Ligase Enzymes

Correct answer: A. To remove phosphate from cut vector

Removing phosphate from vector ends can reduce self-ligation. Correct phosphate ends on insert then help ligation.

Correct answer: A. For transformation into a host cell

Ligase joins vector and insert to make a recombinant plasmid. It can be transferred into a host cell.

Correct answer: A. Sealing a single-strand nick

DNA repair often leaves a discontinuity, called a single-strand nick, in an otherwise correctly paired DNA molecule. The repair pathway fills or restores the missing nucleotide region, but the final interruption in the sugar-phosphate backbone must be sealed. DNA ligase performs this final sealing by forming the missing phosphodiester bond. Therefore option A is correct and represents a genuine repair-related use. Protein folding is assisted by chaperones and is not the role of ligase. Lipid digestion is carried out by lipases, while colouring a cell wall is a laboratory staining action, not a DNA-repair reaction. Ligase repairs structural breaks in nucleic-acid backbones.

Correct answer: A. Ligase joins DNA fragments, whereas helicase unwinds the DNA double helix

DNA ligase joins broken DNA fragments or seals nicks by forming phosphodiester bonds. DNA helicase separates the two strands of the DNA double helix by disrupting hydrogen bonds. Option B incorrectly reverses their functions. Exam tip: remember that ligase performs ligation (joining), while helicase opens the helix.

Correct answer: A. One cuts and the other joins

In recombinant DNA work restriction enzyme cuts DNA and ligase joins DNA. Therefore both tools are studied together.

Correct answer: A. Cut or nicked DNA ends

DNA ligase acts at cut DNA fragments or nicks in DNA. Using energy from ATP, it forms a phosphodiester bond in the sugar-phosphate backbone and joins the broken ends. Replicating single-stranded DNA is not its direct substrate. Exam tip: remember ligase as the enzyme that joins DNA fragments.

Correct answer: C. Pepsin

Pepsin is an enzyme linked with protein digestion. T4 DNA ligase and E coli DNA ligase are actual DNA ligases.

Correct answer: D. It cuts DNA at recognition sites

Cutting DNA at recognition sites is the job of restriction enzymes. T4 DNA ligase is a joining enzyme.

Correct answer: A. DNA ends will not join properly

Inactive ligase cannot form phosphodiester bonds. Therefore recombinant DNA formation may decrease or stop.

Correct answer: A. Compatible sticky ends are present

Compatible sticky ends come together easily due to base pairing. This can increase ligation efficiency.

Correct answer: A. More suitable ligase conditions

The direct answer is A: more suitable ligase conditions may be needed. Blunt ends have no single-stranded overhanging bases. Therefore, unlike sticky ends, they cannot first hold together through complementary base pairing, and ligation is often less efficient. The reaction may be improved by choosing an appropriate DNA ligase, using suitable temperature and buffer conditions, providing ATP or the required cofactor, increasing DNA concentration, or allowing enough reaction time. Option A is correct because it describes the kind of practical improvement that can help. Option B is wrong because DNA is the substrate to be joined; removing it makes ligation impossible. Option C is wrong because boiling generally denatures and destroys enzyme activity rather than improving it. Option D is wrong because sunlight is not the required energy or reaction condition for DNA ligation and may damage biological materials. Blunt-end ligation is possible, just less convenient than sticky-end ligation. Memory cue: sticky ends hold themselves; blunt ends need better conditions and ligase.

Correct answer: A. Cutting is restriction enzyme and joining is ligase

In exams roles of restriction enzyme and ligase are often confused. The easy rule is restriction cuts and ligase joins.

Correct answer: A. Ligase joins Okazaki fragments

Okazaki fragments form on the lagging strand. DNA ligase joins them to make a continuous strand.

Correct answer: A. Joining insert with vector in DNA cloning

In DNA cloning ligase joins compatible ends of insert and vector. This can form a recombinant plasmid.

Correct answer: A. Temperature and buffer condition

Ligase is a protein enzyme and can be affected by reaction conditions. Proper buffer and temperature are important in practicals.

Correct answer: A. Excess heat can denature ligase and cause it to lose its enzymatic activity

Ligase is a protein enzyme that helps form phosphodiester bonds between broken DNA ends. Excess heat can denature its three-dimensional structure, altering the active site and reducing or destroying enzyme activity. A restriction enzyme cuts DNA, whereas ligase joins DNA fragments, so option C is incorrect. Exam tip: For protein enzymes, excessive temperature is commonly associated with denaturation and loss of activity.

Correct answer: A. Joined DNA molecule

The product of ligase action is a joined DNA molecule. It joins fragments by forming a bond in the DNA backbone.

Correct answer: D. completely boiled enzyme

DNA ligase joins DNA ends by forming phosphodiester bonds, so the enzyme must be active. A completely boiled enzyme is usually denatured and loses its catalytic activity; therefore, it is an incorrect condition for a ligase reaction. Compatible DNA ends and an energy source such as ATP support ligation. Exam tip: remember that excessive heat denatures most enzymes and prevents them from functioning.

Correct answer: A. It joins DNA pieces

The term molecular glue is an analogy based on function, not on a sticky physical substance. DNA ligase brings neighbouring DNA ends into a suitable arrangement and catalyses covalent phosphodiester-bond formation between them. This permanently joins DNA fragments or closes a nick in the backbone, much as glue joins two pieces of material. Therefore option A is correct. Restriction enzymes, rather than ligase, cut DNA at particular sites. RNA polymerase produces RNA from a DNA template, and no normal ligase reaction converts DNA into sugar. The analogy is useful because ligase connects pre-existing molecular pieces without creating an entirely new strand from individual nucleotides.

Correct answer: A. phosphodiester bond

DNA ligase forms a phosphodiester bond in the DNA backbone of vector and insert. This makes stable recombinant DNA.

Correct answer: A. ligation by DNA ligase

Sticky ends can make weak association by base pairing. DNA ligase creates a permanent phosphodiester bond.

Correct answer: A. It can seal DNA nicks

The direct answer is option A: ligase can seal DNA nicks. A DNA nick is a small break in one strand of the DNA sugar-phosphate backbone, while the two parts are still held together by the other strand. DNA ligase uses energy to form a phosphodiester bond and close this break. This helps complete DNA replication, repair damaged DNA, and join fragments during cloning. Option A is correct because sealing a nick is a main function of DNA ligase. Option B is wrong because DNA is not changed into sugar; its backbone already contains sugar, and this conversion is not ligase activity. Option C is wrong because cutting DNA at specific recognition sequences is mainly done by restriction endonucleases. Option D is wrong because ligase does not work only on lipids; DNA ligase acts on DNA ends. Memory cue: ligase is the DNA “glue,” whereas a restriction enzyme is the DNA “scissors.”

Correct answer: A. Ligase helps make recombinant DNA by joining different DNA fragments

The direct answer is option A: ligase helps make recombinant DNA by joining different DNA fragments. Recombinant DNA contains DNA pieces obtained from different sources, such as a vector and an inserted gene. First, suitable DNA ends are prepared, often by restriction enzymes. The ends are brought together, and DNA ligase forms phosphodiester bonds in the sugar-phosphate backbone, making the joining stable. Option A is correct because this joining step is essential for constructing recombinant DNA. Option B is wrong because ligase does not burn or destroy DNA; it preserves and joins DNA. Option C is wrong because making a cell wall is a cellular or biosynthetic function, not the function of DNA ligase. Option D is wrong because oxygen transport is performed by molecules such as haemoglobin, not ligase. Exam cue: restriction enzymes cut, ligase joins.

Correct answer: A. It is a protein enzyme

Ligase is an enzyme and enzymes are generally protein in nature. Thus conditions can affect activity.

Correct answer: A. Ligase forms the bond

DNA ligase is a joining enzyme whose central action is sealing a discontinuity in a DNA backbone. It aligns a 3′ hydroxyl end with a neighbouring 5′ phosphate end and, using suitable chemical energy, catalyses formation of the phosphodiester bond between them. Thus option A states the correct relation. The enzyme does not merely break this bond; its characteristic role in replication, repair, and recombinant-DNA construction is to create the missing linkage. Colouring a bond is a staining or visualisation concept, not catalysis, and claiming no relation directly contradicts the known function of ligase. The answer therefore follows from the enzyme’s substrate and reaction.