restriction mapping

Restriction mapping is a method used to map an unknown segment of DNA by breaking it into pieces and then identifying the locations of the breakpoints. This method relies upon the use of proteins called restriction enzymes, which can cut, or digest, DNA molecules at short, specific sequences called restriction sites. After a DNA segment has been digested using a restriction enzyme, the resulting fragments can be examined using a laboratory method called gel electrophoresis, which is used to separate pieces of DNA according to their size.

One common method for constructing a restriction map involves digesting the unknown DNA sample in three ways. Here, two portions of the DNA sample are individually digested with different restriction enzymes, and a third portion of the DNA sample is double-digested with both restriction enzymes at the same time. Next, each digestion sample is separated using gel electrophoresis, and the sizes of the DNA fragments are recorded. The total length of the fragments in each digestion will be equal. However, because the length of each individual DNA fragment depends upon the positions of its restriction sites, each restriction site can be mapped according to the lengths of the fragments. The information from the double-digestion is particularly useful for correctly mapping the sites. The final drawing of the DNA segment that shows the positions of the restriction sites is called a restriction map.