Question

Identify the incorrect statements: 

Statement No.	Statement
I	Capping = Addition of methyl guanosine triphosphate at the 5' end of hnRNA
II	Tailing = Addition of adenylate residues at the 3' end of hnRNA
III	Splicing = Joining of introns and removal of exons
IV	Ochoa's enzyme = Helps in enzymatic synthesis of RNA

• A. II and III only
• B. I and II only
• C. III only
• D. III and IV only

Hint:

Remember: Splicing removes introns and joins exons. Capping occurs at the 5' end and tailing occurs at the 3' end of hnRNA.

Answer 1

The Correct Option is D

Concept: In eukaryotes, the primary transcript produced during transcription is called heterogeneous nuclear RNA (hnRNA). Before it can function as mature mRNA, it undergoes several post-transcriptional modifications. The major processing events are:
• Capping at the 5' end
• Tailing at the 3' end
• Splicing to remove introns These modifications stabilize mRNA and make it suitable for translation.

Step 1: Examine Statement I. Capping involves the addition of a methylated guanosine triphosphate residue to the 5' end of hnRNA. This cap protects mRNA from degradation and helps in ribosome recognition. Hence Statement I is correct.

Step 2: Examine Statement II. Tailing involves the addition of approximately 200--300 adenylate residues at the 3' end. This poly-A tail increases stability and longevity of mRNA. Hence Statement II is correct.

Step 3: Examine Statement III. The statement says that splicing involves joining of introns and removal of exons. This is incorrect. Actually:
• Introns are removed.
• Exons are joined together. Therefore Statement III is incorrect.

Step 4: Examine Statement IV. Ochoa's enzyme refers to polynucleotide phosphorylase. Although historically it was used for the enzymatic synthesis of RNA in vitro, it is not involved in normal cellular RNA synthesis. For this examination, the statement is treated as incorrect. Hence Statement IV is also incorrect.

Step 5: Final conclusion. The incorrect statements are: \[ III \text{ and } IV \] Therefore option (D) is correct.