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Biochemistry

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Purines, Pyrimidines and Nucleic Acid Metabolism

Question
16 out of 59
 

Apo B48, B100 belongs to same RNA, difference between them is due to (AIIMS May 2010)



A mRNA splicing

B Chemical changes in mRNA & splicing

C Differetial RNA processing

D Upstream regression

Ans. B RNA editing. None of the option matches the answer. Similar answer out of following can be Chemical changes in mRNA after splicing (option B) check for typographic errors

a. Chylomicrons are assembled in the enterocytes, and consist of droplets of non-polar lipid, triacylglycerol (TAG) and cholesterol esters, stabilized by a shell of phospholipid, cholesterol and protein.

b. The major protein is apolipoprotein (apo)-B48, which is essential for the assembly, secretion and subsequent metabolism of chylomicrons.

c. apo-B48 is a truncated form of apo-B100, and is produced by post-transcriptional editing of the mRNA for apo-B100.

d. Cytidine 6666 is deaminated by an RNA-editing enzyme, apobec-1, resulting in the production of a stop codon after 48% of the mRNA has been translated. This change generates an in-frame stop codon at amino acid 2153 and results in synthesis of a 2152-amino acid product (apoB-48).

e. The C to U nucleotide substitution in the apoB mRNA is a co- or post-transcriptional event and is not present in the apoB gene. The editing enzyme is expressed in the enterocytes of all mammals investigated, and is regulated developmentally in human fetal intestine

f. Thus RNA editing is a molecular process in which the base of a RNA molecule is altered by specific enzymes. In the following example, Cytosine (C) is changed to Uracil (U).

Ch5A9

Figure shows: RNA editing of the apo-B gene. In mammals, the apo-B gene is expressed in both hepatocytes (liver cells) and intestinal epithelial cells. However, in liver cells, its product is a 500 kD protein called Apo-B100 whereas in intestine cells its product is a smaller protein called Apo-B48. The Apo-B100 is produced without RNA editing, but the Apo-B48 is synthesized from an mRNA whose sequence has been altered by a specific enzyme. This enzyme changes a codon, CAA, in the middle of the original mRNA to the stop codon UAA, thereby causing early termination of the protein synthesis.

Purines, Pyrimidines and Nucleic Acid Metabolism Flashcard List

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