Biology · Biotechnology: Principles and Processes · NEET
DNA ligase is an enzyme that acts as the "molecular glue" of genetic engineering: it joins two cut DNA pieces by sealing the gap (nick) in the sugar-phosphate backbone. After a restriction enzyme (the "molecular scissors") cuts the vector and the foreign gene, DNA ligase pastes them together to make recombinant DNA. Memory hook: scissors CUT, ligase GLUES.
DNA ligase joins the complementary sticky ends of a cut vector and cut foreign gene by sealing the backbone nick, producing one continuous recombinant DNA molecule.
Your doubts, answered
Does DNA ligase cut DNA or join DNA?
DNA ligase only JOINS DNA. It never cuts. Cutting is done by restriction endonucleases (molecular scissors). Ligase acts on the already-cut ends and seals them by forming a phosphodiester bond in the DNA backbone. Simple rule to remember: scissors = restriction enzyme, glue = ligase.
Why is DNA ligase called the molecular glue?
Because it pastes two DNA fragments into one continuous molecule. When the foreign gene and the plasmid vector have complementary sticky ends, they first pair up by hydrogen bonds, but there is still a break (nick) in the backbone. DNA ligase glues this break shut, so the pieces become one stable recombinant DNA molecule.
Can DNA ligase join sticky ends only, or blunt ends too?
DNA ligase can join both. Sticky ends (overhangs made by enzymes like EcoRI) join easily because the overhangs first pair up and hold the pieces together, so ligase only seals the nick. Blunt ends have no overhang, so joining is harder and slower, but ligase can still do it. NEET usually links ligase with sticky ends made by the same restriction enzyme.
What is the difference between DNA ligase and DNA polymerase?
DNA polymerase ADDS new nucleotides to build or extend a DNA strand (used in PCR with Taq polymerase). DNA ligase does NOT add nucleotides; it only joins two existing DNA ends by sealing the gap between them. So polymerase = builds/extends, ligase = seals/joins.
Where exactly is DNA ligase used in making recombinant DNA?
After the foreign DNA and the plasmid vector are cut with the SAME restriction enzyme, both carry identical complementary sticky ends. DNA ligase then joins these ends to form the recombinant plasmid. It is the joining step of recombinant DNA technology, coming right after cutting.
⚠️ The NEET trap ✗ Thinking any two DNA pieces can be glued directly, or that ligase itself creates the sticky ends. ✓ The foreign DNA and vector must first be cut by the SAME restriction endonuclease so they have complementary sticky ends; only then does DNA ligase join them. Ligase joins the ends but does not make them. 🧠 Same restriction enzyme, then ligase - do not skip the enzyme match.
Real NEET questions
2016
A foreign DNA and plasmid cut by the same restriction endonuclease can be joined to form a recombinant plasmid using
A · Eco RI
B · Taq polymerase
C · Polymerase III
D · Ligase ✓
Solution: When a foreign DNA and a plasmid are cut with the same restriction endonuclease, both carry identical complementary sticky ends. DNA ligase acts on these cut DNA molecules and joins their ends end-to-end to form a recombinant plasmid. EcoRI cuts (does not join), and polymerases synthesise DNA rather than seal the nick. NCERT Ch 9: DNA ligase acts on cut DNA molecules and joins their ends.
2020
Choose the correct pair from the following:
A · Nuclease - Separate the two strands of DNA
B · Exonuclease - Make cuts at specific positions within DNA
C · Ligases - Join the two DNA molecules ✓
D · Polymerases - Break the DNA into fragments
Solution: DNA ligase acts on cut DNA molecules and joins their ends, so 'Ligases - Join the two DNA molecules' is correct. Nucleases cut rather than separate strands; exonucleases remove nucleotides from ends (endonucleases cut internally); polymerases synthesise DNA rather than break it.
2019
Match the enzymes with their functions: (a) Restriction endonuclease (b) Restriction exonuclease (c) DNA ligase (d) Taq polymerase with (i) Joins the DNA fragments (ii) Extends primers on genomic DNA template (iii) Cuts DNA at specific position (iv) Removes nucleotides from the ends of DNA
A · a-iii, b-i, c-iv, d-ii
B · a-iii, b-iv, c-i, d-ii ✓
C · a-iv, b-iii, c-i, d-ii
D · a-ii, b-iv, c-i, d-iii
Solution: Restriction endonuclease cuts DNA at a specific position (iii); exonuclease removes nucleotides from the ends (iv); DNA ligase joins the cut DNA fragments (i); Taq polymerase extends primers on the genomic DNA template (ii). Correct match: a-iii, b-iv, c-i, d-ii.
Solved Biotechnology: Principles and Processes NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
DNA ligase is the enzyme that joins two cut DNA fragments by sealing the nick in the sugar-phosphate backbone, forming recombinant DNA. It is called the molecular glue.
Is DNA ligase a restriction enzyme?
No. Restriction enzymes cut DNA (molecular scissors). DNA ligase does the opposite - it joins DNA. They are used one after the other in genetic engineering: cut first, then ligate.
What bond does DNA ligase form?
It forms a phosphodiester bond, sealing the gap between the 3' and 5' ends of the cut DNA strands so the backbone becomes continuous again.
Is DNA ligase used in DNA replication too?
Yes. In replication, DNA ligase joins the short Okazaki fragments of the lagging strand into a continuous strand. So it acts as a joining enzyme in both replication and recombinant DNA technology.
Why must the same restriction enzyme cut both the vector and the gene?
So that both produce identical complementary sticky ends. Matching sticky ends pair up correctly, and then DNA ligase can seal them into one recombinant molecule.
