The plasmid shown has a gap in one DNA strand, ending in 3′ OH and 5′ OH. Which enzymes are needed to fill the gap and form a covalently closed circular plasmid?
[P] DNA ligase; [Q] alkaline phosphatase; [R] DNA polymerase; [S] polynucleotide kinase.
DNA polymerase extends the available 3′ OH using the intact strand as template, filling the missing nucleotides. The 5′ OH at the other edge must be phosphorylated by polynucleotide kinase to provide a 5′ phosphate. DNA ligase can then seal the remaining nick. Alkaline phosphatase would remove phosphate and hinder ligation. Thus P, R and S, B.
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