Section 3: Genetics, Cellular and Molecular Biology
DNA replication, transcription, translation
Subtopic: Fundamentals of DNA replication, transcription, translation
Question Statement:
Which one of the following BLAST search programs is used to identify homologs of a
genomic DNA query in a protein sequence database?
Options:
(A) blastp
(B) blastn
(C) blastx
(D) tblastn
GATE 2018 Biotechnology
Official Answer Key:
C
Step-by-Step Solution & Explanation
### **Official Solution & Step-by-Step Derivation (GATE BT 2018 Q.17)**
**1. Governing Concept & Question Classification**:
- **Section**: BT
- **Question Type**: **MCQ** (1 Mark)
- **Topic**: Bioinformatics (BLAST Algorithms & Search Strategies)
**2. Detailed Step-by-Step Analysis**:
- The BLAST (Basic Local Alignment Search Tool) family comprises distinct programs tailored to query and database sequence types:
1. **blastn**: Compares a nucleotide query against a nucleotide database.
2. **blastp**: Compares an amino acid (protein) query against a protein database.
3. **blastx**: Translates a nucleotide query in all 6 reading frames and compares it against a protein database.
4. **tblastn**: Compares a protein query against a nucleotide database translated dynamically in all 6 reading frames.
5. **tblastx**: Translates both a nucleotide query and a nucleotide database in all 6 frames to perform protein-level comparisons.
- The question asks: *"Which one of the following BLAST search programs is used to identify homologs of a genomic DNA query in a protein database?"*
- Query: Genomic DNA (nucleotide) $ o$ translated into 6 protein frames.
- Database: Protein.
- This translation of a nucleotide query to search against a protein database is performed by **blastx**.
- **Option-by-Option Evaluation**:
- **Option (A) [INCORRECT]**: *blastp* searches a protein query against a protein database.
- **Option (B) [INCORRECT]**: *blastn* searches a nucleotide query against a nucleotide database.
- **Option (C) [CORRECT]**: *blastx* translates a nucleotide query in all six frames to search against a protein database.
- **Option (D) [INCORRECT]**: *tblastn* searches a protein query against a translated nucleotide database.
**3. Final Answer / Verification Summary**:
- **Official GATE Answer Key**: **C**
- **Scoring Criteria**: +1.0 Mark (Negative marking: -0.33 marks).
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