Practice question

When linear DNA is cut by two enzymes producing three fragments, each fragment has two ends derived from cleavage events. Ends originating from natural termini or restriction cuts may be blunt or sticky depending on enzyme specificity. If only 2 kb fragment becomes labeled after end-filling with Klenow and radiolabeled dNTPs, it must be the only fragment possessing a 5' overhang with recessed 3' OH that polymerase can extend using template strand. Other fragments bearing blunt ends or 3' overhangs cannot incorporate label efficiently, thus selective labeling marks fragments with appropriate cohesive terminus aiding mapping orientation.