Q.12 Which part of the genomic DNA contains the sequence corresponding to the 5′ untranslated region (5′ UTR)?

(A) Exon
(B) Intron
(C) Upstream of the transcription start site
(D) Upstream of the promoter

The sequence corresponding to the 5′ untranslated region (5′ UTR) in genomic DNA is found in the exon.
This region starts at the transcription start site (TSS) and extends to just before the start codon, becoming part of the mature mRNA after splicing.​
Correct Answer: (A) Exon

Option Analysis

• (A) Exon: The 5′ UTR sequence resides in the first exon(s) of eukaryotic genes, beginning at the TSS (+1 position) and ending upstream of the ATG start codon. During RNA processing, introns are removed, leaving this exonic sequence in the mature mRNA’s 5′ UTR, which regulates translation via elements like uORFs and secondary structures.​

• (B) Intron: Introns are non-coding sequences spliced out before mRNA maturation, so they do not contribute to the final 5′ UTR. While ~35% of human genes have introns in the 5′ UTR region of pre-mRNA, these are removed, and the 5′ UTR derives solely from exons.​

• (C) Upstream of the transcription start site: This area (5′ to TSS, positions -1 and beyond) contains promoter elements like the TATA box but is not transcribed into mRNA. The 5′ UTR begins exactly at the TSS, making this option incorrect.​

• (D) Upstream of the promoter: Regions far upstream (enhancers, silencers) regulate transcription initiation but are neither transcribed nor part of the 5′ UTR, which is downstream of the promoter and starts at the TSS.​

The 5′ UTR genomic DNA location is a critical concept in molecular biology, especially for CSIR NET Life Sciences aspirants tackling gene structure questions. This region, from the transcription start site to the start codon, regulates mRNA translation efficiency and stability.​

Gene Structure Basics

Eukaryotic genes consist of promoters, exons, and introns. Transcription begins at the TSS within the first exon, producing pre-mRNA. Splicing removes introns, retaining exonic sequences—including the 5′ UTR—as part of mature mRNA.​
In humans, 5′ UTRs average 100-200 nucleotides and often contain regulatory elements like Kozak sequences or uORFs.​

Why Exons Hold 5′ UTR Sequences

• The 5′ UTR spans the 5′ end of exon 1 (post-TSS) to pre-ATG, directly transcribed and retained post-splicing.

• Introns in this area (~35% of genes) are excised, confirming exonic origin.​

• Promoter/TSS distinction ensures no upstream confusion.​

MCQ Relevance for CSIR NET

Questions test understanding of genomic vs. mature mRNA contexts. Option traps like “intron” mislead due to pre-mRNA introns, but genomic 5′ UTR sequences are exonic.​

Functions and Implications

5′ UTRs form secondary structures (hairpins) influencing ribosome scanning and translation. Mutations here link to diseases like Alzheimer’s via IRE dysregulation.​
For competitive exams, visualize: Promoter → TSS (exon start) → 5′ UTR → CDS → 3′ UTR.​