Question

Which of the following statements are correct with reference to a transcription unit in DNA? A. A transcription unit in DNA is defined primarily by three regions: promoter, structural gene and terminator.
B. The promoter is said to be located towards the $5'$-end of the structural gene.
C. The promoter is a DNA sequence that provides binding site for RNA polymerase.
D. The promoter defines the template and coding strands.
E. The terminator is located towards the $3'$-end of the coding strand and it defines the end of the process of transcription.

• A. A, C, D and E only
• B. A, B, C, D and E
• C. A, B, C and D only
• D. B, C, D and E only

Hint:

Crucial Rule: All "ends" ($5'$ or $3'$) in transcription unit descriptions are given with respect to the \textbf{Coding Strand}, even though the coding strand doesn't actually code for anything!

Answer 1

The Correct Option is B

The transcription unit is the segment of DNA that takes part in transcription. All five statements accurately describe its architecture:
• Components (A): It consists of the Promoter (start), the Structural Gene (actual code), and the Terminator (end).
• Promoter Location (B & C): The promoter is located upstream ($5'$-end) of the structural gene (referenced to the coding strand). It is the specific DNA sequence where RNA polymerase binds to initiate the process.
• Strand Definition (D): By its position and orientation, the promoter determines which strand will act as the template strand ($3' \rightarrow 5'$) and which will be the coding strand ($5' \rightarrow 3'$).
• Terminator (E): The terminator is located downstream ($3'$-end) relative to the coding strand and signals the end of transcription. Since all statements (A, B, C, D, and E) are factually correct according to NCERT standards, option (b) is the right choice.