What is a restriction enzyme and how does it facilitate gene cloning?

• A. An enzyme that cuts DNA at specific sequences; creates compatible ends to insert a fragment into a vector.
• B. A protein that unwinds double-stranded DNA.
• C. An RNA molecule that carries amino acids during translation.
• D. An enzyme that joins DNA fragments with hydrogen bonds.

Answer: (A)

The key idea is how restriction enzymes enable gene cloning by cutting DNA at specific sequences to create fragments that can be inserted into a vector. These enzymes recognize short, specific sites in DNA (often palindromic) and cleave the molecule at those sites. The cut can leave sticky ends with overhanging bases or blunt ends. Sticky ends are particularly useful because they have complementary overhangs that pair with other fragments cut with the same enzyme, guiding correct insertion.

In gene cloning, you cut both the DNA fragment you want to clone and the cloning vector with the same restriction enzyme to produce compatible ends, then seal the backbone with DNA ligase. This ligase forms phosphodiester bonds to join the DNA fragments, creating recombinant DNA that can be inserted into host cells for replication.

This differs from other options: unwinding DNA is done by helicase, not restriction enzymes; tRNA is an RNA adaptor in translation; joining DNA fragments uses ligase (which forms phosphodiester bonds, not hydrogen bonds), not the restriction enzyme itself.