Which of the following statements about the processing of the primary transcript in eukaryotes are correct?
A. The primary transcripts contain both the exons and the introns and are non-functional.
B. In splicing, the exons are removed and the introns are joined in a defined order.
C. In tailing, adenylate residues are added at the 3′-end in a template-independent manner.
D. The fully processed hnRNA, now called mRNA, is transported out of the nucleus for translation.
Choose the correct answer from the options given below.
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → A, C and D only
In eukaryotes, RNA polymerase II makes the precursor of mRNA, the heterogeneous nuclear RNA (hnRNA). It has to be processed before it can work.
A. The primary transcripts contain both the exons and the introns and are non-functional. — True. Eukaryotic genes are split genes. The first copy keeps the introns (intervening sequences), so it cannot yet be used.
B. In splicing, the exons are removed and the introns are joined in a defined order. — False. It is the other way round. The introns are removed and the exons are joined, because the exons carry the coding sequence that appears in mature RNA.
C. In tailing, adenylate residues are added at the 3′-end in a template-independent manner. — True. A poly-A tail is added at the 3′-end without using any template. In capping, a methyl guanosine triphosphate is added at the 5′-end.
D. The fully processed hnRNA, now called mRNA, is transported out of the nucleus for translation. — True. After splicing, capping and tailing, the RNA is mature mRNA. It moves to the cytoplasm, where translation takes place.
NCERT Extract:
Types of RNA and the Process of Transcription – “… (ii) The second complexity is that the primary transcripts contain both the exons and the introns and are non-functional. Hence, it is subjected to a process called splicing where the introns are removed and exons are joined in a defined order. hnRNA undergoes additional processing called as capping and tailing. In capping an unusual nucleotide (methyl guanosine triphosphate) is added to the 5′-end of hnRNA. In tailing, adenylate residues (200-300) are added at 3′-end in a template independent manner. It is the fully processed hnRNA, now called mRNA, that is transported out of the nucleus for translation (Figure 5.11).”