Genetic Engineering

Restriction enzymes recognise a specific DNA sequence and cut at that site, leaving short single-stranded overhangs called sticky ends. The pattern of sticky ends depends entirely on which restriction enzyme was used.

If the same restriction enzyme cuts both the donor DNA and the plasmid, both cuts produce the same pattern of sticky ends. These ends are complementary to each other, meaning they can pair up through hydrogen bonding (A–T, C–G).

If different restriction enzymes were used, the sticky ends would have different base sequences and would not be complementary. They would not bind together, so the gene could not be inserted into the plasmid. Once the sticky ends bond, DNA ligase seals the joins to form recombinant DNA.

The correct reason is that using the same restriction enzyme ensures complementary sticky ends are produced on both the gene and the plasmid, allowing them to bond together correctly.

The question asks why the same restriction enzyme must be used on both the donor DNA and the plasmid — so the answer must relate to a property shared between both cuts.

The option about cutting speed is incorrect. The speed of cutting is not relevant to whether the gene can be inserted into the plasmid. Restriction enzymes are not matched by speed.

Restriction enzymes do not destroy the plasmid — they cut it open at a specific site. Using the same enzyme on the plasmid is not about protecting the plasmid from destruction.

Restriction enzymes cut at specific sequences, not to produce equal-length fragments. The length of the resulting pieces is irrelevant to whether the gene can be joined to the plasmid.

The remaining option — that the same enzyme produces complementary sticky ends — is correct. Complementary sticky ends allow the gene and plasmid to join together so DNA ligase can seal them permanently.

DNA ligase is described as 'molecular glue'. Its role is to join DNA fragments together by forming covalent bonds between the sugar-phosphate backbones of the DNA strands.

After the insulin gene and the plasmid have been cut with the same restriction enzyme and their sticky ends have bonded through base pairing, DNA ligase seals the joins permanently, creating a stable recombinant plasmid.

The correct answer is that DNA ligase permanently seals the insulin gene into the plasmid. The other steps (cutting, carrying, and copying) are performed by restriction enzymes, vectors, and PCR respectively.

The question asks specifically about the role of DNA ligase in the genetic engineering process. Each distractor describes a different tool used in the process.

Cutting DNA is the role of restriction enzymes, not ligase. Ligase is associated with joining, not cutting.

Carrying DNA into a host cell is the role of a vector (such as the plasmid itself or a virus). Ligase does not transport DNA.

Making millions of copies of a DNA sequence is the role of PCR (Polymerase Chain Reaction). This happens before insertion into the plasmid, not during joining.

The correct answer is that DNA ligase seals the insulin gene into the plasmid. It acts as molecular glue, forming permanent covalent bonds to complete the recombinant DNA.