Ligase Enzymes

DNA ligase joins cut or nicked DNA ends by forming phosphodiester bonds. Therefore, it is used in recombinant DNA technology to join an inserted DNA fragment with a vector. The function in option B belongs to a restriction enzyme, which cuts DNA at specific recognition sequences. Exam tip: remember ligase as the “molecular glue” of DNA.

DNA ligase permanently joins two DNA fragments by forming phosphodiester bonds between them. In genetic engineering, it is used to insert a desired gene into vector DNA. Cutting DNA at specific sites is the role of restriction enzymes, not ligase. Exam tip: remember ligase as the “molecular glue” that joins DNA fragments.

The governing concept is the complementary division of labour between restriction enzymes and DNA ligase in recombinant-DNA technology. A restriction enzyme cuts DNA at a particular recognition sequence and produces fragments, often with complementary sticky ends. Those ends may base-pair, but base pairing alone is not a permanent covalent connection. DNA ligase catalyses formation of phosphodiester bonds between adjacent nucleotides, sealing the breaks in the sugar-phosphate backbone and producing a continuous recombinant DNA molecule. Therefore option A is correct. DNA ligase does not colour DNA, digest fat, or generate light. Restriction digestion prepares the pieces, whereas ligation joins and stabilises them.

The governing concept is DNA ligation during recombinant-DNA construction. A restriction enzyme can create compatible, often sticky, ends on a vector and a foreign DNA insert. These ends may pair temporarily through complementary base pairing, but pairing alone does not make the recombinant molecule stable. DNA ligase catalyses formation of phosphodiester bonds in the sugar-phosphate backbone, thereby sealing the nicks and joining the fragments permanently. Thus option A is correct. Without ligase, the insert may detach during handling or transformation. The other options describe unrelated biological effects and are not consequences of omitting ligase.

Restriction enzymes and DNA ligase perform different steps in recombinant-DNA construction. A restriction enzyme cleaves DNA at a particular recognition site, producing fragments with compatible or blunt ends. DNA ligase then seals the adjacent DNA strands by catalysing formation of phosphodiester bonds in the sugar-phosphate backbone. This creates a stable recombinant DNA molecule. A ribosome translates messenger RNA into protein, chlorophyll captures light during photosynthesis, and glucose is a carbohydrate; none of these joins DNA fragments. Therefore option A, DNA ligase, is the only correct answer.

Ligase joins DNA fragments by forming bonds between them. For exams remember it as the joining enzyme.

DNA ligase forms phosphodiester bonds in the sugar-phosphate backbone. This permanently joins DNA fragments.

Ligase joins the cut vector and foreign DNA. This is an essential step in making recombinant DNA.

Ligase joins DNA pieces so it can be called DNA glue. In exams distinguish cutting and joining enzymes.

Direct answer: C, the restriction enzyme cuts DNA and ligase joins DNA. Restriction enzymes act like precise molecular scissors. They recognize particular DNA sequences and cleave the phosphodiester backbone, producing sticky or blunt ends. DNA ligase acts like molecular glue: it joins DNA ends by forming phosphodiester bonds in the backbone, usually after compatible ends have aligned. Option C is correct because it states both roles in the correct order. Option A reverses the functions, so it is wrong. Option B is wrong because these enzymes do not merely make proteins; they act on DNA, and protein synthesis is performed by a different cellular system. Option D is wrong because neither enzyme produces light. In cloning, a restriction enzyme opens the vector and cuts the insert, while ligase seals the desired recombinant DNA molecule. Memory cue: restriction enzyme cuts; ligase links.

DNA ligase forms a phosphodiester bond between the 3′-OH end and the 5′-phosphate end of adjacent DNA nucleotides. This seals a nick or break in the sugar-phosphate backbone. A peptide bond is formed between amino acids, so option B is not correct. Exam tip: remember ligase as the enzyme that joins broken DNA ends.

In ligation DNA fragments are joined with the help of ligase. It connects the insert with the vector in cloning.

DNA ligase can join compatible sticky and blunt ends. Joining is usually easier with sticky ends.

Sticky ends can first pair temporarily by complementary bases. This makes joining by ligase easier.

Blunt ends do not have single-stranded overhangs. Therefore they do not hold easily by base pairing first.

DNA ligase joins Okazaki fragments during replication. In exams connect it with the lagging strand.

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

DNA ligase is an enzyme that joins adjacent DNA segments by sealing a nick in the sugar-phosphate backbone. During replication, the lagging strand is produced discontinuously as Okazaki fragments. After the RNA primers are removed and the gaps are filled with DNA, breaks may still remain between neighbouring fragments. Ligase catalyses formation of the missing phosphodiester bond, using an activated intermediate and cellular energy, so the strand becomes continuous and stable. Option A is correct. Ribosome production, fat storage and light capture involve other cellular structures or pathways and are unrelated to the repair of the DNA backbone.

A nick is a small break in the sugar-phosphate backbone of one DNA strand. Ligase can seal such breaks.

T4 DNA ligase uses energy from ATP to form a phosphodiester bond between the 5′-phosphate and 3′-OH ends of DNA. This is why it is widely used in recombinant DNA cloning. Some bacterial DNA ligases use NAD+, but T4 DNA ligase is ATP-dependent, not NAD+-dependent. Exam tip: remember T4 DNA ligase as an ATP-dependent ligase.

E coli DNA ligase generally uses NAD plus. In exams remember the energy difference between T4 ligase and E coli ligase.

T4 DNA ligase is the DNA-joining enzyme associated with bacteriophage T4, a virus that infects Escherichia coli. The enzyme catalyses the formation of phosphodiester bonds between adjacent DNA ends and is widely used in recombinant-DNA experiments. Its name identifies its biological source: T4 refers to the bacteriophage, while DNA ligase describes the enzyme activity. Therefore option A is correct. Human liver, plant roots and fungal caps are not the source indicated by the name and are not the standard biological source of T4 DNA ligase. The enzyme may be purified and used in laboratories, but its origin remains linked to phage T4.

Ligases join two molecules by forming a new covalent bond, usually coupled with the breakdown of an energy source such as ATP. For example, DNA ligase forms phosphodiester bonds between broken DNA ends. Enzymes that break bonds using water are hydrolases, so option C is not correct. Exam tip: associate “joining” or “sealing” of molecules with ligase.

DNA ligase joins cut or broken ends of a DNA molecule. It restores continuity in a DNA strand by forming a phosphodiester bond between adjacent nucleotides. RNA, proteins, and lipids are not its usual substrates. Exam tip: remember ligase as the enzyme that joins DNA fragments.

T4 DNA ligase is a famous enzyme used in cloning. Pepsin and trypsin are linked with digestion.