Four different transcription units are shown below. Choose the option with the correct image.
Show Hint
To solve any transcription unit orientation problem instantly, remember this single rule: All structural positions (Promoter and Terminator) are defined entirely with respect to the coding strand. Promoter is always at the $5'$-end of the coding strand; Terminator is always at the $3'$-end of the coding strand.
Step 1: Understanding the Question:
The problem presents four different structural arrangements of a transcription unit in DNA. We must identify which schematic diagram correctly establishes the structural conventions governing the orientation of the promoter, terminator, template strand, and coding strand. Step 2: Key Formula or Approach:
The convention for defining the structural components of a transcription unit is strictly based on the orientation of the coding strand (non-template strand running in the $5' \rightarrow 3'$ direction):
1. The Promoter is located upstream, towards the $5'$-end of the coding strand.
2. The Terminator is located downstream, towards the $3'$-end of the coding strand.
3. The Template strand has a polarity of $3' \rightarrow 5'$ because RNA polymerase synthesizes RNA only in the $5' \rightarrow 3'$ direction.
4. The Coding strand runs in the $5' \rightarrow 3'$ direction and shares an identical sequence with the synthesized mRNA (except U replaces T). Step 3: Detailed Explanation:
Let's evaluate the structural logic of the diagrams based on these rules:
* Look closely at diagram (C):
* The top strand runs from left to right as $3' \leftarrow 5'$ (which means $5'$ is on the right and $3'$ is on the left). It is explicitly labeled as the Template strand.
* The bottom strand runs from left to right as $5' \rightarrow 3'$ (with $5'$ on the left and $3'$ on the right). It is labeled as the Coding strand.
* Since the coding strand has its $5'$-end on the left side, the Promoter must be placed on the left side. The diagram correctly shows the Promoter box on the left.
* Since the coding strand has its $3'$-end on the right side, the Terminator must be placed on the right side. The diagram correctly shows the Terminator box on the right.
* The template strand running $3' \rightarrow 5'$ from right to left provides the correct sequence template for an RNA polymerase molecule to read as it moves from left to right.
Let us check why other options are structurally invalid:
* In option (A), the promoter is placed at the $5'$-end of the template strand instead of the coding strand, violating international convention.
* In option (B), the template and coding polarities do not match the standard upstream position of the promoter at the coding strand's $5'$ terminus.
* In option (D), the positions of the template and coding labels are mixed up relative to the promoter location.
Therefore, diagram (C) represents the mathematically and structurally sound orientation of a transcription unit. Step 4: Final Answer:
The transcription unit with the completely correct structural labels is given in option (C).
