This set of Molecular Biology Multiple Choice Questions & Answers (MCQs) focuses on “RNA is Processed in Several Ways”.
1. mRNA of which of the following organism does not undergo processing?
Explanation: Most newly synthesized RNAs must be modified in various ways to the converted to their functional forms. Bacterial mRNAs are an exception; they are used immediately as templates for protein synthesis while still being transcribed.
2. rRNA and tRNA of prokaryotes does not undergo any post transcriptional processing.
Explanation: rRNA and tRNA of prokaryotes do undergo post transcriptional processing such as splicing and structural modification. Only bacterial mRNAs are an exception; they are used immediately as templates for protein synthesis while still being transcribed.
3. Splicing of mRNA occurs after they arrive the cytoplasm.
Explanation: Primary transcript of eukaryotic mRNAs undergo extensive modifications, including the removal of introns by splicing, before they are transported from the nucleus to the cytoplasm to serve as templates for protein synthesis. Regulation of these processing steps provides an additional level of control of gene expression, as does regulation of the rates at which different mRNAs are subsequently degraded within the cell.
4. Which of the following is not a type of RNA processing?
a) Polyadenylation at the 3’ end
b) Capping of 5’ end
c) Removal of exons
Explanation: The processing events include the following:
i) Capping of the 5’ end of RNA.
ii) Splicing for removing the introns.
iii) Polyadenylation of the 3’ end of the RNA.
5. Elongation of transcription and RNA processing are all interconnected.
Explanation: One elongation factor (hSPT5) also recruits and stimulates the 5’ capping enzyme. In another case, elongation factor TAT – SF1 recruits components of the splicing machinery. Thus it seems that elongation, termination of transcription, and RNA processing are interconnected to ensure their proper coordination.
6. The first RNA processing event is __________
Explanation: The first RNA processing event is capping. This involves the addition of the modified guanine base to the 5’ end of the RNA.
7. Capping is done by the addition of __________
a) Methylated A
b) Methylated T
c) Methylated G
d) Methylated C
Explanation: The addition of methylated guanine to the RNA transcript by an unusual 5’ – 5’ linkage involving three phosphates. The 5’ cap is created in three enzymatic steps. In the first step, a phosphate group is moved from the 5’ of the transcript. Then, the GTP is added. And in the final step, that nucleotide is modified by the addition of a methyl group.
8. Capping occurs after the mRNA synthesis is complete.
Explanation: The RNA is capped when it is still only some 20 – 40 nucleotides long – when the transcription cycle has progressed only to the transition between the initiation and elongation phases. After capping, dephosphorylation of Ser5 within the tail repeats leads to dissociation of the capping machinery and further phosphorylation causes recruitment of the machinery needed for RNA splicing.
9. As the polymerase reaches the end of RNA which of the following event does not occur as a response?
a) Transfer of Polyadenylation enzyme
b) Cleavage of the RNA
c) Addition of poly A at the 3’ end
d) Termination of transcription
Explanation: Once polymerase reaches the end of a gene, it encounters specific sequences that, after being transcribed into RNA, trigger the transfer of the polyadenylation enzymes to that RNA. This leads to three events:
i) Cleavage of the RNA
ii) Addition of many adenine residues at the 5’ end
iii) Subsequent termination of transcription by polymerase.
10. About how many “A” are added to the nascent RNA in the 5’ end during Polyadenylation?
Explanation: Polyadenylation is mediated by an enzyme called poly – A polymerase, which adds about 200 adenines to the RNA’s 3’ end product by the cleavage. This enzyme uses ATP as a precursor and adds the nucleotides using the same process as RNA polymerase, but does not require a template.
11. The long tail of As is unique to the RNA synthesized by the RNA polymerase __________
Explanation: It is noteworthy that the long tail of As is unique to eukaryotic mRNAs. Thus the long poly A tail is added to the RNA transcript produced by RNA polymerase II. This feature allows the experimental isolation of protein coding mRNAs by affinity chromatography.
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