Evidence for Relationships between Organisms

• 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.

• 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.

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.