Why is energy required in a DNA ligation reaction?

Correct answer: To form a new phosphodiester bond

DNA ligase joins DNA ends by catalysing formation of a new phosphodiester bond in the sugar-phosphate backbone. Creating this covalent linkage is not energetically free, so the ligase reaction uses an activated substrate and chemical energy. Depending on the enzyme source, the energy may be supplied through ATP or NAD+; for example, many common laboratory ligases use ATP, whereas some bacterial ligases use NAD+-dependent activation. Option A is correct because the energy supports formation of the new DNA backbone bond. DNA colouring, freezing water and dissolving fat are unrelated to the biochemical purpose of ligation.