Related molecules of DNA, such as different versions of the same gene from two related organisms, normally have very similar sequences. Consequently they will have similar restriction maps. However, occasional differences in base sequence will result in corresponding differences in restriction sites. Each restriction enzyme recognizes a specific sequence (usually of four, six or eight bases). If even a single base within this recognition sequence is altered, the enzyme will no longer cut the DNA (Fig. 22.08). Consequently, restriction sites that are present in one version of a sequence may be missing in its close relatives.
If two such related but different DNA molecules are cut with the same restriction enzyme, segments of different lengths may be produced. Consequently, a difference
Sal I recognition site
FIGURE 22.08 Single Base Changes Prevent Cuffing by Restriction Enzymes
The recognition sequence for a particular restriction enzyme is extremely specific. Changing a single base will prevent recognition and cutting. The example shown is for Sa/I, whose recognition sequence is GTCGAC.
NNNNGT CGACNNN NN N N C~A"GC~T]G N NN
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