UPSC Prelims 2019 Q99: Science & Technology – Biotechnology & Genetics
- (a) A molecular scissors used in targeted gene editing
- (b) A biosensor used in the accurate detection of pathogens in patients
- (c) A gene that makes plants pest - resistant
- (d) A herbicidal substance synthesized in genetically modified crops
Answer: (a) A molecular scissors used in targeted gene editing
Explanation
Cas9 Protein: CRISPR-Cas9 Gene Editing
| Option | Claim | Verdict | Detail |
|---|---|---|---|
| (a) | Molecular scissors used in targeted gene editing | CORRECT | Cas9 (CRISPR-associated protein 9) is a bacterial RNA-guided DNA endonuclease that cuts DNA at a location specified by a guide RNA. It is the workhorse enzyme of the CRISPR-Cas9 gene editing system - hence 'molecular scissors'. |
| (b) | Biosensor for pathogens | PARTIAL/WRONG | While CRISPR-derived platforms SHERLOCK (Cas13-based) and DETECTR (Cas12-based) are used as nucleic-acid biosensors for pathogens like SARS-CoV-2 and Zika, Cas9 itself is a DNA-cutting endonuclease used for gene editing, not diagnostics. Calling Cas9 a biosensor is incorrect. |
| (c) | Gene making plants pest-resistant | WRONG | Cas9 is an enzyme, not a resistance gene |
| (d) | Herbicidal substance in GM crops | WRONG | herbicides are chemicals (glyphosate) |
Cas9 is the molecular scissors of CRISPR-based gene editing. Answer: (a).
Answer: (a) A molecular scissors used in targeted gene editing.
Why each option is right or wrong
- (a) CORRECT. Cas9 is a bacterial enzyme repurposed as the 'molecular scissors' of CRISPR-based targeted gene editing. Nobel Prize 2020 (Charpentier, Doudna).
- (b) Incorrect. Cas9 is primarily an editor, though CRISPR variants like Cas13/Cas12 are used in biosensors.
- (c) Incorrect. Cas9 is an enzyme, not a plant resistance gene like Bt.
- (d) Incorrect. Herbicides are chemical substances (glyphosate), not proteins like Cas9.
Key point
Cas9 = CRISPR-Cas9 gene editing enzyme (molecular scissors). Nobel Chemistry 2020: Charpentier + Doudna. gRNA guides Cas9 to specific DNA sequence. Applied in medicine, agriculture, research.
Remember
Cas9 is the molecular scissors used in CRISPR-based targeted gene editing. It is an RNA-guided DNA endonuclease from bacteria that cuts DNA at a location specified by a guide RNA. Emmanuelle Charpentier and Jennifer Doudna won the 2020 Nobel Prize in Chemistry for developing CRISPR-Cas9 as a gene editing tool.
Common mistakes
- Confusing Cas9 with Bt (Bacillus thuringiensis) toxin used in pest-resistant GM crops - unrelated.
- Assuming Cas9 is a natural human protein - it is from Streptococcus pyogenes bacteria, adapted for genome editing.
- Not knowing CRISPR-Cas9 has enabled the first approved gene-editing therapy (Casgevy 2023 for sickle cell disease).
Exam tip
CRISPR toolkit: Cas9 (DNA cutting, most used). Cas12 (DNA cutting, DETECTR biosensor). Cas13 (RNA cutting, SHERLOCK diagnostic). Base editing (Liu, no double-strand break). Prime editing (Liu, precise insertions). Applications: gene therapy, agriculture, diagnostics.
Sources
- Nobel Prize 2020 - CRISPR-Cas9 Chemistry citation
- Nature / Science - Foundational CRISPR-Cas9 papers (Jinek et al., 2012)