Base sequences in certain sections of DNA can vary quite considerably between individuals. We can exploit these differences using restriction enzymes. We can distinguish between the DNA from two individuals by exposing the two samples to the same restriction enzyme. Let's say that in the particular section of DNA we're analysing, the DNA from individual A has the recognition sequence for this restriction enzyme but individual B does not. After treatment with the restriction enzyme the two samples (from individuals A and B) are going to consist of fragments of different lengths (since the restriction enzyme only cut the DNA of individual A). We can exploit the differing fragment lengths using gel electrophoresis. Smaller fragments are going to migrate at a faster speed through the gel matrix than larger fragments. As a result, if the DNA fragments from the two samples are of differing length, different banding patterns will result from the two samples.
The fact that the restriction enzyme acted on the DNA from these two individuals differently (and that the resulting DNA fragments were of different lengths) is referred to as a restriction fragment length polymorphism. "Polymorphism" in this context just means that there will be differences in the DNA fragment lengths (following restriction digest) between individuals
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