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The Structure of DNA - Deoxyribonucleic acid

  • DNA = a polynucleotide - made up of lots of nucleotides joined together
  • Nucleotide = made  up of a deoxyribose sugar, a phosphate group and a nitrogenous base
  • Each nucleotide has the same sugar and phosphate, but base can vary
  • Four bases: Adanine (A), Thymine (T), Cytosine (C), Guanine (G)
  • A+T= purines
  • C+G=pyramidines

The Structure of RNA - Ribonucleic acid

  • Similar to DNA - both made of nucleotides - sugar, nitrogenous base and phosphate
  • Make proteins from instructions in DNA
  • Nucleotides form polynucleotide strand, joined between sugar and phosphate

Differences between DNA and RNA

  • Sugar in RNA nucleotides - ribose sugar, not deoxyribose sugar
  • Nulceotides form a single polynucleotide strand (not double)
  • Uracil (a pyramidine) replaces thymine as a base. Uracil always pairs with adanine in RNA

The formation of a DNA molecule

  • DNA nucleotides join together = polynucleotide strands
  • Nucleotides join together between phosphate of one nucleotide and the sugar of another
  • Two DNA polynucleotide strands join by hydrogen bonding between bases
  • Adanine always pairs with thymine and cytosine always pairs with guanine - this is complementary base pairing
  • Two hydrogen bonds form between A and T. Three hydrogen bonds form between C and G
  • Two anti-parallel polynucelotide strands twist forms DNA double helix


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