Following statements were made about mRNA splicing: A. Involvement of a cis-acting branchpoint site (or branchpoint sequence) present near 3' end of each exon is essential for splicing. B. In the first step of splicing reaction, 2'-OH of the conserved U at the branch-site acts as a nucleophile to attack the phosphoryl group of the conserved G in the 5' splice site. C. The newly liberated intron adapts shape of a lariat due to joining of the 5' end of the intron to the branchpoint. D. During the splicing process, there is no net gain in the number of chemical bonds. E. Prp22 (a DEAD-box helicase) is required for stripping the spliced mRNA from the spliceosome. Which one of the following options shows combination of all correct statements?
C, D, E
mRNA Splicing Statements Analysis
mRNA splicing is a crucial step in gene expression where introns are removed from the precursor mRNA (pre-mRNA) and exons are joined together to form mature mRNA. This process is carried out by a complex molecular machine called the spliceosome. Let's analyze each statement provided about mRNA splicing.
Branchpoint Site in mRNA Splicing
- Statement A: Involvement of a cis-acting branchpoint site (or branchpoint sequence) present near 3' end of each exon is essential for splicing.
This statement is incorrect. While the branchpoint site is indeed cis-acting and essential for splicing, it is typically located within the intron, specifically upstream of the 3' splice site. It is not found near the 3' end of each exon.
mRNA Splicing Reaction Steps
- Statement B: In the first step of splicing reaction, 2'-OH of the conserved U at the branch-site acts as a nucleophile to attack the phosphoryl group of the conserved G in the 5' splice site.
This statement is incorrect. In the first step of splicing, the nucleophile is the 2'-OH group of a conserved Adenosine (A) residue at the branchpoint site, not a Uracil (U). This 2'-OH attacks the phosphodiester bond at the 5' splice site.
Lariat Formation in mRNA Splicing
- Statement C: The newly liberated intron adapts shape of a lariat due to joining of the 5' end of the intron to the branchpoint.
This statement is correct. Following the first transesterification reaction, the 5' end of the intron is cleaved and becomes attached to the branchpoint adenosine via a 2'-5' phosphodiester bond. This creates a branched structure within the intron, causing it to adopt a characteristic loop-and-tail shape known as a lariat.
Chemical Bonds During mRNA Splicing
- Statement D: During the splicing process, there is no net gain in the number of chemical bonds.
This statement is correct. mRNA splicing involves two sequential transesterification reactions. In the first step, one phosphodiester bond (at the 5' splice site) is broken, and one 2'-5' phosphodiester bond is formed (creating the lariat). In the second step, one phosphodiester bond (at the 3' splice site) is broken, and one standard 3'-5' phosphodiester bond is formed (joining the exons). Thus, two phosphodiester bonds are broken, and two are formed, resulting in no net change in the total number of phosphodiester bonds.
Prp22 Helicase and Spliceosome
- Statement E: Prp22 (a DEAD-box helicase) is required for stripping the spliced mRNA from the spliceosome.
This statement is correct. Prp22 is a known ATP-dependent DEAD-box RNA helicase involved in the later stages of splicing. It plays a crucial role in releasing the spliced mRNA product from the spliceosome complex and facilitating the disassembly of the spliceosome after the second transesterification reaction.
Identifying Correct mRNA Splicing Statements
Based on the analysis, the correct statements are C, D, and E.
Statement C is correct because it accurately describes the formation of the lariat intermediate. Statement D is correct as splicing involves transesterification reactions that result in no net change in the number of phosphodiester bonds. Statement E is correct, as Prp22 helicase is indeed involved in releasing the spliced mRNA and recycling the spliceosome components.
Important Questions from RNA synthesis and processing
Which one of the following statements related to transcription and processing of mRNA is INCORRECT?
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?
