Evidence of relationships between organisms

Genetic Comparisons using DNA and Proteins

There are 4 ways which scientists can see an organisms evolutionary history:

Using DNA: 

  • DNA Base Sequences
  • DNA Hybridisation 

Using Proteins:

  • Amino Acid Sequence in Bases
  • Immunological Comparisons of Proteins

Comparing the DNA and proteins of different species can help scientists determine the evolutionary relationships between them

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Comparisons of DNA Base Sequences

When one species evolves into another, they will have very similar DNA. Due to MUTATIONS, the DNA would have changed. Overtime, there will be more differences between the species, and more differences in their DNA.

So species that are similar, and MORE CLOSELY RELATED to have more SIMILAR DNA BASE SEQUENCES.

They can use this to show common evolutionary ancestors, see how far back or close that was 

There are millions of base sequences in every organism, so DNA contains alot of material that can show an organisms evolutionary history.

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DNA Hybridisation

When DNA is HEATED, its double strands separates into its two complimentary singles strands. When COOLEDcomplimentary bases on each strand recombine with each other, reform strand.

DNA HYBRIDISATION:

  • DNA for 2 SPECIES is extracted, purified and cut into short pieces
  • One species's DNA is radioactively labelled, then mixed with other species's DNA 
  • Mixture is HEATED to break the strands, then COOLED to allow them to combine again
  • Some of the strands that form will be made up of BOTH SPECIES DNA. 
  • These HYBRID strands are separated out. Some of the bases have PAIRED up on these
  • These strands are heated, temperature increased in stages

  MORE BASES PAIRED UP = MORE BONDS = HARDER TO BREAK                               = MORE CLOSELY RELATED

The HIGHER the TEMPERATURE needed to break the bonds joining the strands together, the MORE CLOSELY RELATED the species are

The LOWER the TEMPERATURE needed to break the bonds joining the strands together, the LESS CLOSELY RELATED the species are

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Comparisons of Amino Acid Sequences in Proteins

The order of AMINO ACIDS in a PROTEIN reflects the SEQUENCE OF BASES IN A GENE.

The degree of similarity in a the amino acid sequence in a gene will show how closely related the two species are.

Once a protein has been chosen from both species, the two sequences are COMPARED

This is done either by:

  • Counting the NUMBER OF SIMILARITIES IN THE SEQUENCES
  • Counting the NUMBER OF DIFFERENCES IN THE SEQUENCES
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Immunological Comparisons of Proteins

The proteins of different species can be compared IMMUNOLOGICALLY to other species to see how closly related they are, in evolutionary terms.

This can be done by using a SPECIFIC ANTIBODY REPSONSE of one species (A) to ANTIGENS on the PROTEINS of another species (B): EG. human and monkeys

  • Inject a rabbit (Species A) with human (Species B) ALBUMIN -> PROTEIN
  • Rabbit (A) lymphocytes produce antibodies specific to the human (B) albumin antigens
  • Collect a serum from the rabbit (A) (contains ANTIBODIES) and purify the antibodies
  • Add serum from the rabbit (A) to samples of albumin from other species.
  • This forms a PRECIPITATE, with ANTIGEN-ANTIBODY complexes. This can be compared to humans (B) to see evolutionary relationships. 

          The MORE ANTIGENS = MORE PRECIPITATE        = MORE CLOSELY RELATED THE SPECIES ARE

The LESS ANTIGENS = LESS PRECIPITATE = LESS CLOSELY RELATED THE SPECIES ARE

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Courtship Behaviours

Why is it necessary?: 

1. Recognise Members of Their Own Species

  • Ensures mating only happens between members of the same species as they will be able to produce fertile offspring

2. Identify a Mate That is Capable of Breeding

  • Both partners need to be sexually mature, fertile and wanting to mate

3. Form a Pair Bond

  • Leads to a successful mating and raising of offspring, some birds mate for life

4. Synchronise Mating

  • Takes part for optimum probability of mating
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