Question

Which enzyme is used to join two DNA fragments in genetic engineering?

• A. Restriction endonuclease
• B. DNA polymerase
• C. RNA polymerase
• D. DNA ligase

Hint:

Think of genetic engineering as a two-step process: cutting DNA at target sites, then sealing the cut ends back together into one molecule. Recall which class of enzyme handles the sealing step, specifically the one that closes a nick in the DNA backbone by forming a new bond.

Answer 1

The Correct Option is D

The enzyme used to join two DNA fragments in genetic engineering is DNA ligase. Here's how it works:

1. Function of DNA Ligase: DNA ligase is an enzyme that creates phosphodiester bonds between DNA strands, thus sealing breaks in the DNA backbone. It is crucial for connecting DNA fragments during the formation of recombinant DNA.
2. Role in Genetic Engineering: In genetic engineering, DNA ligase is employed to join DNA fragments that have been cut by restriction enzymes at specific sites. After restriction endonucleases cut the DNA, DNA ligase helps to recombine these fragments back together to form new sequences.
3. Comparison with Other Options:
   • Restriction Endonuclease: This enzyme cuts DNA at specific sequences, but does not join DNA fragments.
   • DNA Polymerase: This enzyme synthesizes new DNA strands by adding nucleotides, but does not join fragments.
   • RNA Polymerase: This enzyme synthesizes RNA from a DNA template, but is not involved in joining DNA fragments.
4. Conclusion: Given these functions, DNA ligase is the correct and appropriate choice for joining DNA fragments in genetic engineering. The role of DNA ligase is irreplaceable in most cloning processes, where precise joining of DNA is required to produce a stable recombinant DNA molecule.

Answer 2

Step 1: Understanding genetic engineering.
Genetic engineering involves cutting DNA at specific sites and then joining desired DNA fragments to form recombinant DNA. This process requires specialized enzymes.
Step 2: Role of DNA ligase.
DNA ligase is an enzyme that joins DNA fragments by forming phosphodiester bonds between adjacent nucleotides. It seals the sugar-phosphate backbone, making the DNA continuous.
Step 3: Analysis of the given options.
(A) Restriction endonuclease: These enzymes cut DNA at specific recognition sites.
(B) DNA polymerase: This enzyme synthesizes new DNA strands but does not join fragments.
(C) RNA polymerase: It synthesizes RNA from a DNA template.
(D) DNA ligase: Correct — it joins two DNA fragments by forming phosphodiester bonds.
Step 4: Conclusion.
The enzyme used to join two DNA fragments in genetic engineering is DNA ligase.

Answer 3

Concept:

• Genetic engineering builds recombinant DNA in two stages: first cutting DNA at target sites, then rejoining the pieces at their cut ends.
• Cutting is carried out by restriction enzymes, which leave a nick in the sugar-phosphate backbone at the joint between two DNA fragments.
• A separate enzyme is needed to seal this nick and restore one continuous strand.

Step 1: Identify which stage of the process the question is about.
The question asks about the joining step, not the cutting step. Two DNA fragments (for example, a gene of interest and a plasmid vector) are placed together at matching cut ends and must be permanently sealed into a single continuous molecule.

Step 2: Recall which enzyme performs the sealing, and how.
DNA ligase forms a new phosphodiester bond between the 3-prime hydroxyl end of one nucleotide and the 5-prime phosphate end of the adjacent nucleotide. This closes the nick between the two DNA pieces, fusing them into one unbroken strand.

Step 3: Match this function to the given options.
Restriction endonuclease cuts DNA at specific sequences and does not join anything. DNA polymerase extends an existing strand by adding new nucleotides during replication; it does not repair a break between two separate fragments. RNA polymerase makes an RNA copy of a DNA template and has no role in joining DNA. Only the enzyme that forms a phosphodiester bond across the nick matches the description in the question.

Final Answer: DNA ligase is the enzyme used to join two DNA fragments in genetic engineering.