Practice question

Klenow fragment retains polymerase and 3' to 5' exonuclease but lacks 5' to 3' exonuclease activity, making it ideal for labeling. For end labeling, it fills recessed 3' ends templated by 5' overhangs incorporating alpha-32P labeled nucleotides efficiently. Blunt ends possess recessed 3' terminus after transient breathing and can be labeled by exchange reaction under certain conditions with high deoxynucleotide and enzyme concentrations. While sticky 5' overhangs are preferred substrates for fill-in, blunt-end labeling is classic method to create uniformly labeled probes for restriction mapping and Southern analysis when cohesive ends are unavailable.