10.10 Restriction endonucleases are used to construct restriction maps of linear or circular pieces of DNA. The DNA usually is produced in large amounts by recombinantDNA techniques. Generating restriction maps is like putting the pieces of a jigsaw puzzle together. Suppose we have a circular piece of double-stranded DNA that has a length of 5,000 bp. If this DNA is digested completely with restriction enzyme I, four DNA fragments are generated: fragment a is 2,000 bp, b is 1,400 bp, c is 900 bp, and d is 700 bp. If, instead, the DNA is incubated with the enzyme for a short time, the result is partial digestion of the DNA: not every restriction enzyme site in every DNA molecule will be cut by the enzyme, and all possiblecombinations of adjacent fragments can be produced. From a partial digestion experiment of this type, fragments of DNA were produced from the circular piece ofDNA that contained the following combinations of the above fragments: a-d-b, d-a-c, c-b-d, a-c, d-a, d-b, and b-c. Lastly, after digesting the original circular DNA to completion with restriction enzyme I, the DNA fragments are treated with restriction enzyme II under conditions conducive to complete digestion. The resultingfragments are 1,400, 1,200, 900, 800, 400, and 300 bp. Analyze all the data to locate the restriction enzyme sites as accurately as possible.

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