Genetic Comparisons

This is a summary on genetic comparisons for Unit 2 AQA biology.

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Genetic Comparisons
Introduction
It's not only physical features which set different species apart, there's several
features in organisms which we can look at to compare how closely related
different species are to one another. We can do this by comparing DNA base
sequences, amino acid sequences and immunological features.
DNA Hybridisation
One way of seeing how closely related a species is to another is
to compare the DNA base sequences by DNA hybridisation.
This is carried out in the following way:
1. DNA is extracted from the two species and cut into short
pieces
2. DNA from one of the species is labelled with a radioactive
tracer
3. The two sets of DNA are added together
4. The mixture is heated to separate the strands of DNA
5. The mixture is left to cool to allow the strands of DNA to
combine with each other and it is analysed. The DNA
which is 50% traced with the radioactive tracer is extracted
(as this is the DNA which will contain one strand of the
unlabelled species and one strand of the labelled
species).
6. These strands are then heated gradually. The more heat
which is required to separate the two strands, the more
closely they are related.
How does this work?
The DNA contains one strand from each species. The more complementary
base pairs the species have to one another, the more hydrogen bonds are
formed and the more energy is required to break the two strands.
Comparison of amino acid sequences in proteins
Another way to compare the relationships between different species is to
compare the amino acid sequences which code for proteins in their bodies. The
disadvantages of this is that it requires all of the organisms studied to contain the
same protein and this isn't always possible if really different organisms are been
compared, for example animals and birds.
This works because the amino acid sequence which makes the protein is coded
for by DNA. Scientists look at the sequences for the protein from different
species and see how they compare. If a species has a more similar code, it will
be more closely related, but if it has a code which isn't similar, it will be more
distantly related.

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As you can see from the following table, species 2 and species 3 are the most
closely related because their amino acid sequence is the most similar.…read more

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