochondria ,plastid and prokaroytes have circular DNA . According to Watson and Crick


  • DNA consist of two polynucleotide chain or strands . These strands are always anti parallel .
  • In each polynucleotide chain the phosphate molecule lie on the outer side of the deoxyribose sugar & nitrogen base inward .
  • The nitrogen bases of the two strands are linked together with the help of hydrogen bonds.
  • In DNA molecule base pairing is very specific . Purine always pair with pyrimidine (A=T) .
  • In the DNA molecule the two strands are twisted around each other and form a double helix around a common central axis .
  • One turn of the helix measures  about 34 A and contains 10 base paired nucleotides . Therefore the distance between adjacent nucleotide is 3.4 A.
  • Diameter of the helix is about 20 A .   .


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