Which option correctly links ligase and the 3′-OH group?

Correct answer: The 3′-OH participates in phosphodiester-bond formation

The governing concept is the chemical structure of the DNA backbone. During ligation, the 3′-hydroxyl group at the end of one DNA strand reacts with a 5′-phosphate group at the adjacent end. DNA ligase catalyses formation of the phosphodiester bond that closes the nick and makes the sugar-phosphate backbone continuous. Consequently, option A is correct. The 3′-OH is not an enzyme and therefore does not make chlorophyll, cut DNA, or digest protein. Cutting is carried out by nucleases or restriction enzymes, while protein digestion is performed by proteases. The group participates as a chemical end of DNA, with ligase facilitating the reaction.