Practice question

Restriction enzymes generating same overhang sequence can cross-ligate despite different recognition sites. BamHI cleaves 5'-G^GATCC-3' leaving 5' protruding GATC, while BglII cleaves 5'-A^GATCT-3' leaving identical GATC tetranucleotide. Because Watson-Crick pairing depends solely on overhang sequence, fragments produced by these enzymes anneal efficiently and are sealable by DNA ligase. The ligation product creates a hybrid sequence, either GGATCT or AGATCC, that is generally not recognized by either parental enzyme, a feature exploited to prevent re-digestion while enabling cloning when BamHI site is unavailable.