A plasmid containing a normal human beta-globin allele is digested with restriction enzymes. The results are shown in lane 2 of the electrophoresis gel. A second plasmid, contain the beta-globin allele that results in sickle-cell disease, is digested in the same way and placed in lane 3. The standard in lane 1 contains DNA fragments in lengths of 100 base pairs (bp), 200 bp, and so on. Based on the results shown, which of these BEST describes the mutation that resulted in the sickle-cell allele?