Which one of the following statements related to transcription and processing of mRNA is INCORRECT?
mRNAs with premature stop codons are degraded by Nonsense-Mediated Decay (NMD) and mRNAs without an in-frame stop codon get accumulated and translated in the cytoplasm.
Transcription and mRNA Processing Analysis
The question asks to identify the statement that is INCORRECT regarding transcription and processing of mRNA. Let's analyze each statement:
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Prokaryotic Transcription: The statement says that during prokaryotic transcription, DNA binding properties of RNA polymerase are altered by sigma factor. In prokaryotes, the core RNA polymerase enzyme has limited DNA binding specificity. The sigma factor associates with the core enzyme to form the holoenzyme. The sigma factor is crucial for recognizing and binding to specific promoter sequences on the DNA, thereby directing the RNA polymerase to the transcription start site. This indeed alters the DNA binding properties, making the binding site-specific. Thus, this statement is correct.
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Eukaryotic Transcription: The statement describes the roles of different RNA polymerases in eukaryotic transcription: RNA polymerase I synthesizes ribosomal RNAs (rRNA), RNA polymerase II synthesizes messenger RNA precursors (pre-mRNA) and some small RNAs, and RNA polymerase III synthesizes transfer RNAs (tRNA), 5S rRNA, and other small RNAs. This distribution of tasks among eukaryotic RNA polymerases is generally correct. While Pol II transcribes pre-mRNA which is then processed into mRNA, the statement accurately assigns the primary classes of RNA synthesized by each polymerase. Thus, this statement is correct.
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RNA Splicing: The statement compares splicing mechanisms in tRNA and pre-mRNA. tRNA splicing often involves specific endonucleases that cleave the tRNA precursor and ligases that join the pieces. This is a successive/sequential cleavage and ligation process. Pre-mRNA splicing, occurring in the spliceosome, involves the formation of a lariat intermediate as introns are removed. This statement accurately describes the distinct mechanisms of splicing observed in these two types of RNA. Thus, this statement is correct.
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mRNA Degradation and Translation: The statement claims that mRNAs with premature stop codons are degraded by Nonsense-Mediated Decay (NMD) and mRNAs without an in-frame stop codon get accumulated and translated in the cytoplasm. The first part, about NMD degrading mRNAs with premature stop codons, is correct. NMD is a quality control mechanism that detects and degrades such aberrant mRNAs. However, the second part, stating that mRNAs without an in-frame stop codon accumulate and are translated, is INCORRECT. Cells have another surveillance mechanism called Non-Stop Decay (NSD) that targets mRNAs lacking a stop codon. When a ribosome reaches the 3' end of an mRNA without encountering a stop codon, it stalls. This stalled ribosome and the associated mRNA are then targeted for degradation by the NSD pathway. Such mRNAs do not typically accumulate and get translated into full-length, functional proteins; they are degraded. Therefore, this statement is incorrect.
Based on the analysis, the incorrect statement is the one about mRNAs without an in-frame stop codon accumulating and being translated.
Important Questions from RNA synthesis and processing
Following statements were made with respect to transcription in eukaryotes:
A. RNA polymerase III synthesises mRNAs in the nucleoplasm
B. The target promoter for RNA polymerase III is usually represented by a bipartite sequence downstream of the transcription start site.
C. The assembly factors TFIIIA and TFIIIC assist the binding of the positioning factor TFIIIB at the precise location.
D. TFIIIB is the last factor that joins the initiation complex.
E. Phosphorylated Ser residues in the C-terminal domain (CTD) of RNA polymerase II serve as binding sites for mRNA processing enzymes.
Which one of the following options represents the correct combination of the statements?
Following statements were made about the post-transcriptional processing of RNA in eukaryotes.
A. Soon after transcription initiation, RNA polymerase II pauses ~30 nucleotides downstream from the site of initiation until the Cap structure is added to the 5' end of the nascent pre- mRNA.
B. The 5' splice sites are functionally divergent whereas the 3' sites are functionally equivalent.
C. In addition to helping in recognition of the splice sites, the exon definition also functions as a splicing regulator by allowing pairing and linking of adjacent 5' and 3' splice sites.
D. The intron definition mechanism applies only to the larger introns (above 500 nucleotides length) and assists in achieving alternate splicing.
E. The splicing reactions carried out in vitro have revealed that the first and second transesterification reactions are reversible.
Which one of the following combination of statements is correct?
Which one of the following statements is NOT a correct feature of Escherichia coli RNA polymerase?
The following statements are related to transcription in bacteria/eukaryotes.
A. During concurrent promoter sequence recognition and melting, melting commences with base flipping where two bases are flipped out into pockets of the primary sigma factor
B. Binding of α-amanitin to RNA polymerase Il permits entry of nucleotides into RNA pol II active site and synthesis of RNA, but prevents translocation
C. RNA polymerase I can use upstream promoters with 3 consensus sequences, as well as internal promoters having a bipartite structure
D. FACT is associated with RNA polymerase during transcriptional elongation and helps displace histone octomers during transcription
Which of the following combinations of statements represents all correct statements?
Which one of the snRNAs given below base pairs with 5' splice site of pre mRNA?
