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More reliable and accurate than mere physical resemblance in determining the genetic relationship between individuals and species, is of course a genetic comparison. This can be achieved either by directly analysing their DNA, or the proteins encoded by it.
There is a correlation between the degree of relatedness between individuals/species, and the degree of similarity between their base sequences in DNA. For example, ACTGGAC and ACTGGAT are more similar than ACTGGAC and GCTGGAA. Hence, it may be deduced that the organism possessing the first sequence is more closely related to the second organism, than to the last organism.
One technique which enables scientists to carry out a DNA comparison is called DNA hybridisation. This involves the joining together of DNA sections from 2 species to be compared. These are the steps involved:
1. DNA samples are collected, cut into smaller sections, then heated to 90 degrees.
2. The heat denatures the DNA molecules by breaking the hydrogen bonds within; the strands of DNA separate.
3. The separated strands from the two species are now put together and allowed to cool.
4. Some strands will join back with their original pair; others will join with strands from the other organism to form hybrid DNA.
The temperature at which these strands re-anneal (bond together) is the clue to the genetic relationship between the two organisms. DNA strands which joined back with their original counterpart from the same organism re-anneal at 87 degrees, as they share a lot of base sequences. The hybrid DNA, on the other hand, is formed at a lower temperature. This is because fewer sequences are shared, and so fewer hydrogen bonds are formed which hold the strands together.
The lower the temperature at which hybrid DNA forms between two organisms, the less genetically related they are. This technique has led to a new classification system being used for plants.
Protein (Amino Acid) Comparisons
Proteins are sequences of amino acids. The exact sequence of amino acids found in a protein in an organism may differ from one to another, usually between different species. Whether it differs, and the degree to which it does, suggests how closely related those organisms/species are.
The sequence may be identical, different by 1 amino acid, or different by 50 amino acids. This information is very helpful in building phylogenetic trees, especially as there are so many different proteins which can be analysed to build a complete picture.