In eukaryotic cells, transcription cannot begin until (A) the two DNA strands have completely separated and exposed the promoter. (B) several transcription factors have bound to the promoter. (C) the 5′ caps are removed from the mRNA. (D) the DNA introns are removed from the template.
The correct answer and explanation is:
The correct answer is:
(B) Several transcription factors have bound to the promoter.
Explanation:
In eukaryotic cells, transcription is a complex process that requires the coordinated action of multiple proteins and enzymes. Before transcription can begin, RNA polymerase II, the enzyme responsible for synthesizing messenger RNA (mRNA), must be correctly positioned at the promoter region of a gene. However, RNA polymerase II cannot directly bind to the promoter by itself. Instead, several transcription factors must first recognize and bind to specific DNA sequences at the promoter.
These transcription factors play crucial roles, including:
- Recognizing the promoter: The promoter is a specific DNA sequence, typically containing a TATA box, where transcription begins. The TATA-binding protein (TBP), a subunit of the transcription factor TFIID, binds to the TATA box.
- Recruiting RNA polymerase II: Other transcription factors, such as TFIIB, TFIIE, TFIIF, and TFIIH, assemble at the promoter to form the pre-initiation complex. This assembly stabilizes RNA polymerase II and positions it correctly.
- Unwinding DNA: TFIIH has helicase activity, which helps unwind the DNA double helix to expose the template strand for RNA synthesis.
- Initiating transcription: Once all necessary factors are in place, RNA polymerase II undergoes phosphorylation, enabling it to start synthesizing RNA.
Without these transcription factors, RNA polymerase II cannot efficiently bind or initiate transcription, making them essential for gene expression.
Why the Other Choices Are Incorrect:
- (A) The two DNA strands do not need to be fully separated—only a small segment near the promoter is unwound.
- (C) The 5′ cap is added after transcription begins, not before.
- (D) Introns are removed from mRNA after transcription, during RNA processing.
Now, I will generate an image illustrating the transcription initiation process.
Here is the scientific illustration of transcription initiation in eukaryotic cells. It visually represents the binding of transcription factors to the promoter region, along with RNA polymerase II preparing for transcription. Let me know if you need any modifications or additional details!
