Ligase Enzymes

E. coli DNA ligase uses NAD⁺ as a cofactor to form phosphodiester bonds between broken DNA ends. In contrast, many viral DNA ligases, such as T4 DNA ligase, are ATP-dependent. FAD and biotin are not cofactors for this ligation reaction. Exam tip: remember bacterial DNA ligase as generally NAD⁺-dependent and T4 DNA ligase as ATP-dependent.

The direct answer is option B: T4 DNA ligase. T4 is a bacteriophage, meaning a virus that infects bacteria. The enzyme named T4 DNA ligase is encoded by and associated with bacteriophage T4. It joins DNA ends by catalysing phosphodiester-bond formation and is widely used in molecular cloning, especially for joining DNA fragments. Option B is correct because its name directly identifies the T4 bacteriophage source or association. Option A, E. coli ligase, is associated with the bacterium Escherichia coli, not T4 phage. Option C, DNA polymerase I, is mainly involved in DNA synthesis and replacement of primers or gaps, not identified as the T4-linked ligase in this question. Option D, amylase, digests starch into smaller carbohydrates and is unrelated to DNA joining. Exam cue: the name “T4 DNA ligase” itself points to T4 bacteriophage.

DNA ligase repairs or creates phosphodiester bonds in the DNA backbone. It is needed for DNA continuity.

Ligase joins Okazaki fragments and makes the lagging strand continuous. This is an important point in replication.

If ligase is absent nicks between Okazaki fragments will not be sealed. The lagging strand will remain incomplete.

DNA ligase forms a covalent phosphodiester bond between 3 prime OH and 5 prime phosphate. This is the chemical basis of ligation.

Removing 5 prime phosphate from vector can reduce joining of its own ends. This helps control self-ligation.

Alkaline phosphatase can remove phosphate from vector ends. This can reduce vector self-ligation.

Dephosphorylated vector reduces self-ligation but proper insert ends help ligation. Chemical ends must be considered.

In directional cloning different ends are designed. Ligase joins compatible insert and vector ends.

Directional cloning helps join the insert in the desired orientation. Ligase then permanently joins the correct ends.

A self-ligated vector may close with its own ends but lacks the desired insert. Therefore it can be unwanted in cloning.

A proper vector and insert ratio can improve desired ligation. Too little insert can form fewer recombinant plasmids.

Low insert can reduce the chance of desired joining with vector. Ratio is a practical factor in cloning experiments.

If DNA ends are compatible ligation can be easier. Incompatible ends can make joining difficult.

Ligase is a protein enzyme and can become inactive by heat. Cold condition helps preserve activity.

Successful ligation requires an active ligase, suitable DNA ends, an appropriate reaction buffer, and the chemical energy needed to activate the DNA ends. If the enzyme has lost its activity because of improper storage, excessive heat, unsuitable pH, or another damaging condition, it cannot catalyse phosphodiester-bond formation even when the DNA fragments are compatible. Thus option A is the correct reason for reaction failure. Compatible ends in B generally promote joining, while a proper buffer in C supports enzyme function. The availability of ATP or another required cofactor in D supports ligation rather than causing failure.

T4 DNA ligase requires ATP to form phosphodiester bonds between DNA ends. Without ATP, enzyme activation and the ligation reaction can stop, so ligation may be greatly reduced or fail. Cutting DNA is the role of a restriction endonuclease, not a ligase. Exam tip: Remember that T4 DNA ligase is ATP-dependent.

Proper buffer gives ligase correct pH and salt conditions. Wrong conditions can reduce activity.

Many ligation buffers contain magnesium ions for enzyme activity. Cofactors can be important in practical conditions.

The direct answer is option A: ligase can denature and become inactive. Ligase is a protein enzyme that joins DNA ends by forming a phosphodiester bond. Every enzyme works best within a suitable temperature range. First, moderate warmth may increase molecular movement and help the reaction. However, excessive heat disrupts the weak bonds that maintain the enzyme’s three-dimensional shape. The active site then changes, so ligase cannot bind and join DNA efficiently. Therefore, ligation usually decreases or stops; the exact effect depends on the enzyme and conditions. Option A is correct because it describes this heat-induced loss of activity. Option B is wrong because ligation will not always increase; too much heat can destroy enzyme activity. Option C is wrong because DNA does not turn into chlorophyll, which is a pigment made through biological pathways. Option D is wrong because ATP is not converted into sugar by heating a ligase reaction. In an experiment, use the recommended temperature and time, and avoid unnecessary heating. Memory cue: mild heat may help a reaction, but excess heat can unfold a protein enzyme.

In replication ligase joins Okazaki fragments and in cloning it joins vector-insert. These are two main contexts.

DNA repair often leaves a nick or discontinuity in the sugar-phosphate backbone after damaged portions have been removed and the correct nucleotides have been inserted. DNA ligase seals this remaining break by catalysing formation of the missing phosphodiester bond, thereby restoring continuity of the strand. Hence option A is correct. Ligase does not generally destroy DNA, so B reverses its actual role. It is not a protein-staining reagent, making C irrelevant, and although water is present in biological reactions, D is scientifically meaningless because ligase requires appropriate substrates, ions, pH, and energy conditions rather than water alone.

DNA ligase joins a nick in the sugar-phosphate backbone of DNA by restoring the phosphodiester bond, making the strand continuous again. Cutting DNA is the role of nucleases, not ligase. Exam tip: remember ligase as the enzyme that “joins” DNA fragments or seals nicks.

A nick is a small break in the sugar-phosphate backbone of one DNA strand. DNA ligase seals this break by forming a phosphodiester bond, so option A is correct. DNA polymerase adds nucleotides, whereas restriction enzymes cut DNA at specific sequences. Exam tip: remember ligase as the enzyme that ‘joins’ broken DNA ends or seals nicks.