Ligase Enzymes

Correct answer: B. Ligase joins DNA fragments and helicase unwinds DNA

DNA helicase unwinds the double helix by separating the complementary strands through disruption of hydrogen bonds. DNA ligase joins adjacent DNA fragments, such as Okazaki fragments, by forming phosphodiester bonds. Therefore, option B is correct. Option A reverses the functions of the two enzymes. Exam tip: remember helicase as the enzyme that opens the helix and ligase as the enzyme that joins fragments.

Correct answer: D. Transformation into a host cell

After ligation the recombinant plasmid can be introduced into a host cell. This process is called transformation.

Correct answer: A. Inactive ligase

The direct answer is option A: inactive ligase is one possible reason for ligase-reaction failure. Ligation means joining DNA fragments by sealing the sugar-phosphate backbone. DNA ligase must be active to catalyse formation of phosphodiester bonds between properly aligned DNA ends. If it is inactive, recombinant DNA formation may be reduced or absent. Option A is correct because it directly removes the enzyme activity required for joining. Option B is wrong because compatible ends can align through base pairing and usually make ligation easier. Option C is wrong because a proper buffer helps maintain the chemical conditions needed by the enzyme. Option D is wrong because an available energy source supports the reaction; it is not normally a reason for failure. However, compatible ends, buffer, and energy cannot compensate for an inactive enzyme, and other issues such as incorrect temperature or poor DNA quality can also matter. Memory cue: no active ligase, no efficient DNA sealing.

Correct answer: B. Joining will be difficult

The direct answer is option B: joining will be difficult. Sticky ends are single-stranded overhangs. Complementary bases on two overhangs can pair through hydrogen bonds, placing the DNA fragments in a suitable position for ligase to seal their sugar-phosphate backbones. Non-complementary sticky ends cannot form the required base pairing, so the fragments do not align and remain together efficiently. Therefore ligation may be reduced or difficult. Option A is wrong because non-complementarity does not make joining easier. Option B is correct because it describes the reduced efficiency expected in this reaction. Option C is wrong because DNA-to-protein conversion is not automatic and requires transcription followed by translation. Option D is wrong because DNA cutting is done by restriction enzymes or other nucleases, whereas ligase joins DNA. Memory cue: complementary ends pair; ligase then repairs the backbone.

Correct answer: C. Joined DNA molecule

DNA ligase forms phosphodiester bonds between DNA fragments and seals the sugar-phosphate backbone. Therefore, its product is a joined DNA molecule. Producing single-stranded DNA is not the function of ligase. Exam tip: Remember ligase as the enzyme that “joins” DNA fragments.

Correct answer: D. As a helper cofactor for enzyme activity

In many ligation reactions magnesium ion can help enzyme activity. Therefore buffer composition is important.

Correct answer: A. When compatible sticky ends are present

The governing concept is complementary-end pairing before enzymatic sealing. Compatible sticky ends contain complementary single-stranded overhangs, so they can hydrogen-bond with one another and temporarily hold the DNA fragments in the correct alignment. This increases the local concentration and correct orientation of the 3′-OH and 5′-phosphate groups, allowing ligase to form the phosphodiester bond more efficiently. Therefore option A is correct. Boiling usually denatures the protein enzyme, absence of an energy source prevents or reduces ligation chemistry, and incompatible ends cannot pair properly. Blunt ends may also be ligated, but they generally require more favourable conditions and are often less efficient than compatible sticky ends.

Correct answer: A. It seals a nick in a damaged DNA strand

During DNA repair, after a damaged section is removed and replacement nucleotides are added, a small break called a nick may remain in the sugar-phosphate backbone. DNA ligase seals this nick by forming a phosphodiester bond, restoring continuity of the DNA strand. Removal of a damaged base is mainly performed by a DNA glycosylase, not ligase. Exam tip: remember ligase as the enzyme that ‘joins DNA breaks’.

Correct answer: B. T4 DNA ligase

T4 DNA ligase is a famous example of a DNA joining enzyme. Pepsin and amylase are linked with digestion.

Correct answer: A. bond formation

Ligase enzymes join two molecules or DNA ends by forming a new covalent bond. DNA ligase specifically forms a phosphodiester bond between adjacent DNA fragments. Bond hydrolysis is the function of hydrolase enzymes, not ligases. Exam tip: remember that the enzyme that joins DNA fragments is ligase.

Correct answer: D. Cut vector and insert will not join permanently

Restriction enzyme can cut but ligase performs permanent joining. Therefore ligase is essential in cloning.

Correct answer: B. Formation of phosphodiester bond

The direct answer is B: formation of a phosphodiester bond. DNA ligase joins two DNA ends by making a covalent phosphodiester bond in the sugar-phosphate backbone. First, the ends are brought close and correctly aligned. Then ligase seals the break by joining the 3′-OH group of one nucleotide to the 5′-phosphate group of the other. Because this is a strong covalent bond, the joining is permanent under ordinary conditions. A is wrong because hydrogen bonds between bases are weak and reversible; they help complementary strands pair but do not seal a backbone break. B is correct because it describes the actual permanent chemical link. C is irrelevant: a colour change is not DNA joining. D is irrelevant: gel solidification is a physical change and does not connect DNA ends. Memory cue: ligase is the DNA “glue,” and its glue is a covalent phosphodiester bond.

Correct answer: C. Ligase joins Okazaki fragments

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

Correct answer: D. It joins DNA ends

The core function of ligase is joining DNA ends. In Class 11 link it with recombinant DNA and replication.

Correct answer: A. Pepsin

Pepsin is a proteolytic enzyme that breaks proteins into smaller peptides in the stomach, so it is not a ligase. In contrast, T4 DNA ligase and E. coli DNA ligase join broken DNA ends by forming phosphodiester bonds. Exam tip: ligases join molecules or DNA fragments, whereas enzymes such as pepsin break down proteins.

Correct answer: B. Cut DNA then ligate DNA then transform

In cloning DNA is first cut then ligated and recombinant DNA enters the host. This is the basic logical order.

Correct answer: C. Compatible ends and active ligase are present

Compatible ends and active ligase help form a recombinant plasmid. This is the joining step of cloning.

Correct answer: D. DNA backbone continuity

DNA ligase restores DNA strand continuity by sealing backbone nicks. It is important in both repair and replication.

Correct answer: B. Joining

The name ligase refers to the action of joining. DNA ligase joins DNA fragments by forming phosphodiester bonds between their ends. Cutting is performed by restriction endonucleases, not by ligase. Exam tip: remember that ‘ligate’ means to join.

Correct answer: A. covalent phosphodiester bond

DNA ligase forms a covalent phosphodiester bond between the 3′-OH end and the 5′-phosphate end of a DNA strand. This permanently seals a nick in the sugar-phosphate backbone. Hydrogen bonds occur between complementary bases, so they do not seal DNA ends. Exam tip: Associate DNA ligase with formation of phosphodiester bonds in the DNA backbone.

Correct answer: C. Joining insert DNA with vector

In laboratory cloning ligase joins insert DNA with vector. This is a main step in making recombinant DNA.

Correct answer: A. डीएनए शर्करा-फॉस्फेट बैकबोन में निक

DNA ligase seals a nick in the sugar-phosphate backbone of DNA by forming a phosphodiester bond. It is important for joining Okazaki fragments during DNA replication and for DNA repair. DNA helicase unwinds the DNA double helix; it does not seal nicks. Exam tip: remember ligase as the enzyme that joins DNA fragments.

Correct answer: B. To join Okazaki fragments

In replication Okazaki fragments form on the lagging strand. Ligase joins them to make a continuous strand.

Correct answer: A. Lagging strand

DNA ligase joins Okazaki fragments of the lagging strand. This is often asked in replication MCQs.

Correct answer: C. DNA ligase

DNA ligase joins pre-existing DNA ends by forming phosphodiester bonds; it does not add new nucleotides. DNA polymerase is the closest distractor because it extends a DNA strand by adding nucleotides. Exam tip: if DNA ends or nicks are being sealed, think of ligase.