Restriction map

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Restriction maps are important tools for analyzing DNA . A restriction map shows the positions of the interfaces of individual restriction enzymes on the DNA of genomes or plasmids . Using the length of the DNA fragments that arise when the DNA is cut by restriction enzymes, DNA sections can be identified in comparison with a restriction map.

Procedures

Basically there are two possibilities to carry out a restriction for mapping, the complete and incomplete restriction digest. The resulting pieces of DNA are determined by gel electrophoresis using a suitable marker .

Complete restriction digest

Here, the DNA fragment is first cleaved in several batches using different restriction enzymes. In addition, the fragment is cleaved with all of the restriction enzymes previously used. An overlapping cleavage is thus obtained, which enables the location of the restriction sites to be determined.

Incomplete restriction digest

In contrast to the method mentioned above, only one restriction enzyme is used here. The restriction is only carried out for a short time, so that many DNA fragments are not completely cut. Here, too, an overlapping split is obtained.

example

Some regions of genomic DNA, called hypervariable regions, differ from person to person. The resulting differences between the restriction maps, so-called restriction fragment length polymorphisms , play an important role in forensic molecular biology and in molecular biological processes to clarify relationships (e.g. paternity test ).

A spectacular example of the use of restriction polymorphisms in forensic molecular biology was the circumstantial trial against OJ Simpson . In the course of this process, Berkeley statistician Terence Speed ​​made it clear that a probability of 1 / 1,000,000 for a particular restriction pattern to occur only indicates that such a segment of DNA is found on average once per million in many millions of people. This probability does not say anything about the individual case, since the probability for a person with this restriction pattern is just as small as for all other people.

This is similar to lottery winners. These exist even though their chances of winning are microscopic. Here, too, the probability only allows you to estimate how many winners there are per drawing. Statistics cannot be used to predict the identity of the winner.

The strength of the use of restriction polymorphisms lies in the determination of persons who are not possible perpetrators. Their use to identify a perpetrator must be viewed with due skepticism .

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