Biology Unit 2 AS

This is for chapter 15 for AQA biology AS, genetic comparisons using DNA & Proteins

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DNA hybridisation depends upon a particular property of the DNA double helix, when DNA is heated, the double strand splits into two complementary single strands. When cooled, the complementary bases on each strand will recombine to make the original double strand.

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  • DNA from two species is extracted, purified and cut into short pieces
  • The DNA from one species is labelled by attaching a radioactive/fluorescent marker to it. It is then mixed with unlabelled DNA from the other species
  • The mixture of both sets of DNA is heated to seperate the strands
  • Mixture is cooled to allow the strands to combine with other strands that have complementary sequences of bases.
  • Some of the double strands will be made of one strand from each species- called hybridisation- the new strands are called hybrid strands.
  • Hybrid strands seperated out and the temperature is increased in stages
  • If the two strands are closely related they will share many nucleotide bases and therefore there will be more hydrogen bonds
  • The greater the number of hydrogen bonds= the stronger the strands= the higher the temperature to break= closely related species.
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Use of DNA base sequencing in plants

Plants use to be classified by the appearance, either monocotyledons (single seeded leaf) or dicotyledons (two seed leaves/broad leaves)

Team of scienctits- kew- recently devised a new classification of the families of flowering plants. This was based on the DNA sequences of the three genes found in all plants.

  • They used 565 species that between them, represented all of the known families of flowering plants in the world.
  • For each plant, the DNA sequences of all three genes was determined
  • The sequences for each species were compared using computer analysis
  • A phylogenetic tree for the families of flowering plants was devised based upon the DNA sequences of the species

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Comparison of amino acids

The sequence of amino acids is determined by DNA. The degree of similarity in the amino acid sequence in two species will therefore show how closely related the two species are.

This can be done by either counting the number of similarities or the number of differences.

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