In both eukaryotes and prokaryotes RNA polymerase recognizes the transcriptional start site by

Question

BlackTom AI · Accepted Answer

Binding to a promoter, a specific sequence in the DNA, is the mechanism by which RNA polymerase locates the transcriptional start site in both prokaryotes and eukaryotes.

Relevant knowledge points and concepts:
- A promoter is a short DNA sequence adjacent to the transcription start site that specifies where RNA polymerase should begin transcription.
- In prokaryotes, core promoter elements include the -35 and -10 (Pribnow) boxes; recognition is mediated by RNA polymerase holoenzyme with a sigma factor that binds these consensus sequences.
- In eukaryotes, promoters often contain a TATA box, initiator (Inr), and other proximal elements; RNA polymerase II is recruited to these sequences through general transcription factors (e.g., TFIID/TBP) and mediator complexes.

Step-by-step reasoning:
1. Prokaryotic mechanism: The sigma factor of the RNA polymerase holoenzyme recognizes and binds the -35 and -10 consensus sequences. This binding positions the polymerase at the +1 transcription start site and facilitates local DNA unwinding to initiate RNA synthesis.
2. Eukaryotic mechanism: General transcription factors first bind promoter elements (for example TBP subunit of TFIID binds the TATA box). These factors assemble a preinitiation complex that recruits RNA polymerase II to the transcription start site and properly orients it for initiation.
3. Both systems rely on specific DNA sequences (promoters) plus accessory proteins to ensure accurate start-site selection; chromatin context and regulatory elements modify efficiency but do not replace promoter recognition.

Why the correct answer is correct:
- Promoter binding directly positions RNA polymerase at the start site in both domains of life, making it the fundamental recognition mechanism.

Why other options would be incorrect (briefly):
- Recognition is not achieved by the start codon or ribosome-binding sites (those are for translation), nor by random binding; DNA methylation or chromatin state influences accessibility but is not the primary locating signal.

Conclusion: The correct answer—Binding to a promoter, a specific sequence in the DNA—matches and is confirmed by the molecular mechanisms described above.

In both eukaryotes and prokaryotes RNA polymerase recognizes the transcriptional start site bySingle choice

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