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*10.28 Katrina purified a clone from a plasmid library made using genomic DNA and sequenced a 500-bp long segment using the dideoxy sequencing method. Hertwin-sister Marina used PCR with Taq DNA polymerase to amplify the same 500-bp fragment from genomic DNA. Marina sequenced the fragment using the dideoxysequencing method, and obtained the same sequence as Katrina did. She then cloned the fragment into a plasmid vector and, following ligation and transformation into E.coli, sequenced several, independently isolated plasmidsto verify that she had cloned the correct sequence. Most of them have the same sequence as Katrina's clone, but Marina finds that about 1/3 of them have a sequence that differs in one or two base-pairs. None of the clones that differ from Katrina's clone are identical. Fearing she has done something wrong, Marina repeats her work, only to obtain the same results: about 1/3 of the fragments cloned from the PCR product have single base-pair differences. Explain this discrepancy.

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