GATE 2022 BT – Question 25

Recombinant DNA Technology and Other Tools in Biotechnology · Recombinant DNA technology: Restriction and modification enzymes, vectors, expression vectors, libraries, gene isolation and cloning, transposons, gene targeting · 1 mark · Multiple choice

The recognition sequences of four Type-II restriction enzymes (RE) are given below. The symbol (↓) indicates the cleavage site. Identify the RE that generates sticky ends.

RE1 - 5̍ G↓GATCC 3̍
RE2 - 5̍ CTG↓CAG 3̍
RE3 - 5̍ CCC↓GGG 3̍
RE4 - 5̍ AG↓CT 3̍

Practise this question in The GATE Grind →

Show answer and explanation

Correct answer: (A) RE1 - 5̍ G↓GATCC 3̍

Explanation

A restriction enzyme produces **sticky (cohesive) ends** when it cuts the two strands at different positions, leaving a single-stranded overhang. It produces **blunt ends** when it cuts both strands at the same position (in the middle of a palindrome).

**RE1, 5′-G↓GATCC-3′:** the cut is after the first base. The complementary strand reads 5′-GGATCC-3′ too, and its cut is also after the first G, so the cuts are 4 bases apart on the two strands, giving a 5′ overhang of GATC. **Sticky.** ✓ (this is BamHI)
**RE2, 5′-CTG↓CAG-3′:** the cut is in the middle of the 6-base site, so both strands are cut at the same place. **Blunt**, as drawn.
**RE3, 5′-CCC↓GGG-3′:** a cut in the middle gives **blunt** ends (SmaI).
**RE4, 5′-AG↓CT-3′:** a cut in the middle of the 4-base site gives **blunt** ends (AluI).

Answer **RE1** (A).