Ligase Enzymes

Correct answer: A. Joining will be difficult

The direct answer is option A: joining will be difficult. Sticky ends are short single-stranded DNA overhangs produced by certain restriction enzymes. They join efficiently when the bases on one end are complementary to the bases on the other end, because complementary bases can hydrogen-bond and hold the fragments together temporarily. DNA ligase can then seal the backbone. If the sticky ends are not complementary, they cannot pair properly, so the fragments are poorly aligned and ligation efficiency decreases. Option A is correct. Option B is wrong because non-complementarity does not make joining always faster; it normally makes it less efficient. Option C is wrong because DNA does not automatically become protein; gene expression requires transcription and translation. Option D is wrong because ligase joins DNA and does not cut it; cutting is done by nucleases. Memory cue: matching sticky ends help ligase; mismatching ends hinder it.

Correct answer: A. 3 prime hydroxyl

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

Correct answer: A. 5 prime phosphate

5 prime phosphate is required for phosphodiester bond formation. Without it ligation can be affected.

Correct answer: A. bond formation

Direct answer: A, bond formation. Ligase enzymes join molecules or parts of the same molecule by catalyzing bond formation. In DNA work, DNA ligase joins adjacent DNA ends and forms phosphodiester bonds in the sugar-phosphate backbone. The reasoning is: DNA ends are aligned, the enzyme positions the reacting groups, energy is used as required, and a covalent bond is formed. Option A is correct because it names the defining action. Option B is wrong because random bond breaking is not ligase action; controlled cleavage is associated with nucleases or restriction enzymes. Option C is wrong because light production is unrelated to ligase chemistry. Option D is wrong because water evaporation is a physical change, not enzyme catalysis. Do not confuse ligase with lyase or nuclease: ligase joins, while nucleases cut nucleic acids. Memory cue: the word ligase sounds like link, so it links pieces.

Correct answer: A. Ligase restores DNA continuity by sealing nicks in the DNA backbone

Ligase joins the broken phosphodiester bond at a nick in the DNA backbone, thereby restoring DNA continuity. Option D describes DNA polymerase, which synthesizes a new strand by adding nucleotides. Exam tip: remember ligase as the enzyme that joins broken DNA ends.

Correct answer: A. Keep it under cold conditions

The governing concept is preservation of protein-enzyme activity. Ligase is a protein, and excessive heat can disrupt its three-dimensional structure, denature the active site, and reduce or destroy catalytic activity. Keeping a laboratory enzyme under the recommended cold conditions, commonly on ice during handling and in a suitable freezer for longer storage, slows degradation and helps retain activity. Thus option A is the best practical answer. Boiling is deliberately destructive to most enzymes, direct sunlight can supply heat and damaging radiation, and mixing the preparation with soil offers no controlled preservation. Exact temperature and buffer depend on the product protocol.

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

In cloning vector and insert are first cut then ligated and recombinant DNA goes into host. This is the logical order.

Correct answer: A. Joined molecule of vector DNA and insert DNA

Ligase can join vector DNA and insert DNA to form a recombinant plasmid. This is the central product of gene cloning.

Correct answer: A. Energy is needed for bond formation

The governing concept is enzyme-catalysed ligation coupled to an energy source. DNA ligase joins a 3′-hydroxyl end to a 5′-phosphate end and forms a phosphodiester bond in the DNA backbone. This bond-forming reaction is not spontaneous under ordinary ligation conditions; the enzyme uses chemical energy supplied through a cofactor or activated intermediate. Depending on the ligase, the energy source may be ATP or NAD⁺, so option A is correct in general terms. Ligase does perform chemistry rather than working without a reaction. Restriction enzymes cut DNA, but energy is not used only for cutting, and ligation does not change DNA colour.

Correct answer: A. Ligase is like glue and restriction enzyme is like scissors

Ligase joins DNA so the glue analogy is correct. Restriction enzyme cuts DNA so the scissors analogy is correct.

Correct answer: A. Which enzyme joins Okazaki fragments

The enzyme that joins Okazaki fragments is DNA ligase. Unwinding is linked with helicase and cutting with restriction enzymes.

Correct answer: A. Sealing a nick in a damaged strand

DNA ligase can seal a nick in a damaged strand during repair. This is important for DNA stability.

Correct answer: A. It is not efficient without compatible and chemically proper ends

The governing concept is substrate-end compatibility in DNA ligation. Ligase does not freely join every pair of DNA ends. The ends must be positioned correctly, and the necessary chemical groups, especially a 5′-phosphate and a 3′-hydroxyl, must be available. Compatible sticky ends often base-pair and hold the fragments together, increasing the chance of sealing; incompatible ends or unsuitable chemistry make ligation inefficient. Therefore option A states a genuine limitation. Options B and C describe normal functions of ligase, while option D is also correct because ligase seals nicks during DNA replication. The limitation concerns reaction suitability, not absence of joining ability.

Correct answer: A. 5 prime phosphate is needed in bond formation

DNA ligase forms a phosphodiester bond between 5 prime phosphate and 3 prime OH. Therefore 5 prime phosphate is important.

Correct answer: A. The 3′-OH participates in phosphodiester-bond formation

The governing concept is the chemical structure of the DNA backbone. During ligation, the 3′-hydroxyl group at the end of one DNA strand reacts with a 5′-phosphate group at the adjacent end. DNA ligase catalyses formation of the phosphodiester bond that closes the nick and makes the sugar-phosphate backbone continuous. Consequently, option A is correct. The 3′-OH is not an enzyme and therefore does not make chlorophyll, cut DNA, or digest protein. Cutting is carried out by nucleases or restriction enzymes, while protein digestion is performed by proteases. The group participates as a chemical end of DNA, with ligase facilitating the reaction.

Correct answer: D. It recognizes palindromic sites and cuts DNA

Recognizing palindromic sites and cutting DNA is the role of restriction enzymes. Ligase is a joining enzyme.

Correct answer: A. DNA joining tool in recombinant DNA construction

In Class 11 biotechnology ligase is a joining tool in recombinant DNA construction. Study it in cloning workflow.

Correct answer: A. Possibility of recombinant DNA formation

Ligase can join compatible vector and insert. This gives a possibility of recombinant DNA formation.

Correct answer: A. continuous lagging strand

Ligase joins Okazaki fragments to make the lagging strand continuous. This is an essential role in replication.

Correct answer: A. DNA ligase

DNA ligase seals a nick in the sugar-phosphate backbone of DNA by forming a phosphodiester bond. DNA polymerase synthesises new DNA, but DNA ligase performs the final sealing of a nick. Exam tip: Choose DNA ligase when the question mentions joining DNA ends or sealing a nick.

Correct answer: A. Ligase forms a covalent phosphodiester bond

Ligase forms covalent phosphodiester bonds to permanently join DNA fragments. This is its catalytic product.

Correct answer: A. It makes cut DNA pieces into a stable recombinant molecule

After restriction digestion ligase is needed to make DNA pieces into a stable molecule. This is an essential step in cloning.

Correct answer: A. phosphodiester bond formation

Permanent joining happens by phosphodiester bond formation. Ligase forms this bond in the DNA backbone.

Correct answer: A. Enzymes that join DNA ends with help of energy

The direct answer is option A: ligase enzymes join DNA ends with the help of energy. DNA has a sugar-phosphate backbone, and a complete joining requires formation of a phosphodiester bond between neighbouring nucleotides. DNA ligase catalyses this bond-forming reaction and uses chemical energy supplied by the reaction system. This is why it is used in DNA repair, replication, and recombinant-DNA cloning. Option A is correct because it states both the action, joining DNA ends, and the need for energy. Option B is wrong because cutting DNA at a particular recognition site is the function of a restriction endonuclease. Option C is wrong because breaking proteins into amino acids is done by proteases, not ligase. Option D is wrong because converting water into oxygen is not the definition of ligase and involves unrelated biological processes. Exam cue: ligase joins; restriction enzyme cuts.

Correct answer: A. restriction enzyme-cut and ligase-join

This pair of restriction enzyme and ligase is very useful in exams. Remember cutting and joining separately.