AQA A Level Chemistry复习笔记7.7.4 DNA



  • The nucleic acid DNA is a polynucleotide – it is made up of many nucleotides bonded together in a long chain
  • A nucleotide is a building block consisting of a phosphate group, 2-deoxyribose and a nitrogenous base such as adenine, guanine, cytosine and thymine



The components that make up nucleotide in DNA




A DNA nucleotide


  • Each DNA polynucleotide strand is made up of alternating deoxyribose sugars and phosphate groups bonded together to form the sugar-phosphate backbone. These bonds are covalent bonds known as phosphodiester bonds
  • DNA molecules are made up of two polynucleotide strands lying side by side, running in opposite directions – the strands are said to be antiparallel
  • The nitrogenous bases of each nucleotide project out from the backbone towards the interior of the double-stranded DNA molecule



A single DNA polynucleotide strand showing the positioning of the ester bonds


Hydrogen bonding

  • The two antiparallel DNA polynucleotide strands that make up the DNA molecule are held together by hydrogen bonds between the nitrogenous bases
  • These hydrogen bonds always occur between the same pairs of bases:
    • The purine adenine (A) always pairs with the pyrimidine thymine (T) – two hydrogen bonds are formed between these bases
    • The purine guanine (G) always pairs with the pyrimidine cytosine (C) – three hydrogen bonds are formed between these bases
    • This is known as complementary base pairing
    • These pairs are known as DNA base pairs



A section of DNA – two antiparallel DNA polynucleotide strands held together by hydrogen bonds


Double helix

  • DNA is not two-dimensional as seen in the diagram above
  • DNA is described as a double helix
  • This refers to the three-dimensional shape that DNA molecules form


DNA molecules form a three-dimensional structure known as a DNA double helix


  • The weaker hydrogen bonds between the base pairs are more easily broken than the covalent bonds in the sugar-phosphate strands
  • This is what allows the strands to break apart during cell division
  • New nucleotides from the cells move in to pair up with the exposed bases, which then link by sugar-phosphate bonding and the helix molecules are replicated