Question

The activities happening during RNA editing are:
(A). A change in RNA base sequence after transcription
(B). Usual addition or occasional deletion of 'U' residues
(C). Occasional addition of 'C' residues
(D). Proceeding with the editing in the 5'-3' direction of mRNA
Choose the correct answer from the options given below:

• A. (A), (B) and (C) only.
• B. (A), (B) and (D) only.
• C. (A), (C) and (D) only.
• D. (B), (C) and (D) only.

Hint:

Remember that guide RNA-mediated RNA editing (such as U-insertion in trypanosomes) proceeds from the 3' end to the 5' end of the mRNA, not in the standard 5' to 3' direction.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
RNA editing is a post-transcriptional modification process that alters the nucleotide sequence of an RNA molecule relative to its DNA template.
This molecular mechanism can change the coding instructions of the mRNA, leading to the synthesis of proteins that are different from those predicted by the genomic sequence.

Step 2: Detailed Explanation:
Let us analyze each statement about RNA editing:
1. Statement (A) is correct: The fundamental definition of RNA editing is the post-transcriptional alteration of the RNA base sequence, which can occur via substitution (e.g., C-to-U or A-to-I) or insertion/deletion of nucleotides.
2. Statement (B) is correct: In the mitochondria of kinetoplastid protozoa (such as trypanosomes), RNA editing commonly involves the extensive insertion (addition) or deletion of uracil ('U') residues. This process is mediated by guide RNAs (gRNAs).
3. Statement (C) is correct: In addition to 'U' insertion, other forms of insertional editing occur, such as the occasional insertion of cytosine ('C') residues, which has been observed in some slime molds (like Physarum polycephalum) and plant mitochondrial transcripts.
4. Statement (D) is incorrect: In guide-RNA-mediated U-insertion/deletion editing, the editing machinery generally initiates at the 3' end of the mRNA and proceeds in the 3' to 5' direction.
This occurs because the initial gRNA anchors near the 3' end of the unedited transcript, and subsequent editing creates the complementary target site for the next upstream gRNA.
Therefore, statements (A), (B), and (C) are correct, while statement (D) is incorrect.

Step 3: Final Answer:
The correct option is (A), which includes (A), (B) and (C) only.