Problem 1 - Campbell Biology

Question

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 \(5^{\prime}\) caps are removed from the mRNA. (D) the DNA introns are removed from the template.

Step-by-Step Solution

Answer

The correct answer is (B).

Step 1: Understand the question

The question asks which condition must be met for transcription to begin in eukaryotic cells. Review each choice to determine which is correct.

Step 2: Analyze Choice (A)

Choice (A) states that the two DNA strands must completely separate and expose the promoter. This is not accurate since only a part of the DNA near the promoter needs to be unwound for transcription to start.

Step 3: Analyze Choice (B)

Choice (B) states that several transcription factors must have bound to the promoter. This is correct because transcription factors are essential for the assembly of the transcription initiation complex.

Step 4: Analyze Choice (C)

Choice (C) states that the 5' caps must be removed from the mRNA. This does not make sense since the 5' cap is added to mRNA after transcription begins, not before.

Step 5: Analyze Choice (D)

Choice (D) states that DNA introns must be removed from the template. This is incorrect because intron removal occurs during RNA processing, not before transcription starts.

Step 6: Conclude the correct answer

The analysis indicates that transcription in eukaryotic cells cannot begin until several transcription factors have bound to the promoter. Therefore, the correct answer is (B).

Key Concepts

transcription factors

Transcription factors are essential proteins in the process of transcription initiation in eukaryotic cells. These factors help to regulate the expression of genes by binding to specific DNA sequences. They function by:

• Recognizing and binding to the promoter region of the gene
• Recruiting and stabilizing RNA polymerase
• Facilitating the assembly of the transcription initiation complex

Transcription factors are highly specific, recognizing unique sequences in the promoter region. Their binding can be influenced by other proteins and signals, making them pivotal in the regulation of eukaryotic gene expression.

promoter

The promoter is a critical DNA sequence located upstream of a gene. It acts as a control region, where transcription factors and RNA polymerase II assemble to initiate transcription. Key features of a promoter include:

• The TATA box: a conserved DNA sequence found in many promoters
• Binding sites for transcription factors
• Regions that interact with enhancer or silencer elements to modulate gene expression

A properly functioning promoter is essential for the accurate and efficient transcription of eukaryotic genes.

eukaryotic gene expression

Eukaryotic gene expression is a complex process involving multiple stages, from DNA transcription to protein synthesis. It can be regulated at various levels, such as:

• Transcriptional regulation: controlled by transcription factors and promoter regions
• Post-transcriptional regulation: including RNA splicing, capping, and polyadenylation
• Translational regulation: affecting how mRNA is translated into proteins

Each of these stages ensures that genes are expressed at the right time, place, and in the right amount, which is crucial for the proper functioning of eukaryotic organisms.

transcription initiation complex

The transcription initiation complex is a key structure formed during the initiation of eukaryotic transcription. It consists of:

• General transcription factors: essential for all genes transcribed by RNA polymerase II
• RNA polymerase II: the enzyme responsible for synthesizing mRNA from the DNA template
• Mediator complex: a multi-protein complex that facilitates the interaction between transcription factors and RNA polymerase

The assembly of the transcription initiation complex begins with the binding of transcription factors to the promoter. This process brings RNA polymerase II to the DNA, positioning it to start RNA synthesis. The formation of this complex is a crucial step in the regulation of eukaryotic gene expression.