Question

Which of the following is the basis for the intestine-specific expression of apoprotein B-48 ?

• A. DNA rearrangement and loss
• B. DNA rearrangement and loss
• C. RNA alternative splicing
• D. RNA editing

Answer 1

The Correct Option is D

Key fact: apoB-48 is the textbook example of RNA editing in humans, so spotting that phrase is enough to pick option 4.
Here is the mechanism. One apoB gene serves two tissues. In the liver the mRNA is translated in full to give apoB-100. In the intestine a deaminase changes a single cytosine to uracil in the mRNA after it is transcribed, turning a CAA glutamine codon into a UAA stop codon. Translation halts early, yielding apoB-48, which is roughly 48 percent the size of apoB-100, hence the name.
Eliminate the others: DNA rearrangement and transposition reshuffle the genome (as in immunoglobulin genes), and alternative splicing chooses different exons. None of these explains apoB-48, which is purely a post-transcriptional base edit. So RNA editing is the answer.
Ref: Lippincott's Biochemistry, 5th edn.