Splicing Mechanism of Pre-mRNA

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Questions and Answers

What is the role of complementary sequences between snRNAs and splice sites in pre-mRNA?

They prevent spliceosome assembly.

They facilitate spliceosome assembly. (correct)

They enhance transcription of pre-mRNA.

They degrade the intron lariat.

Which components are initially involved in recognizing the 5′ splice site during pre-mRNA splicing?

U5 snRNP and branch point complexes.

Intron lariat and pre-catalytical spliceosomes.

U1 and U2 snRNPs. (correct)

U4 and U6 snRNPs.

What happens to the intron lariat after it is excised during the splicing process?

It is immediately reinserted into pre-mRNA.

It undergoes catalysis to form exons.

It remains intact and is used in further splicing.

It is degraded. (correct)

What is the term used for the complex after the excised intron lariat is removed?

Disassembled spliceosomal complex. Signup and view all the answers

Which splicing complex is formed before the catalytic action takes place?

Complex A. Signup and view all the answers

What initiates the splicing process in the spliceosome cycle?

Binding of U1 snRNP to the 5′ splice site Signup and view all the answers

Which complex is involved in the unwinding of the U4/U6 snRNA duplex?

Brr2 helicase Signup and view all the answers

What is the role of hnRNPs in the spliceosome cycle?

To facilitate the formation of the H complex Signup and view all the answers

During the second trans-esterification reaction, what is positioned to attack the phosphodiester bond?

2′OH group of the BP Signup and view all the answers

What is the significance of the H complex in the spliceosome cycle?

It indicates the initial commitment to splicing Signup and view all the answers

What type of RNA is primarily responsible for the translation process?

mRNA Signup and view all the answers

Which of the following is classified as non-coding RNA?

tRNA Signup and view all the answers

What is the main role of small nuclear RNA (snRNA)?

Intron removal during splicing Signup and view all the answers

What stimulates the formation of RNA helices?

Positively charged molecules Signup and view all the answers

Which of these is NOT a type of small non-coding RNA?

mRNA Signup and view all the answers

Double-stranded RNA is primarily involved in which of the following processes?

RNA interference Signup and view all the answers

Interrupted genes, which contain introns, are found in what type of organisms?

Eukaryotic organisms only Signup and view all the answers

Which type of RNA is primarily involved in the regulation of gene expression?

long non-coding RNA Signup and view all the answers

What is the primary function of the spliceosome in pre-mRNA processing?

It splices exons together. Signup and view all the answers

Why are introns significant in eukaryotic genes?

They can contain regulatory elements and additional genes. Signup and view all the answers

Which of the following is NOT a function of introns mentioned?

Removing unwanted mRNA sequences. Signup and view all the answers

What type of splicing does NOT require a spliceosome?

Self splicing (Type/group I and II) Signup and view all the answers

How does alternative splicing contribute to eukaryotic diversity?

By allowing different coding regions to be included or excluded. Signup and view all the answers

What is the primary function of U6 snRNA in the spliceosome cycle?

To pair with U2 snRNA to form the catalytic active site Signup and view all the answers

What role does the endonuclease play in tRNA processing?

It cleaves the tRNA precursors at both ends of the intron Signup and view all the answers

Which snRNPs are included in the C complex of the spliceosome?

U2, U5, and U6 Signup and view all the answers

What role do cis-acting elements found in introns play?

They promote transcription by binding transcription factors. Signup and view all the answers

What happens to mRNA after it is released by the spliceosome?

It is exported to the cytoplasm Signup and view all the answers

Which statement about introns is true?

Introns can evolve new proteins through gene duplication. Signup and view all the answers

What is the result of intron removal during tRNA processing?

Release of two half-tRNAs with unusual ends Signup and view all the answers

What is a key characteristic of trans-splicing compared to cis-splicing?

It combines sequences from two different transcripts Signup and view all the answers

What is the primary reason for intron removal during mRNA processing?

To facilitate mRNA export from the nucleus. Signup and view all the answers

Which RNA polymerase is responsible for transcribing the precursor rRNA?

RNA polymerase I Signup and view all the answers

What is the role of small nucleolar RNAs (snoRNAs) in rRNA processing?

They help in the modification of other RNAs Signup and view all the answers

Which type of splicing uses conserved sequences in introns and spliceosomes?

Cis-splicing Signup and view all the answers

In which organism is trans-splicing best characterized?

C. elegans Signup and view all the answers

What factors are involved in the cotranscriptional processing of rRNA?

Cotranscriptional processing factors and ribosomal proteins Signup and view all the answers

What is a potential application of trans-splicing?

Gene therapy techniques Signup and view all the answers

Which of the following is NOT a product of tRNA processing?

5S rRNA Signup and view all the answers

Which statement about splicing is incorrect?

Trans-splicing is the most common form of RNA splicing Signup and view all the answers

How are rRNA molecules released from their precursor?

Via cleavage by endonucleases Signup and view all the answers

Which snoRNA is required for the first cleavage event in rRNA processing?

U3 snoRNA Signup and view all the answers

Study Notes

RNA and Gene Expression

RNA is a central molecule in gene expression.
Life may have evolved from an RNA world.
RNA molecules are negatively charged.
RNA tends to repel one another in solution.
Stable RNA secondary structures are formed in solution.
RNA helix formation is stimulated by positive ions (e.g., Mg2+).

Types of RNA

RNA is categorized as non-coding (ncRNA) or coding.
Non-coding RNA includes housekeeping, regulatory, long, and small non-coding RNAs.
Coding RNA includes mRNA, circular RNA (circRNA), microRNA (miRNA).
Small ncRNAs include small nuclear RNA (snRNA), small nucleolar RNA (snoRNA).
siRNAs (small interfering RNAs) and piRNAs (PIWI-interacting RNAs) are also present.

RNA Structures

RNA has primary, secondary, and tertiary structures.
Hydrogen bonding between complementary bases creates secondary structure.
Secondary structures are formed hierarchically.
Tertiary structures are built upon secondary ones.
Elements of RNA structure include duplex, stem loops, bulges, internal loops.

RNA Processing: Intron Removal-Splicing

Eukaryotic genes are interrupted (interrupted genes, interrupted genes).
Exons are spliced together into a pre-mRNA by the spliceosome.
Introns are not junk; they have regulatory elements and genes.
Introns are longer than the final cytoplasmic mRNA in most eukaryotic genes.
Intron removal plays a crucial role (removal, role) in RNA processing.
RNA, polymerase transcription, gene expression, and timing are involved.

Alternative Splicing

Alternative splicing occurs in most multicellular eukaryotes.
Different splicing modes like intron retention or exon skipping contribute to protein diversity.
Alternative splicing affects gene expression.
Alternative splicing affects structural and functional diversity in gene products.

Types of RNA Splicing

Nuclear RNA encoding proteins (cis-splicing) which requires conserved sequences.
Trans-splicing of nuclear RNA, self-splicing, and type/group or type group II are also considered.

Introns in Eukaryotic Gene Expression

Introns enhance eukaryotic gene expression by containing cis-acting elements that promote transcription.
Introns can contain protein-coding or non-coding RNA genes, such as snoRNAs, miRNAs.
Introns contribute to gene expression timing.
Introns have an important role in the evolution of new proteins.

Capping of mRNA

About 30 nt downstream of the initiation site in the transcribed mRNA, the RNA polymerase is paused.
Pol II is paused, awaiting the recruitment of capping enzymes to complete its function after transcription initiation.
A 5' terminal G-base is added to the mRNA by guanylyl transferase and linked to the terminal phosphate of the mRNA.
Important for mRNA stability, splicing and export, and translation.

The Spliceosome

A large, macromolecular complex, consisting of several non-coding RNAs and dozens of auxiliary proteins (snRNPs), is required for splicing.
U1 to U6 are snRNAs, high in uridine residues.
Components like Sm proteins and splicing factors are key players.
The complex assembles and disassembles on each intron during RNA expression.

The Three Variant Spliceosomes

The major spliceosome recognizes introns with a 5'-GT...AG-3' sequence.
It has the essential snRNPs: U1, U2, U4/U6, and U5.
The minor spliceosome recognizes introns with a 5'-AT...AC-3' sequence.
Trans-spliceosome: facilitates splicing of separate pre-mRNA molecules transcribed from different genes.

Rules for Intron and Exon Definition

Introns <200-500 nt: intron definition.
Introns >500 nt: exon definition (critical for alternative splicing and various diseases).
Exon definition is important for size, flanked by 5′ and 3′ splice sites, and gene expression control.

Splicing: A Two-Step Chemical Reaction

A lariat is formed by intron cleavage and joining to a branch site adenosine's 2′ hydroxyl.
The 3' end of the intron joins the 5' end of the next exon in a transesterification reaction.
Splicing generates an intron lariat that's released and the exons are connected.

Splice Sites

High conservation of nucleotides is observed within introns and at splice sites.
Different splicing patterns are categorized based on the machinery involved.
These patterns influence protein production.

Intron Removal Overview: Spliceosome Assembly

ATP consumption helps assemble the spliceosome for lariat formation and intron release.
The spliceosome undergoes multiple steps to remove introns: from precursor to active, then inactive form.
Important reaction for releasing introns.

Spliceosome Assembly and Commitment

Complementary sequences between snRNAs and pre-mRNA splice sites guide spliceosome assembly.
The spliceosome assembles on the pre-mRNA, proceeds through various complexes, and ultimately catalyzes splicing reactions.
Splicing proceeds through various molecular complex modifications to perform catalysis.

The Precursor Messenger RNA (Pre-mRNA) Splicing Cycle

Splicing cycle involves multiple steps including pre-mRNA processing, intron excision, and final mRNA maturation.
The cycle involves various regulatory proteins to ensure accurate mRNA sequence.

Spliceosome Cycle Steps 1-3

Initiation of splicing involves association of pre-mRNA and hnRNPs (splicing regulators) to snRNPs (snRNA and proteins).
Formation of the E-complex, based on the binding of U1 snRNP to the 5' splice site.
Formation of the A-complex, which involves replacing splicing factors with U2 snRNP, which interacts with the branch site.

Spliceosome Cycle Steps 4-5 & 7

Formation of the B-complex with the addition of U4/U6 and U5 snRNPs facilitates further processing of the spliceosome.
Splicing catalysis occurs in the C-complex, where U6 snRNP pairing initiates the bond formation, U2 snRNP pairing enables the branch site, and U5 snRNP interaction connects the exons.
Cycle concludes with the release of the snRNPs for reuse in future splicing events.

Different Types of RNA Splicing

Nuclear RNA encoding proteins (cis-splicing) requires conserved sequences in introns and spliceosomes.
Trans-splicing of nuclear RNA involves the splicing of separate pre-mRNA molecules.
Self-splicing (Type I/II) occurs through intron folding and interactions, and is independent of proteins.
tRNA and rRNA undergo special splicing mechanisms.

Trans-Splicing of Nuclear RNAs

The product is a chimeric mRNA molecule assembled from two different transcripts.
Trans-splicing is a rare but important process with roles in gene expression that generate great diversity.

Production of rRNAs

rRNA synthesis involves multiple steps, not true splicing.
The large and smaller rRNAs are created from a precursor rRNA by cleavage.

rRNA Processing by snoRNAs

snoRNAs, a class of small RNA molecules, influence rRNA modification.
Pre-rRNA undergoes cleavage, methylation, and other modifications assisted by snoRNAs.
Cleavage, nucleases (various types) involvement is key for rRNA processing.

Other Variants of Introns: U12-Dependent Introns

Alternative splicing mechanism involving U12 snRNPs.
Minor splicing class has AT-AC rules associated with it.
Involves a unique set of factors like U12 snRNP.

tRNA Processing by RNases and Splicing

tRNA processing uses specific enzymes and depends on the tRNA's secondary structure.
An endonuclease, acting at the intron's edges in the mature tRNA, catalyzes a splicing-like event.
Involves RNases and other enzymes to create mature tRNA, with specific base modification in the intermediate stage.

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Description

This quiz explores the intricate process of pre-mRNA splicing, focusing on the roles of snRNA, splice sites, and spliceosome complexes. Answer questions about the catalytic steps, complexes involved, and the significance of various components during the splicing cycle.