Both genetic and physical maps provide information about the relative positions and distances between genes, molecular markers, and chromosome segments. Genetic maps are based on rates of recombination and are measured in percent recombination, or centimorgans. Physical maps are based on the physical distances and are measured in base pairs.

Restriction mapping determines the relative positions of restriction sites on a piece of DNA. When a piece of DNA is cut with a restriction enzyme and the fragments are separated by gel electrophoresis, the number of restriction sites in the DNA and the distances between them can be determined by the number and positions of bands on the gel (p. 000 in Chapter 18), but this information does not tell us the order or the precise location of the restriction sites. To map restriction sites, a sample of the DNA is cut with one restriction enzyme, and another sample is cut with a different restriction enzyme. A third sample is cut with both restriction enzymes together (a double digest). The DNA fragments produced by these restriction digests are then separated by gel elec-trophoresis, and their sizes are compared. Overlap in size of fragments produced by the digests can be used to position the restriction sites on the original DNA molecule.

EcoRI BamH1 EcoRI + Size alone alone BamHI standards (double digest)

EcoRI BamH1 EcoRI + Size alone alone BamHI standards (double digest)


19.4 Restriction sites can be mapped by comparing DNA fragments produced by digestion by restriction enzymes used alone and in various combinations. A sample of a linear piece of DNA was first digested with EcoRI alone. Another sample was digested by BamHI alone, and finally a third sample was digested by both EcoRI and BamHI. The resulting fragments were separated by gel electrophoresis and stained with ethidium bromide.

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