Ligase Enzymes

Correct answer: A. Joining DNA and forming a joined DNA molecule

Ligase joins DNA ends to form a joined DNA molecule. It is important in recombinant DNA construction.

Correct answer: A. Phosphodiester bonds will not form between DNA ends

DNA ligase joins DNA fragments by forming phosphodiester bonds in their sugar-phosphate backbones. If the ligase is inactive, these bonds cannot form, so the vector and insert cannot be permanently ligated. Complementary sticky ends may pair temporarily, but this is not a stable covalent join. Exam tip: remember that ligase seals nicks in the DNA backbone.

Correct answer: A. Buffer provides a proper environment for enzyme activity

Ligation buffer helps ligase activity by providing pH salts and cofactors. Do not ignore buffer in practical questions.

Correct answer: A. ligation reaction

The direct answer is A: the ligation reaction. Ligase works when two DNA ends are close enough and properly aligned. Step by step, compatible ends come together, ligase recognizes the nearby nick or break, and it forms a phosphodiester bond in the DNA backbone. This is why closeness is essential: the enzyme cannot join ends that are far apart. A is correct because ligation specifically means joining DNA fragments. B, photosynthesis, is the light-driven process by which plants make carbohydrates; it does not require ligase to join DNA ends. C, starch cooking, involves heat and changes in starch, not DNA joining. D, fat storage, concerns storing lipids in cells and has no role in ligase action. Do not confuse a reaction’s name with every biological process that may occur in a cell. Exam cue: “ligation” means “joining”; ligase joins adjacent DNA ends.

Correct answer: A. Removal of 5 prime phosphate makes vector ends less able to join themselves

Removing 5 prime phosphate can reduce vector self-ligation. This is a strategy to reduce unwanted background in cloning.

Correct answer: A. missing 5 prime phosphate

Missing 5 prime phosphate can prevent phosphodiester bond formation. The other options help ligation.

Correct answer: A. DNA backbone becomes continuous

Ligase makes the DNA backbone continuous by forming a phosphodiester bond. This is the final effect of ligation.

Correct answer: A. DNA ligase

DNA ligase seals the nicked backbone and restores continuity. Link it with both repair and replication.

Correct answer: A. Because it permanently joins vector and insert

The direct answer is option A: ligase joins the vector and insert to form a stable recombinant DNA molecule. A vector is a DNA carrier, and an insert is the desired DNA fragment, such as a gene. In cloning, compatible ends of these two pieces are aligned first. DNA ligase then forms phosphodiester bonds in the backbone, so the fragments become covalently connected and stable under suitable reaction conditions. “Permanently” here means stable covalent joining, not that the molecule can never be cut by any later enzyme. Option A is correct. Option B is wrong because running a gel is done by electrophoresis equipment and an electric field. Option C is wrong because random cutting is not ligase activity; nucleases cut DNA. Option D is wrong because ligase does not create a host cell; the host is an existing living cell used to receive the vector. Exam cue: vector plus insert plus ligase gives recombinant DNA.

Correct answer: A. DNA-joining enzyme

The governing concept is distinguishing enzymes by the reaction they catalyse. DNA ligase is best identified as a DNA-joining enzyme because it seals breaks or nicks in the sugar-phosphate backbone by catalysing phosphodiester-bond formation between suitable DNA ends. Therefore option A is correct. A DNA-cutting enzyme is a nuclease, with restriction endonucleases being a familiar example; such enzymes should not be confused with ligase. Protein-digestion enzymes are proteases, and starch-digestion enzymes are amylases or related carbohydrases. Ligase is used in DNA replication and recombinant-DNA work, but its short exam identity remains “DNA-joining enzyme.”

Correct answer: A. DNA cloning

In DNA cloning ligase forms a covalent phosphodiester bond between vector and insert. This makes a stable recombinant plasmid.

Correct answer: D. Ligase creates Okazaki fragments by cutting DNA

During DNA replication, Okazaki fragments are formed on the lagging strand. DNA ligase seals the nick between adjacent fragments by forming a phosphodiester bond, making the strand continuous. Therefore, option D is incorrect; cutting DNA to create fragments is not the function of ligase. Exam tip: remember ligase as the enzyme that joins broken DNA ends.

Correct answer: A. glue

Ligase joins DNA fragments so the glue analogy is correct. The scissors analogy is for restriction enzymes.

Correct answer: A. After ligation recombinant DNA can enter a host cell

Ligation makes recombinant DNA and transformation can take it into a host cell. They are sequential steps in cloning workflow.

Correct answer: A. Joins DNA by forming phosphodiester bonds

DNA ligase is fundamentally a joining enzyme. It brings adjacent DNA ends together and catalyses formation of a phosphodiester bond between the 3′-hydroxyl group and the 5′-phosphate group of the DNA backbone. This action seals a nick or joins compatible DNA fragments, which is essential during recombinant-DNA construction and DNA repair. Therefore option A is correct. Restriction enzymes, represented by option B, cut DNA at particular recognition sequences rather than joining it. Options C and D describe unrelated processes involving carbohydrate digestion and plant pigment formation, so they do not identify ligase activity.

Correct answer: A. Recombinant DNA and DNA replication

Ligase joins fragments in recombinant DNA and joins Okazaki fragments in replication. Both exam contexts are important.

Correct answer: A. They join DNA ends with the help of energy

The direct answer is option A: ligase enzymes join DNA ends with the help of energy. During ligation, DNA ends are brought close together and the enzyme catalyses formation of phosphodiester bonds in the sugar-phosphate backbone. Energy is needed for this bond-forming reaction. This function is useful in DNA repair, replication, and cloning, where separate DNA pieces must become one continuous molecule. Option A is correct because it gives the essential action of DNA ligase. Option B is wrong because cutting DNA at recognition sites is done by restriction enzymes, not ligase. Option C is wrong because protein digestion into amino acids is performed by proteolytic enzymes. Option D is wrong because glucose does not become chlorophyll through ligase activity; chlorophyll synthesis is a different biochemical pathway. Memory cue: ligase is the molecular glue of DNA.

Correct answer: A. DNA ligase

DNA ligase forms phosphodiester bonds between cut vector and insert. This can make stable recombinant DNA.

Correct answer: B. permanent DNA joining

Without ligase permanent phosphodiester joining will not occur. Sticky ends can only pair temporarily.

Correct answer: C. DNA ligase

Direct answer: C, DNA ligase. A nick is a small break in one strand of the DNA sugar-phosphate backbone where neighboring nucleotides are not covalently connected, even though the bases may be paired. DNA ligase recognizes the properly aligned DNA ends and catalyzes formation of the missing phosphodiester bond, thereby sealing the nick. This occurs when Okazaki fragments are joined during replication and when an insert is joined to a vector during cloning. Option C is correct. Option A, DNA helicase, unwinds the double helix by separating base pairs; it does not seal nicks. Option B, a restriction enzyme, cuts DNA at particular recognition sequences and generally creates the break rather than repairing it. Option D, RNA polymerase, makes RNA from a DNA template and does not join DNA fragments. A helpful sequence is: helicase opens, polymerase synthesizes, restriction enzyme cuts, and ligase seals. Memory cue: ligase is the DNA repair and joining enzyme.

Correct answer: D. Joining DNA fragments

DNA ligase joins two DNA fragments by forming phosphodiester bonds between them. Therefore, it is used in the ligation step of recombinant DNA formation. A restriction enzyme cuts DNA, whereas gel electrophoresis separates DNA fragments. Exam tip: remember “ligase” as the enzyme that “joins” DNA fragments.

Correct answer: A. Phosphodiester bond between vector and insert

Ligase forms a phosphodiester bond between vector and insert. This makes the recombinant plasmid stable.

Correct answer: B. complementary base pairing

Sticky ends have complementary overhangs. They can first come together by base pairing and then ligase joins them.

Correct answer: C. They have no overhang

Blunt ends do not have complementary overhangs. Therefore they lack initial pairing support like sticky ends.

Correct answer: A. ATP

T4 DNA ligase uses ATP to form a phosphodiester bond between adjacent DNA ends. Therefore, ATP is correct. NAD⁺-dependent ligases are typically found in bacteria, whereas the T4 bacteriophage ligase is ATP-dependent. Exam tip: Remember T4 DNA ligase with ATP and bacterial DNA ligase commonly with NAD⁺.