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Restriction fragment length polymorphisms (RFLPs) are differences in the lengths of restriction fragments from two individuals whose DNA (or a subset thereof) have been restricted (cut) with the same restriction enzyme. They arise because (pseudo)random genetic variation causes different restriction sites to appear (and at different loci, or places) in otherwise 'identical' organisms.

Prominent use: DNA fingerprinting

Restriction fragment length polymorphism (RFLP)is able to sample about 5% of any gene. This technique was developed in the early 1980's and was the first method of genetic analysis available to molecular biologists. It was replaced by direct DNA sequencing.

Cutting a gene with a restriction enzyme allowed biologists to compare differences in the lengths of restriction fragments; DNA sequencing allowed biologists to read the exact order of nucleotide bases in the gene itself.

RFLP's, what does this all mean?

In as short an explanation as I can manage -

Assume five people have left a small sample of DNA. How can you tell the difference?

One way is via RFLP analysis. To do this you need a restriction enzyme, this will 'cut' DNA in a very specific place. To be more precise it cuts at a specific nucleotide sequence.

If this enzyme always cuts (cleaves) at the same place, if you have several identical DNA sequences it will cut in an identical fashion in all sequences.

So if at this cleavage point, the DNA sequence of any of the 5 people mutates then the enzyme will not cut. Alternatively if a mutation occurs in a different place, a cleavage point may be created, (either scenario will lead to a different restriction pattern).

If any of the above is true then a difference can be detected using RFLP analysis. Using this method (combined with PCR) I have been able to trace back maternal family lineage through hundreds of years and across several continents.

To reveal the results of such an analysis, these 'cut'  fragments need to be separated. This will show the length of the restricted fragments. To do this gel electrophoresis is used. This pulls apart all these DNA lengths that have been cut up allowing all to be revealed.

Summary

Restriction enzymes cut very specific DNA sequences called restriction sites. Each persons DNA is different and will therefore have different numbers of restriction sites in different places. As a result, each persons DNA will be cut in to varying lengths when exposed to the enzyme; it is this variance in length that is detected.

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