Question

Choose the correct sequence of events as they occur during a bacterial transcription
A. RNA polymerase sigma (\(\sigma\)) subunits recognizes promoters
B. DNA double helix unwind
C. Sigma (\(\sigma\)) subunit dissociates from the holoenzyme
D. Formation of hairpin secondary structure
Choose the correct answer from the options given below:

• A. A, B, C, D
• B. A, C, B, D
• C. B, A, D, C
• D. C, B, D, A

Hint:

Think of transcription like a train journey. \textbf{A}: The conductor (\(\sigma\)-factor) finds the station (promoter). \textbf{B}: The doors open (DNA unwinds). \textbf{C}: The train leaves the station and the conductor gets off (\(\sigma\)-factor dissociates). \textbf{D}: The train reaches its destination and the brakes are applied (hairpin forms for termination).

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Bacterial transcription involves three main stages: initiation, elongation, and termination. The question asks to arrange the key events of these stages in their correct chronological order.
Step 2: Detailed Explanation:
Let's analyze the sequence of events:


Initiation (Step A): The process begins when the RNA polymerase holoenzyme, specifically its sigma (\(\sigma\)) subunit, recognizes and binds to the promoter sequence on the DNA.

Initiation (Step B): After binding, the RNA polymerase unwinds a short segment of the DNA double helix at the promoter region, forming an "open complex" and exposing the template strand.

Elongation (Step C): As the RNA polymerase moves along the DNA template synthesizing RNA (after about 10 nucleotides), the sigma (\(\sigma\)) subunit is no longer needed for promoter binding and dissociates from the core enzyme. The core enzyme continues elongation.

Termination (Step D): The process ends at a terminator sequence. The formation of a stable hairpin secondary structure in the newly synthesized RNA is characteristic of Rho-independent termination, which signals the RNA polymerase to dissociate from the DNA template.

Step 3: Final Answer:
The logical and chronological sequence of events is A \(\rightarrow\) B \(\rightarrow\) C \(\rightarrow\) D.