Following statements were made about catalytic introns: A. Group I introns may undergo self-splicing by transesterification. B. Group II introns do not require any factor/protein for autosplicing either in vivo or in vitro C. Certain introns of both the group I and II classes may contain open reading frames which are translated into protein. D. Generally, group I introns migrate by DNA-mediated mechanisms, whereas group II introns migrate by RNA-mediated mechanisms. E. Ribonuclease P (RNase P) is essential for bacteria and archaea but not eukaryotes. Which one of the following combinations represents statements which are all correct?
A, C, D
Catalytic Introns and RNase P: Understanding Key Features
This question asks us to evaluate several statements about catalytic introns, specifically Group I and Group II, and Ribonuclease P (RNase P) to identify the correct combination.
Let's analyze each statement carefully:
Statement A: Group I Introns and Self-Splicing
Statement A says: Group I introns may undergo self-splicing by transesterification.
Analysis: Group I introns are a class of ribozymes that can catalyze their own excision from an RNA molecule. This process occurs through a series of transesterification reactions, typically initiated by an external guanosine cofactor. This self-splicing occurs without the need for protein enzymes. Therefore, statement A is correct.
Statement B: Group II Introns and Splicing Factors
Statement B says: Group II introns do not require any factor/protein for autosplicing either in vivo or in vitro.
Analysis: Group II introns can also undergo self-splicing through transesterification reactions, forming a lariat structure. This autosplicing can occur in vitro under specific conditions (high salt concentrations). However, in vivo, splicing of Group II introns is often dependent on or significantly enhanced by various protein factors, including maturases encoded within the intron itself, as well as host proteins. Therefore, stating they require *no* factors *either* in vivo *or* in vitro is incorrect.
Statement C: Introns Containing Open Reading Frames (ORFs)
Statement C says: Certain introns of both the group I and II classes may contain open reading frames which are translated into protein.
Analysis: Both Group I and Group II introns can contain ORFs within their sequences. These ORFs often encode proteins, such as maturases (which assist in splicing of the intron itself) or reverse transcriptases and endonucleases (involved in their mobility). These proteins are translated and function to promote the intron's survival or spread. Therefore, statement C is correct.
Statement D: Intron Migration Mechanisms
Statement D says: Generally, group I introns migrate by DNA-mediated mechanisms, whereas group II introns migrate by RNA-mediated mechanisms.
Analysis: Intron migration, also known as homing or retrotransposition, is a process by which introns spread to new locations. Group I introns often migrate via a DNA-mediated mechanism involving intron-encoded homing endonucleases that create double-strand breaks in target DNA. Group II introns migrate via an RNA-mediated retrotransposition mechanism, using an intron-encoded reverse transcriptase to synthesize a cDNA copy of the spliced RNA, which is then inserted into the genome. Therefore, statement D is correct.
Statement E: Ribonuclease P (RNase P) Essentiality
Statement E says: Ribonuclease P (RNase P) is essential for bacteria and archaea but not eukaryotes.
Analysis: Ribonuclease P (RNase P) is a ribozyme (a ribonucleoprotein complex where the RNA component has the catalytic activity) that is crucial for processing the 5' leader sequence of precursor transfer RNAs (pre-tRNAs) in all three domains of life: bacteria, archaea, and eukaryotes. It is essential for cell viability in bacteria, archaea, and typically eukaryotes as well, as mature tRNAs are required for protein synthesis. Therefore, stating it is not essential for eukaryotes is incorrect.
Identifying the Correct Combination
Based on the analysis:
- Statement A is correct.
- Statement B is incorrect.
- Statement C is correct.
- Statement D is correct.
- Statement E is incorrect.
The combination of correct statements is A, C, and D.
Looking at the options provided, the option that contains only statements A, C, and D is the correct one.
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?
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?
