Ligase Enzymes

E. coli DNA ligase uses NAD⁺ as the energy cofactor while sealing nicks in DNA, so option A is correct. In contrast, many bacteriophage ligases, such as T4 DNA ligase, are ATP-dependent; therefore, option B is a close but incorrect choice for E. coli. Exam tip: commonly remember bacterial DNA ligase as NAD⁺-dependent and T4 ligase as ATP-dependent.

DNA ligase acts on nicked DNA or DNA with cut ends. It joins them by forming phosphodiester bonds.

The most important identity of ligase is joining DNA. Study it as the opposite role of restriction enzyme.

DNA ligase is commonly associated with joining DNA fragments in recombinant-DNA technology. After a vector and an insert have been cut and aligned, ligase seals the breaks in their sugar-phosphate backbones and forms a recombinant DNA molecule. It also joins Okazaki fragments during normal DNA replication, but the central biotechnology association in cloning is DNA-fragment joining. Option A is therefore correct. Respiratory gas exchange is a physiological function of organs, transpiration is water loss from leaves, and bone formation is a process of animal tissue development. None of these processes describes the molecular action of DNA ligase.

DNA ligase joins cut DNA fragments or seals nicks by forming phosphodiester bonds in the sugar-phosphate backbone. Therefore, option A is correct. Helicase separates DNA strands, whereas DNA polymerase synthesizes new DNA. Exam tip: remember ligase as the enzyme that “joins” DNA pieces.

Restriction enzyme cuts DNA and later ligase joins the cut ends. This is the common order in cloning workflow.

Complementary ends of a vector and an insert can first pair through complementary base pairing, which holds the two DNA molecules in the correct alignment. This pairing alone is not a permanent covalent connection. DNA ligase then seals the remaining discontinuities in the sugar-phosphate backbones by catalysing formation of phosphodiester bonds, producing a stable recombinant DNA molecule. Option A is therefore correct. A ribosome synthesises proteins, chlorophyll participates in light absorption during photosynthesis, and amylase hydrolyses starch; none of them joins DNA fragments. Complementary ends assist ligation, but the enzyme responsible for the permanent join is DNA ligase.

A ligase joins the ends of two DNA fragments by forming phosphodiester bonds between them. Therefore, it is correctly described as a joining enzyme. Restriction endonucleases cut DNA, whereas DNA polymerase synthesizes DNA. Exam tip: remember that “ligate” means to join or bind.

DNA ligase forms a bond between 3 prime OH and 5 prime phosphate. It completes the DNA backbone.

The governing concept is that DNA nucleotides are connected through a sugar-phosphate backbone. A phosphodiester bond links the 3′ hydroxyl group of one deoxyribose sugar to the 5′ phosphate group of the next nucleotide. DNA ligase catalyses formation of this covalent bond when it joins adjacent DNA ends or seals a nick. Therefore, option A is correct: the bond becomes part of the DNA sugar-phosphate backbone. A cell wall is a structural layer outside some cells, a fat layer is made mainly of lipids, and a ribosome surface is not the structure being repaired or joined by ligase. Ligase joins DNA; it does not make any of those distractor structures.

Successful ligation needs active enzyme and proper DNA ends. An energy source also helps ligase action.

The direct answer is A: ATP can provide energy in a ligase reaction. DNA ligase must join two DNA ends by forming a phosphodiester bond in the backbone. This bond-forming reaction needs chemical energy. Some ligases, including commonly used T4 DNA ligase, use ATP for activation and joining. ATP does not become the DNA sequence; its energy supports the reaction. Option A is correct. Option B is wrong because cutting DNA is done by restriction enzymes or other nucleases, not by ATP in a ligase reaction. Option C is wrong because gel staining uses dyes such as nucleic-acid stains, not ATP as a colouring agent. Option D is wrong because protein digestion is carried out by proteases, not ligase and ATP. A careful detail is that not every ligase uses ATP: some bacterial ligases use NAD+ instead. The question says “can be,” so ATP is a valid possible energy source. Memory cue: ligase joins; ATP or another cofactor supplies reaction energy.

T4 DNA ligase depends on ATP while E coli DNA ligase is generally linked with NAD plus. This is a direct exam difference.

The governing principle of recombinant-DNA construction is that a vector and an insert must be joined into one continuous DNA molecule. After restriction enzymes create compatible ends, DNA ligase brings the ends together and catalyses phosphodiester-bond formation in the sugar-phosphate backbones. Thus option A, DNA ligase, performs the joining step and helps produce the recombinant plasmid. DNA helicase separates strands rather than joining them. RNA polymerase synthesises RNA from a DNA template, and ribonuclease degrades RNA. Those enzymes may have important cellular functions, but none carries out covalent joining of the plasmid vector and inserted DNA in this procedure.

During replication of the lagging strand, DNA polymerase synthesises short pieces called Okazaki fragments because synthesis proceeds only in the 5′ to 3′ direction. After primer removal and replacement, adjacent fragments can still have a nick in the sugar-phosphate backbone. DNA ligase seals that nick by catalysing a phosphodiester bond, making the lagging strand continuous. Therefore option A is correct. A chloroplast is a photosynthetic organelle, lipase digests fats, and haemoglobin transports oxygen; none is a DNA-joining enzyme. Ligase does not synthesise the fragments initially, but it completes their covalent connection.

If ligase is absent Okazaki fragments will not join properly. The lagging strand will remain incomplete.

DNA ligase is an enzyme that joins DNA strands by forming the missing phosphodiester bond in the sugar-phosphate backbone. A nick is a small break in one strand where the bases may still be paired correctly but the backbone is not continuous. During DNA repair, ligase seals such nicks after another repair enzyme has removed damaged material and a polymerase has filled the gap. Therefore, option A, DNA repair, is correct. This sealing restores continuity and helps preserve the stability of the genome.

Ligase is also useful in biotechnology because it joins compatible DNA fragments during recombinant-DNA construction. However, the question asks specifically about nick sealing, and DNA repair is the relevant biological process. Fat digestion uses digestive enzymes, photosynthesis uses light-driven reactions, and water evaporation is a physical change without DNA ligase. Thus, the supplied explanation and answer correctly connect ligase with repair of breaks in the DNA backbone.

The direct answer is A: ligase joins fragments during DNA replication. Replication produces the leading strand continuously but the lagging strand discontinuously, in short pieces called Okazaki fragments. After each fragment is made and its RNA primer is replaced with DNA, small breaks remain between neighbouring fragments. DNA ligase forms phosphodiester bonds and seals these breaks, producing one continuous strand. Option A is correct. Option B is wrong because making food in leaves is mainly photosynthesis, involving chlorophyll and light. Option C is wrong because calcium storage in bone involves cells and mineral tissue, not DNA ligase. Option D is wrong because oxygen transport in blood is mainly performed by haemoglobin in red blood cells. Ligase can also join DNA fragments in genetic engineering, but the replication example is a clear biological role. Memory cue: lagging strand leaves gaps; ligase links them.

The direct answer is option A: DNA ends must be close so that ligase can form a bond between them. DNA ligase seals a break in the sugar-phosphate backbone by joining a nearby 3-prime hydroxyl end to a 5-prime phosphate end, when the ends are properly aligned and reaction conditions are suitable. Bringing compatible ends close first may involve complementary base pairing, especially with sticky ends. Option A is correct because physical proximity and correct alignment are needed for bond formation. Option B is wrong because ligation is not meant to burn DNA. Option C is wrong because protein digestion is unrelated to joining DNA ends. Option D is wrong because the gel is only a separation medium and does not disappear as a result of ligation. Compatible ends, active ligase, suitable buffer, and required energy all support the reaction, but distance or poor alignment can prevent it. Memory cue: ligase is a molecular sealant; the ends must meet before it can seal.

5 prime phosphate is needed to form a phosphodiester bond. Its absence can reduce ligation efficiency.

Ligation needs compatible ends active ligase and an energy source. These conditions help permanent joining.

T4 DNA ligase joins DNA fragments by forming phosphodiester bonds between 3′-OH and 5′-phosphate ends. Therefore, it is used in recombinant DNA and cloning experiments to join vector DNA with foreign DNA. Option B describes a restriction endonuclease, which cuts DNA. Exam tip: remember, ligase joins DNA, whereas restriction enzymes cut DNA.

Ligase joins matching ends. Insert orientation often depends on restriction enzyme design and ends.

In self-ligation a vector can join its own ends without insert. This can be an unwanted result in cloning.

Two different ends can reduce the chance of vector self-joining. This is useful in directional cloning.