Ligase Enzymes

During DNA repair, a nick may remain in the sugar-phosphate backbone of one DNA strand. DNA ligase forms a phosphodiester bond to seal this nick and restore strand continuity. Separating DNA strands is the function of helicase, not ligase. Exam tip: remember ligase as the enzyme that “joins” or “seals” DNA fragments and nicks.

The core function of ligase is DNA joining. Other details relate to energy source and bond formation.

Ligase enzymes form bonds between DNA ends with the help of energy. In Class 11 link them with recombinant DNA.

Ligase joins broken DNA fragments by forming phosphodiester bonds between them, so option A is correct. Restriction endonucleases cut DNA, while helicase separates the two DNA strands. Exam tip: remember “restriction enzyme cuts; ligase joins.”

DNA ligase is used to join a cut plasmid and gene fragment. It helps make a recombinant plasmid.

DNA ligase catalyzes joining between DNA ends. This reaction is linked with phosphodiester bond formation.

Without ligase the permanent phosphodiester bond will not form. Therefore stable recombinant DNA will not form.

Complementary overhangs allow sticky ends to pair first. Active ligase then makes a permanent bond.

Blunt ends lack overhangs so initial base pairing support is low. Therefore ligation can be less efficient.

T4 DNA ligase can join both sticky and blunt DNA ends. Therefore it is popular in cloning labs.

T4 DNA ligase obtains energy from ATP to join nicks or broken DNA ends. It is ATP-dependent: AMP is transferred during the ligation process, helping formation of the phosphodiester bond. NAD⁺ is used by some bacterial DNA ligases, not by T4 DNA ligase. Exam tip: remember T4 DNA ligase as ATP-dependent, whereas many bacterial DNA ligases are NAD⁺-dependent.

The governing concept is the source and nomenclature of enzymes used in recombinant-DNA technology. T4 DNA ligase is obtained from bacteriophage T4, so option A is correct. The designation “T4” identifies the bacteriophage associated with the enzyme; it does not refer to a plant, human tissue, or fungus. T4 DNA ligase joins compatible DNA ends by catalysing formation of phosphodiester bonds in the DNA backbone, using ATP as an energy source. This activity makes it useful for ligation of an insert into a vector during cloning. Mango leaf, human bone, and fungal cap are not the standard biological sources indicated by the enzyme’s name and therefore are implausible distractors.

The governing concept is discontinuous synthesis of the lagging DNA strand. DNA polymerase can extend a new strand only in the 5′→3′ direction, so the lagging strand is produced as short Okazaki fragments. After RNA primers are removed and the gaps are filled with DNA, a remaining nick exists between adjacent fragments. DNA ligase seals this nick by forming a phosphodiester bond between a 3′-hydroxyl group and a 5′-phosphate group. Therefore option A is correct. Helicase unzips DNA, primase makes RNA primers, and DNA polymerase selects and adds complementary nucleotides.

DNA ligase joins Okazaki fragments formed on the lagging strand. This makes the DNA strand continuous.

DNA ligase seals a single-strand nick in the sugar-phosphate backbone of DNA by forming a phosphodiester bond. Thus, it directly restores continuity of the DNA strand. Complete chromosome loss cannot be repaired simply by the action of ligase. Exam tip: remember that ligase joins broken DNA ends or seals nicks.

Polymerase extends a strand by adding nucleotides. Ligase seals backbone nicks between fragments.

Ligase joins broken DNA fragments by forming phosphodiester bonds, for example while joining Okazaki fragments. Helicase uses ATP energy to separate the two strands of the DNA double helix. Therefore, option A is correct. In option B, the functions of the two enzymes are interchanged. Exam tip: remember that helicase opens DNA, whereas ligase joins DNA pieces.

DNA ligase forms phosphodiester bonds to join broken DNA ends or seal nicks. Hence, it joins Okazaki fragments and joins a vector with an insert during recombinant DNA formation. Cutting DNA at a restriction site is performed by a restriction endonuclease, not by ligase. Exam tip: remember restriction enzyme for cutting and ligase for joining.

Ligase forms covalent phosphodiester bonds between foreign DNA and vector backbone. This makes recombinant DNA stable.

Buffer gives the right environment for ligase activity. Wrong buffer can reduce ligation efficiency.

In many ligation reactions magnesium ion is important for enzyme activity. Practical buffers may include such ions.

Ligase makes recombinant DNA and then it can be introduced into a host cell. This process is called transformation.

In self-ligation the vector can join its own ends. This does not form the desired recombinant plasmid.

Directional cloning uses different ends. Ligase can then join the insert in the correct orientation.

The direct answer is option A: inactive ligase is a possible reason for ligation failure. DNA ligase is the enzyme that seals compatible DNA ends by forming phosphodiester bonds in the backbone. If the enzyme has lost its activity because of heat, poor storage, an unsuitable reaction mixture, or another problem, the required bonds will not form efficiently. Option A is correct because an inactive joining enzyme directly prevents ligation. Option B is wrong because compatible ends usually help the reaction by bringing matching DNA molecules together. Option C is wrong because a proper buffer generally supplies suitable chemical conditions rather than causing failure. Option D is wrong because T4 DNA ligase uses ATP, and available ATP supports its activity when other conditions are correct. These factors do not guarantee success alone, but they are supportive, not normal failure causes. Memory cue: failed joining—check ligase activity, ends, buffer, temperature, and energy.