Question

Which of the restriction enzyme does produce fragments with blunt end upon restriction digestion?

• A. BamH
• B. HindIII
• C. NotI
• D. SmaI

Hint:

SmaI is a classic blunt-end cutter that cleaves the sequence 5'-CCC$\downarrow$GGG-3' symmetrically in the middle.
Cohesive (sticky) ends are produced by enzymes like BamHI, EcoRI, and HindIII.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
Restriction endonucleases are bacterial enzymes that cleave double-stranded DNA at specific palindromic recognition sequences.
Based on their cleavage pattern, they produce either sticky ends (with single-stranded overhangs) or blunt ends (with no overhangs).

Step 2: Detailed Explanation:
Let us analyze the cleavage patterns of the listed restriction enzymes:
1. BamHI: Recognizes the sequence 5'-G$\downarrow$GATCC-3' and cleaves between the two guanine residues.
This asymmetrical cut produces 5'-cohesive (sticky) overhangs:
\[ \text{5'-G} \quad \text{and} \quad \text{5'-GATCC-3'} \]
2. HindIII: Recognizes 5'-A$\downarrow$AGCTT-3' and cuts asymmetrically, producing 5'-cohesive overhangs.
3. NotI: Recognizes the 8-base pair sequence 5'-GC$\downarrow$GGCCGC-3' and cuts asymmetrically, producing 5'-sticky ends.
4. SmaI: Recognizes the sequence 5'-CCC$\downarrow$GGG-3' and cleaves symmetrically directly at the center of the recognition site, between the internal cytosine and guanine residues.
This symmetrical cut leaves no single-stranded overhangs, producing blunt-ended fragments:
\[ \text{5'-CCC-3'} \quad \text{and} \quad \text{5'-GGG-3'} \]

Step 3: Final Answer:
The restriction enzyme that produces blunt ends upon digestion is SmaI, which corresponds to Option (D).