Evidence for Relationships between Organisms

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Comparison of DNA base sequences

  • During evolution - DNA of new species will be similar to that of the old species.
  • Mutations in the DNA nucleotide sequence means that different amino acids are coded for and that means that and different polypeptide chain is constructed- changes the structure of the protein

Therefore, species that are more closely related have more similarities in the nucleotide base sequences that those who aren't more closely related.

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DNA Hybridisation

  • DNA from two species is extracted, purified and cut into short pieces.
  • DNA is labelled with flourescent DNA probe. It is then mixed with unlabelled DNA from another species.
  • The mixture of the two sets of DNA is heated to separate the double helix into single strands of DNA.
  • The mixture cools which allows the strands to recombine with other strands that have complementary base sequences.
  • Some strands will recombine with strands of the other species (hybrid strands)- these are easily identified beacause it's 50% labelled (with flourescent marker).
  • Hybrid strands are heated and the temperature increases in stages.
  • At each temperature stage the degree to which the two strands are still linked together is measured.
  • If the two species are closely related they will share many complementary bases.
  • There will therefore be more HYDROGEN BONDS holding them together- the more hydrogen bonds the more strong the strand is.
  • The stronger the bonds the more temperature required to split them apart- the more closely related the species are.
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Use of DNA base sequencing in classifying plants

Scientists at the Royal Botanical Gardens, Kew devised a new classification of the families of flowering plants. This was based on the DNA sequences of three genes found in all plants:

  • They used 565 species that represented all the known families of flowering plants in the world.
  • For each plant, the DNA sequences of all three genes were 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 sequence of species used. 
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