Jl

I kb

We now know that the BamHI site lies between these two EcoRI sites. Considering the four fragments produced by the double digest, there are several possible arrangements by which a BamHI site could fit in between the two EcoRI sites. To determine which of the arrangements is correct, compare the results of the EcoRI digestion with the double digest. When the original 13-kb DNA fragment was cut by EcoRI alone, the three fragments produced were 8 kb, 3 kb, and 2 kb in length. The 2-kb and 3-kb bands are also present in the double digest, indicating that these fragments do not contain a BamHI site. The 8-kb fragment present in the EcoRI digest disappears in the double digest and is replaced by the 7-kb fragment and the 1-kb fragment, indicating that the 8-kb fragment has the BamHI site. Thus the 7-kb and 1-kb fragments must lie next to each other, and the 2-kb and 3-kb fragments are on the ends. Thus, the correction arrangement of the restriction sites is:

EcoRI site

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