Which Part of Genomic DNA Contains the Sequence Corresponding to the 5′ Untranslated Region (5′ UTR)?

Understanding the 5′ Untranslated Region of a Gene

The 5′ untranslated region, commonly abbreviated as the 5′ UTR, is an important part of a mature messenger RNA molecule. It is transcribed from genomic DNA but is not translated into the amino acid sequence of a protein.

The 5′ UTR begins at the transcription start site and extends to the translation start codon. In most protein-coding genes, the translation start codon is AUG in the mature mRNA and corresponds to ATG in the coding strand of genomic DNA.

The term “untranslated” can sometimes create confusion. Untranslated does not mean untranscribed. The 5′ UTR is transcribed into RNA and remains present in the mature mRNA, but ribosomes do not translate its main sequence into the protein encoded by the primary open reading frame.

Because the 5′ UTR is retained in the mature RNA after pre-mRNA splicing, the genomic DNA sequences corresponding to the mature 5′ UTR are classified as exonic sequences. Therefore, among the given options, the correct answer is exon.

Where Is the 5′ UTR Located in a Eukaryotic Gene?

A typical eukaryotic protein-coding gene contains several functional regions arranged in a specific order. The promoter is located upstream of the transcription start site and helps recruit the transcription machinery. Transcription begins at the transcription start site, commonly represented as +1.

After transcription begins, the RNA sequence between the transcription start site and the translation start codon forms the 5′ untranslated region. The coding sequence begins at the translation start codon and continues until a stop codon is reached. The transcript also generally contains a 3′ untranslated region downstream of the stop codon.

Therefore, the basic arrangement can be understood as follows: promoter, transcription start site, 5′ UTR, translation start codon, protein-coding sequence, stop codon, and 3′ UTR.

The 5′ UTR is therefore located downstream of the transcription start site and upstream of the translation start codon. Since it is part of the transcribed region and is retained in the mature mRNA, it is represented by exon sequence in genomic DNA.

Why Is the 5′ UTR Considered Part of an Exon?

Exons are commonly described as the parts of a gene that remain present in the mature RNA after RNA splicing. Although exons often contain protein-coding sequences, an exon does not necessarily have to encode amino acids.

This distinction is essential for understanding the correct answer. The first exon of a protein-coding gene may contain only the 5′ UTR, or it may contain both a portion of the 5′ UTR and the beginning of the protein-coding sequence.

During pre-mRNA processing, introns are removed and exons are joined together. The 5′ UTR sequences that are retained in the mature mRNA therefore belong to exons.

Thus, the correct biological definition is that exons are retained sequences, whereas introns are removed during splicing. Since the mature 5′ UTR is retained in the mature mRNA, its corresponding genomic sequence is exonic.

Transcription and Translation Are Different Processes

Understanding the difference between transcription and translation is central to solving this question. Transcription is the process by which information in DNA is copied into RNA. Translation is the process by which the ribosome reads the coding sequence of mRNA and synthesizes a polypeptide.

The 5′ UTR participates in the transcript but does not form part of the main protein-coding sequence. Therefore, it is transcribed but untranslated.

The promoter and sequences upstream of the transcription start site are generally not part of the RNA transcript. In contrast, the 5′ UTR begins at or downstream of the transcription start site and is included in the RNA molecule.

This is why the 5′ UTR cannot be placed upstream of the transcription start site or upstream of the promoter. Its defining feature is that it forms part of the transcribed RNA.

What Are the Major Functions of the 5′ UTR?

Although the 5′ UTR does not encode the main amino acid sequence of a protein, it can play important regulatory roles in gene expression. Its nucleotide sequence and structural features can influence how efficiently an mRNA is translated.

The 5′ UTR can contain binding sites for RNA-binding proteins and may form secondary structures that influence ribosome recruitment and scanning. In some transcripts, the 5′ UTR also contains upstream open reading frames, commonly called uORFs, which can regulate translation of the main coding sequence.

The length and sequence composition of the 5′ UTR can also influence mRNA stability, localization, and responsiveness to cellular signals. Therefore, untranslated regions are not biologically unimportant simply because they do not encode the main protein sequence.

Detailed Analysis of Option (A)

(A) Exon

This option is correct because the sequence corresponding to the mature 5′ UTR is retained in the mature mRNA and is therefore exonic.

An exon is a sequence represented in the mature RNA after pre-mRNA processing. Exons can contain protein-coding regions, untranslated regions, or a combination of both.

For example, the first exon of a gene may contain the entire 5′ UTR and part of the coding sequence. In another gene, the 5′ UTR may extend across more than one exon. Regardless of the exact gene structure, sequences retained in the mature 5′ UTR are exonic.

The fact that the 5′ UTR is not translated does not make it an intron. Translation status and exon status are different concepts. A sequence can be untranslated and still be part of an exon.

Hence, option (A) is the correct answer.

Detailed Analysis of Option (B)

(B) Intron

This option is incorrect because introns are generally removed from the pre-mRNA during RNA splicing and are not present in the final mature mRNA.

The 5′ UTR, in contrast, is a defined region of the mature mRNA located before the translation start codon. Because it remains in the mature transcript, its corresponding genomic sequences are exonic rather than intronic.

Some genes can contain an intron within the broader genomic region that gives rise to the 5′ untranslated portion of the transcript. Such an intron is removed during processing. The sequences that remain as the mature 5′ UTR are still exonic.

Therefore, an intron does not represent the mature 5′ UTR sequence.

Hence, option (B) is incorrect.

Detailed Analysis of Option (C)

(C) Upstream of the Transcription Start Site

This option is incorrect because the 5′ UTR is part of the RNA transcript and therefore cannot be located upstream of the transcription start site.

The transcription start site marks the nucleotide where RNA synthesis begins. Sequences upstream of this site are generally not copied into the primary RNA transcript.

The 5′ UTR begins at the 5′ end of the transcript and extends toward the translation start codon. Therefore, it lies downstream of the transcription start site rather than upstream of it.

Because the 5′ UTR is present in mature mRNA, it must originate from a transcribed region of genomic DNA.

Hence, option (C) is incorrect.

Detailed Analysis of Option (D)

(D) Upstream of the Promoter

This option is incorrect because a region upstream of the promoter is even farther away from the transcribed region of the gene.

The promoter is a regulatory DNA region that helps recruit RNA polymerase and other transcription factors. It is generally positioned upstream of the transcription start site.

The 5′ UTR is produced only after transcription begins. Therefore, its corresponding genomic sequence is located downstream of the transcription start site and cannot be located upstream of the promoter.

Although distant regulatory elements may occur upstream of a promoter, such sequences are not the 5′ untranslated region of the mature mRNA.

Hence, option (D) is incorrect.

Relationship Between Exons, Introns and Untranslated Regions

A mature eukaryotic mRNA can contain both coding and noncoding exonic sequences. The 5′ UTR is located before the translation start codon, while the 3′ UTR is located after the translation stop codon. Both regions are untranslated but are generally retained in the mature mRNA.

Introns, in contrast, are removed from the pre-mRNA during splicing. Therefore, the distinction between exon and intron depends primarily on RNA processing and retention in the mature transcript, not simply on whether a DNA sequence encodes amino acids.

This concept explains why untranslated regions are classified as exonic. A sequence does not need to encode a protein to be part of an exon.

Final Answer

The 5′ untranslated region (5′ UTR) is transcribed into RNA and remains present in the mature mRNA, but it is located before the translation start codon and is therefore not translated into the main protein sequence.

Because the genomic DNA sequence corresponding to the mature 5′ UTR is retained in the processed mRNA, it is classified as an exonic sequence. It is not an intron and is not located upstream of the transcription start site or promoter.

Therefore, the part of genomic DNA containing the sequence corresponding to the 5′ UTR is the exon.

Correct Option: (A) Exon