AQA AS Level Biology Unit 2 - Evidence for relationships between organisms

The evidence for relationships betwen organisms topic of unit 2 as aqa biology

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Genetic comparisons using DNA and proteins

  • When organisms evolve it is not only their visible internal and external features that adapt and change but also the moecules of which they are made.
  • DNA determines the proteins of an organism including enzymes and proteins determines the features of an organism.
  • It follows that changes in the features of an organism are due to changes in its DNA .
  • Comparing the DNA and proteins of different species helps scientists to determine the evolutionary relationships between them
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Comparison of DNA bases sequences

  • When one species gives rise to another species during evolution, the DNA of the new species will initially be very similar to that of the species that gave rise to it.
  •  Due to mutations, the sequences of nucleotide bases in the DNA of the new species will change.
  • Consequently, over time, thenew species will accumulate more and more differences in its DNA.
  • As a result, we would expect species that are more closely related to show more similarity  in their DNA base sequences than species that are more distantly related.
  • As their are millions of base sequences in every organism, DNA contains a vast amount of information about the evolutionary history of all organisms.

One way to determine similarities between the DNA of different organisms is to use a technique called DNA hybridisation.

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DNA hybridisation

  • DNA hybridisation depends up a particular property of the DNA double helix.
  • When DNA is heater, its double strand separates into 2 complementary stands.
  • When cooled, the complementary bases on each strand recombine with each other to reform the original double strand.
  • Given suffiecient time, all strands in a mixture of DNA will pair up with their partners.
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The process of DNA hybridisation

Using this propety, DNA hybridisation can be used to compare the DNA of two species in the following manner:

  • DNA from two species is extracted, purifies and cut into short pieces
  • The DNA from one of the species is labelled by attaching a radioactive or 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 separate their strands.
  •  The mixture is cooled to allow the strands to combine with other strands that have a complementary sequence of bases
  • Some of the double strands that reform will be made up of one strand from each species. This is called hybridisation and the enw strands are call hybrid strands. These can be identified because they're 50% labelled
  • These hybrid strands are separated out and the temperature is increase in stages
  • At each temperature 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 nucleotide bases
  • There will therefore be more hydrogen bonds linking them together in the hybrid strand
  • The greater the number of hydrogen bonds, the stronger the hybring strand will be
  • The stronger the hybrid strand the higher the temperature needed to separate them into two single strands
  • The higher the temperature at which the hybrid strand splits the more closely the two species are related
  • The lower the temperature at which it splits the more distantly related
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Use of DNA base sequencing in classifying plants

Until recently the classification of flowering plants had been bases on the appearance of a plants physical features. This led to flowering plants being placed in one of two groups. The monocotyledons that have a single seed leaf (generaly have thin narrow leaves) and the dictyledons that have two seed leaves (and generally have broad leaves)

A team of scientists recently devised a new classification of the famillies of flowering plants. This was based on the DNA sequences of three genes found in all plants. Their work was carried out as follows:

  • They used 565 species that between them represented all 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 of flowering plants was devised based upon the DNA of the species used

The hylogenetic tree that the scientists produced showed how species have evolved into natural groups. These groupings represent evolutionary relationships better than any previous form of classification has ever done

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Comparison of amino acid sequences in proteins

  • The sequence of amino acids in proteins is determined by DNA.
  • The degree of similarity in the amino acid sequence of the same protein in two species will therefore reflect how closely related the two species are.
  • Once the amino acid sequence for a chosen protein has been determined for two species the two sequence are compared.
  • This can be done by counting either the number of similarities or the number of differences in each sequence.
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Immunological comparisons of proteins

The proteins of different species can also be compared using immunological techniques. The principle behind this methos is the fact that antibodies of one species and antigens on proteins, such as albumin in the d serum of another. This process is carried out as follows:

  • Serum albumin from species A is injected into species B
  • Species B produces antibodies specific to all the antigen sites on the albumin from species A
  • Serum is extracted from species B this serum contains antibodies specific to the antigens on the albumin from species A
  • Serum from species B is mixed with serum from the blood of a third species C
  • The antibodies respond ti their corresponding antigens on the albumin in the serum of species C
  • The response is the formation of a precipitate
  • The greater the number of similar antigens, the more precipitate is formed and the more closely related the species
  • The fewer the number of similar antigens, the less precipitate is formed and the more distandtly the species are related
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