# DBT JRF 2020 — Question 96

Exam: DBT JRF 2020 · Section: B · Topic: Molecular Biology · Difficulty: Medium

## Question

Which one of the following involves RNA Editing?

## Options

- **A.** deletion, insertion or chemical modification of nucleotides that are present in the mRNA
- **B.** joining of exons from one pre-mRNA molecule to form mRNA
- **C.** deletion, insertion or chemical modification of nucleotides in the gene encoding the mRNA
- **D.** joining of exons from two different pre-mRNA molecules to form mRNA

## Answer

Option A

## Explanation

RNA editing is a post-transcriptional process that alters the nucleotide sequence of an RNA molecule, changing it from the sequence encoded by the genomic DNA. Mechanisms: (1) Adenosine-to-Inosine (A-to-I) editing: ADAR enzymes deaminate adenosine → inosine (read as guanosine by ribosomes), changing codon meaning (e.g. AMPA receptor GluA2 subunit: Q607R edit prevents Ca $^{2+}$ permeability); (2) Cytidine-to-Uridine (C-to-U) editing: APOBEC1 enzyme complex deaminates cytidine → uridine (e.g. apolipoprotein B: CAA → UAA stop codon in intestine → ApoB-48 instead of full-length ApoB-100); (3) Insertion/deletion of uridines in kinetoplastid mitochondria. RNA editing changes mRNA sequence POST-transcriptionally, distinct from alternative splicing (which rearranges exons) or mutation (which changes DNA).

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