Section 3: Genetics, Cellular and Molecular Biology
CRISPR-Cas9 gene editing
Subtopic: Fundamentals of CRISPR-Cas9 gene editing
Question Statement:
Which component of the CRISPR/Cas9 gene editing system is NOT of natural
origin?
Options:
(A) CAS9 protein
(B) CRISPR repeats
(C) PAM sequence
(D) sgRNA
Official Answer Key:
D
Step-by-Step Solution & Explanation
### **Official Solution & Concept Analysis (GATE BT 2026 Q.26)**
**1. Governing Concept**:
- **Topic**: Genetic Engineering & CRISPR/Cas9 Genome Editing
- In the native bacterial type II CRISPR adaptive immunity system of *Streptococcus pyogenes*:
- The targeting guide consists of two separate, natural RNA molecules: **crRNA** (CRISPR RNA, containing the 20 nt protospacer target) and **tracrRNA** (trans-activating crRNA, which forms a scaffold duplex with crRNA to recruit Cas9).
- The **Cas9 endonuclease** is a natural bacterial protein containing RuvC and HNH nuclease domains.
- In 2012, Jennifer Doudna and Emmanuelle Charpentier engineered an artificial synthetic fusion: a single continuous chimeric RNA linking crRNA and tracrRNA via a synthetic hairpin loop, termed the **single guide RNA (sgRNA)**.
**2. Option Analysis**:
- **Option (A) [Cas9 protein] is INCORRECT**: Naturally occurring bacterial endonuclease.
- **Option (B) [crRNA] is INCORRECT**: Naturally transcribed and processed from the genomic CRISPR repeat-spacer array.
- **Option (C) [tracrRNA] is INCORRECT**: Naturally occurring small RNA in *S. pyogenes*.
- **Option (D) [Single guide RNA (sgRNA)] is CORRECT**: An artificial, human-engineered chimeric fusion of crRNA and tracrRNA not found in natural bacteria.
**3. Final Answer**:
- **Correct Choice**: **D**
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