IBDP Biology: Biochemical Comparisons for Classification --- Part 2

• This allows for the comparison of whole DNA molecules rather than short sequences.
• DNA sample is taken from one species and radioactively labelled.
• DNA sample is taken from another species.
• These are mixed.
• The mixture is heated to break the hydrogen bonds holding the two DNA strands in the double helix.
• Some of the strands are partially complementary and new hydrogen bonds reform between them.
• The hybrid strands are identified because they have 50% of the radioactivity.
• The are separated from the mixture.
• Heat is then used to separate the strands.
• The temperature at which the two strands separate is measured.
• The more closely related tow species:
  • The greater the number of similarities between the two species DNA;
  • The greater the number of complementary base pairs form;
  • The higher the number of hydrogen bonds;
  • The greater the heat required to separate them.

• When it comes to IBDP Biology,  the primary structures of proteins are genetically determined.
• The degree of similarity in the amino acid sequences of the proteins reflects the genealogy between two species.
• For example, one of the most studied amino-acid sequences was that of cytochrome c.
• This is an ancient protein common to all aerobic organisms.
• The more differences in the amino acid sequence of the protein, the more distantly related the species.
• The amino acid sequence of human cytochrome c is identical to that of the chimpanzee, but differs from the dog's by 13 amino acids, and from the tuna by 31 amino acids.
• The sequence of amino acids can be compared with sequences of the protein from other species.
• From the degree of similarity between species, phylogenetic trees can be hypothesized.

• Albumins and antibodies are both proteins.
• Antibodies produced by the immune system are very sensitive to differences in the structure of molecules they are exposed to.
• The antibodies of one species will react when exposed to blood serum, containing the proteins, of another.
• A range of antibodies will be created that respond to the antigenic binding sites on the albumin.
• Albumin from one of the species being investigated is injected into another ‘outgroup’ species.
• When it comes to IBDP Biology,  the antibodies that respond against albumin, are collected from this outgroup species.
• These antibodies, in the form of an antiserum are then exposed to the blood proteins of each of the other species under investigation.
• The reaction is quantified by measuring the mass of the precipitate formed.
• If an organism is very distantly related to outgroup species a strong reaction will take place between the blood serum.
• Species more closely related to the outgroup will exhibit a weaker reaction.
• This procedure is used less frequently today, due to the advancement of the other molecular methodologies.

That's the end of this topic.

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