5. In eukaryotic genes, DNA sequences that define gene promoters occur:
   (1) only in the regions upstream of the transcription start sites.
   (2) only in the regions that represent the transcribed parts of the genes.
   (3) only in the regions downstream of the transcription termination sites.
   (4) either in the regions upstream of the transcription start site or within the transcribed regions of the gene.

Where Are Promoter Sequences Found in Eukaryotic Genes?

Slug: eukaryotic-gene-promoter-sequence-location

Meta Description:
Learn where promoter sequences are located in eukaryotic genes and why they can occur both upstream of the transcription start site and within transcribed regions. Essential reading for molecular biology and CSIR NET Life Science exam prep.

Correct Answer

(4) Either in the regions upstream of the transcription start site or within the transcribed regions of the gene.

Detailed Explanation

In eukaryotic genes, promoter sequences are critical for the initiation and regulation of transcription. Traditionally, promoters were thought to be confined to the region immediately upstream of the transcription start site (TSS), where they serve as the binding site for RNA polymerase and general transcription factors. This core promoter region includes well-known elements such as the TATA box, initiator (Inr), and other motifs that are essential for transcription initiation.

Beyond the Upstream Region: Promoters Within Transcribed Regions

Recent advances in genome-wide mapping of transcription start sites have revealed that promoter elements can also be located within the transcribed region of a gene—that is, downstream of the TSS but still functionally important for transcription initiation. For example, the downstream promoter element (DPE) is found within the initial portion of the transcribed region (typically from +28 to +32 relative to the TSS), especially in genes lacking a TATA box. This means that parts of the promoter can overlap with the beginning of the gene’s coding sequence.

Key Points from Research and Exam Resources

• Upstream Promoters: Most core and proximal promoter elements are found upstream of the TSS, serving as the main regulatory hub for transcription initiation.

• Within Transcribed Regions: Some promoter elements, like the DPE, are located within the transcribed region, and these are essential for the regulation of certain genes, particularly those without a TATA box.

• Genome-Wide Studies: Large-scale analyses have confirmed that transcription can initiate from multiple sites, and promoter elements are not strictly limited to upstream regions.

Why Other Options Are Incorrect

• Option (1): Promoter elements are not restricted only to upstream regions; some are found within the transcribed region.

• Option (2): Promoters are not found exclusively within the transcribed parts; they are also present upstream.

• Option (3): Promoters are not located downstream of the transcription termination site; such regions do not play a role in transcription initiation.

Conclusion

Promoter sequences in eukaryotic genes can be found either upstream of the transcription start site or within the transcribed region of the gene. This nuanced understanding is crucial for interpreting gene regulation and for success in competitive exams like CSIR NET Life Science