Ligase Enzymes

T4 DNA ligase is ATP dependent and E coli DNA ligase is generally NAD plus dependent. This is a common comparison.

Ligases join two molecules by forming a covalent bond, usually with energy supplied by ATP. For example, DNA ligase forms a phosphodiester bond between broken DNA ends. Enzymes that break bonds belong to different enzyme classes, not ligases. Exam tip: remember “ligation” means joining.

DNA ligase is a sealing and joining enzyme. It can close nicks in a DNA strand, join Okazaki fragments during lagging-strand synthesis, and connect a vector with a foreign DNA insert during recombinant-DNA formation. These functions involve creating phosphodiester bonds in the DNA backbone. Therefore option D is the wrong role. Cutting DNA at a restriction site is carried out by a restriction endonuclease, which recognises a specific sequence and cleaves the DNA. The distinction is important: restriction enzymes cut, whereas ligase joins or seals the resulting DNA ends.

In recombinant DNA DNA fragments from different sources are joined. Ligase makes this joining permanent.

In cloning practical fewer desired recombinants can indicate a ligation problem. It is not always the only reason.

Ligase joins DNA ends to form a joined DNA molecule. This product can be useful in both cloning and repair.

The expression molecular glue is an analogy for the joining action of DNA ligase. When two DNA ends are correctly aligned, ligase catalyses formation of covalent phosphodiester bonds in the sugar-phosphate backbone, converting separate fragments into one continuous DNA molecule. Thus option A gives the best reason. This joining is not merely a weak physical association; it is a chemically stable bond-forming reaction. Option B describes restriction enzymes or other nucleases, while C concerns staining methods used for visualisation. Option D is opposite to ligase function because ligase preserves and assembles DNA rather than removing it.

The most basic identity of DNA ligase is that it is an enzyme that joins DNA strands or fragments. It catalyses phosphodiester-bond formation between suitably aligned DNA ends, thereby sealing nicks and producing a continuous DNA backbone. Consequently, option A is correct and is the simplest reliable definition. Option B is a common confusion: DNA cutting is mainly associated with restriction endonucleases, not ligase. Option C refers to enzymes involved in carbohydrate breakdown, and option D is not an enzyme at all but a category describing a plant pigment such as chlorophyll. These distinctions make A the only scientifically appropriate answer.

Compatible ends come close and make joining easier for ligase. Incompatible ends can reduce ligation efficiency.

DNA ligase joins adjacent DNA ends by forming a phosphodiester bond. Many DNA ligases require an energy source such as ATP for this reaction. Amylase, not ligase, is the main enzyme involved in starch digestion. Exam tip: remember ligase as the enzyme that joins DNA fragments.

The leading strand is made continuously while Okazaki fragments form on the lagging strand. Therefore ligase fragment joining role is more prominent on the lagging strand.

Ligase forms a phosphodiester bond between DNA backbones of vector and insert. This makes stable recombinant DNA.

In cloning ligation makes recombinant DNA. Then transformation can introduce it into a host cell.

In Class 11 link ligase with DNA joining in cloning and Okazaki fragment joining in replication. This is the safest summary.

The core fact of ligase is DNA joining and phosphodiester bond formation. Keep it separate from the cutting role of restriction enzyme.

Ligase enzymes join DNA fragments with the help of energy. DNA ligase completes the DNA backbone by forming phosphodiester bonds.