Ligase Enzymes

DNA polymerase and DNA ligase have different catalytic roles. A polymerase extends a pre-existing primer by adding individual nucleotides, normally to a free 3′ hydroxyl group, so it lengthens a DNA chain. Ligase does not perform that nucleotide-by-nucleotide extension. Instead, it joins already existing DNA ends or seals a nick, using energy to form a phosphodiester bond between neighbouring nucleotides. Consequently, option A correctly describes what ligase does not do. Options B, C, and D describe its actual joining function or the chemical bond produced. The question is about mode of action, not about whether ligase participates in DNA synthesis overall.

A typical cloning workflow first isolates the vector and insert, then cuts them or otherwise prepares compatible DNA ends. The ends must be available and correctly aligned before the joining reaction can be performed. DNA ligase is therefore added at the ligation stage, often near the end of the in-vitro assembly sequence, to seal the compatible vector and insert ends by forming phosphodiester bonds. Option A is correct. Ligase does not cut DNA; restriction enzymes usually perform that preparation. It does not make a gel, and it is an enzyme rather than a host cell. The phrase “near the last step” refers to the order of assembly, not an absolute requirement for every protocol.

An enzyme’s substrate is the molecule or structure on which it acts. DNA ligase recognises DNA ends that are already close together but have a discontinuity in one sugar-phosphate strand; this structure is called nicked DNA. The enzyme seals the nick by forming the missing phosphodiester bond, thereby restoring a continuous backbone. Therefore option A is the best answer. Glucose is a carbohydrate, fatty acid is a lipid component, and chlorophyll is a photosynthetic pigment. None provides adjacent DNA nucleotide ends or the nucleic-acid backbone required for ligase activity. Ligase joins pre-existing DNA structures rather than metabolising ordinary cellular nutrients or pigments.