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| Modern Genetic Analysis Chapter 10. Recombinant DNA Technology Using Cloned DNA Figure 10-27. Restriction mapping by comparing elec-trophoretic separations of single and multiple digests. In this simplified example, digestion with enzyme 1 shows that there are two restriction sites for this enzyme but does not reveal whether the 3-kb segment is in the middle or on the end of the digested sequence, which is 17 kb long. Combined digestion by both enzyme 1 and enzyme 2 leaves the 6- and 8-kb segments intact but cleaves the 3-kb fragment, showing that enzyme 2 cuts at a site within this enzyme 1 fragment. If the 3-kb section were on the outside of the sequence being studied, digestion by enzyme 2 alone would yield a 1- or 2-kb fragment. Because this is not the case, of the three restriction fragments produced by enzyme 1, the 3-kb fragment must lie in the middle. That the RE2 site lies closer to the 6-kb section than the 8-kb section can be inferred from the 7- and 10-kb lengths of the enzyme 2 digestion.

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