Question

Which of the following phosphate is removed from the incoming nucleotide during the bacterial transcription?

(1). alpha
(2). gamma
(3). 5 prime
(4). 3 prime

• A. alpha
• B. gamma
• C. 5 prime
• D. 3 prime

Hint:

Remember that both replication and transcription involve nucleophilic attack by a 3'-OH group on the \( \alpha \)-phosphate of an incoming nucleotide triphosphate.
The \( \beta \)- and \( \gamma \)-phosphates are always released as inorganic pyrophosphate (\( \text{PP}_i \)).

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
Transcription is the process of synthesizing RNA from a DNA template.
The substrates for RNA polymerase are ribonucleoside triphosphates (rNTPs: ATP, CTP, GTP, UTP), which contain three phosphate groups attached to the 5' carbon of the ribose sugar.
These phosphate groups are designated as \( \alpha \) (nearest to the ribose), \( \beta \), and \( \gamma \) (outermost).

Step 2: Detailed Explanation:
The chemical reaction catalyzed by RNA polymerase during chain elongation proceeds through a nucleophilic attack:
1. The 3'-hydroxyl (3'-OH) group at the terminus of the growing RNA chain acts as a nucleophile.
2. It attacks the electrophilic phosphorus atom of the \( \alpha \)-phosphate of the incoming ribonucleoside triphosphate (rNTP).
3. This attack results in the formation of a phosphodiester bond, and the monophosphate nucleotide (containing the \( \alpha \)-phosphate) is covalently incorporated into the RNA chain.
4. The high-energy anhydride bond between the \( \alpha \)-phosphate and the \( \beta \)-phosphate is cleaved during this reaction, releasing a pyrophosphate group (\( \text{PP}_i \)).
5. This pyrophosphate consists of the \( \beta \)- and \( \gamma \)-phosphate groups of the incoming rNTP.
Among the options provided, the \( \gamma \)-phosphate is the specific phosphate group that is cleaved and removed from the incoming nucleotide as a component of the pyrophosphate leaving group.

Step 3: Final Answer:
The \( \gamma \)-phosphate group is cleaved and removed, which corresponds to option (B).