13. We are studying a protein-coding gene in an organism whose genome is not
sequenced. The nucleotide sequence of 20 bases at the 5’ end of its 900 base exon is
known. What is the most efficient way to find the entire coding sequence of the gene?
a. Use NGS to compare the known 20-base sequence to the transcriptome.
b. PCR amplify from purified genomic DNA using a 5’ primer against the known
sequence, and a random hexamer primer. Then do Sanger sequencing.
c. Reverse transcribe, then PCR amplify the resulting DNA using a 5’ primer against
the known sequence, and oligo-dT primers. Then do Sanger sequencing.
d. Do in-situ hybridization using a fluorescently labeled RNA probe against the
known sequence, and compare with a control RNA against a random sequence.
CSIR NET Solved: Finding Entire Coding Sequence with Known 5′ End
The most efficient way to determine the full coding sequence of a protein-coding gene when only the first 20 bases of its 900-base exon are known in an unsequenced genome is option c: Reverse transcribe RNA, PCR amplify using a 5′ primer against the known sequence and oligo-dT primers, then Sanger sequence. This targets mature mRNA to capture the complete exon directly.
Option Analysis
Option a (NGS transcriptome comparison): Next-generation sequencing of the transcriptome identifies transcripts matching the 20-base sequence but requires substantial sequencing depth, computational mapping, and assembly without a reference genome. This approach is resource-intensive and less targeted for a single gene.
Option b (PCR from genomic DNA with random hexamer): Amplifying genomic DNA with a specific 5′ primer and random hexamer generates products from the known sequence into unknown downstream genomic regions, including introns. Since eukaryotic genes typically contain introns, this yields contaminated sequences needing complex splicing prediction, making it inefficient for coding sequence recovery.
Option c (RT-PCR with oligo-dT, Sanger sequencing): Reverse transcription using oligo-dT primers binds the poly-A tail of mRNA, producing cDNA from the 3′ end toward the 5′ end. PCR with the specific 5′ primer and oligo-dT (or anchored variants) amplifies the full-length cDNA of the transcript, bypassing introns. Sanger sequencing then provides the precise coding sequence, ideal for targeted, low-input gene discovery.
Option d (In-situ hybridization): Fluorescent RNA probes detect the transcript’s location in tissues via hybridization but reveal no nucleotide sequence information. This visualizes expression patterns, not the coding sequence itself.
The challenge of finding the entire coding sequence (CDS) of a protein-coding gene arises frequently in molecular biology, especially for organisms with unsequenced genomes. When only 20 bases at the 5′ end of a 900-base exon are known, targeted amplification becomes essential for CSIR NET Life Sciences exam preparation.
Why RT-PCR with Oligo-dT Excels
Reverse transcription PCR (RT-PCR) using a gene-specific 5′ primer and oligo-dT primers directly captures the mature mRNA, yielding intron-free cDNA for Sanger sequencing. Oligo-dT anchors to the poly-A tail, ensuring 3′ completeness, while the known 5′ sequence provides specificity. This method outperforms genomic PCR, which includes introns, and NGS, which demands high coverage without references.
Comparison of Methods
| Method | Targets | Advantages | Limitations |
|---|---|---|---|
| NGS Transcriptome | All RNAs | High-throughput discovery | Costly, no reference assembly issues |
| Genomic PCR + Random Primer | gDNA | Simple primers | Introns contaminate CDS |
| RT-PCR + Oligo-dT (Correct) | mRNA cDNA | Targeted, full CDS | Needs RNA quality |
| In-situ Hybridization | Tissue RNA | Localization | No sequence data |
CSIR NET Exam Insights
This question tests understanding of RACE-like strategies (Rapid Amplification of cDNA Ends) adapted for 3′ extension from known 5′ sequence. Random hexamers suit incomplete RT but risk non-specific products; oligo-dT ensures poly-A driven completeness for protein-coding genes. Practice similar queries on gene cloning techniques for competitive exams.
