Transcription start site vs. start codon: In gene expression, where does transcription initiate relative to the translation start codon (AUG)?

Introduction / Context:
Understanding the spatial relationship between transcription and translation landmarks is fundamental. The mRNA contains untranslated regions (UTRs) that are transcribed but not translated. This question probes whether you know that transcription begins before the coding region where translation starts.

Given Data / Assumptions:

• Start codon (usually AUG) marks translation initiation.
• Promoters and transcription start sites (TSS) lie on DNA upstream of the coding sequence.
• mRNAs have 5′ UTRs that precede the start codon.

Concept / Approach:
RNA polymerase initiates at the promoter near the TSS, producing a primary transcript that includes the 5′ UTR, the coding sequence, and often a 3′ UTR. Because 5′ UTR sequences lie before the start codon, the TSS is upstream relative to the coding AUG. Therefore, transcription begins upstream from the translation start codon.

Step-by-Step Solution:
Recall: Promoter → TSS → 5′ UTR → AUG (start codon).Transcription initiates at TSS, not at AUG.Therefore, transcription begins upstream of the start codon.

Verification / Alternative check:
Mapping cDNA to genomic DNA reveals that the 5′ end of mRNA aligns upstream of the start codon, confirming transcription starts earlier.

Why Other Options Are Wrong:

• At the start codon: that is where translation begins, not transcription.
• Downstream from the start codon: would omit 5′ UTRs, which contradicts abundant experimental evidence.
• None of these: incorrect because “upstream” is correct.

Common Pitfalls:
Conflating “start” of transcription with “start” of translation due to similar terminology. Always distinguish TSS from AUG.

Final Answer:
upstream from the start codon