Pre-mRNA Processing Overview

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Questions and Answers

What is the first step in the processing of eukaryotic premature mRNA transcripts?

Exon Splicing

3’ Polyadenylation

Intron Removal

5’ Capping (correct)

What is the role of hnRNP in relation to pre-mRNA?

To prevent folding of pre-mRNA into secondary structures (correct)

To synthesize pre-mRNA

To degrade pre-mRNA

To facilitate pre-mRNA folding into secondary structures

Which type of RNA polymerase synthesizes eukaryotic pre-mRNA?

RNA polymerase I

RNA polymerase II (correct)

RNA polymerase III

None of the above

What term refers to the molecular complexes formed by pre-mRNA and nuclear proteins?

hnRNP Signup and view all the answers

What is one of the primary functions of hnRNA?

To be associated with nuclear proteins Signup and view all the answers

What is the primary outcome of trans-splicing in eukaryotes?

Generation of a single mature mRNA from multiple primary RNA transcripts Signup and view all the answers

Which of the following best describes alternative splicing?

The generation of different mature mRNAs from the same pre-mRNA Signup and view all the answers

Why is RNA editing considered a rare phenomenon in higher eukaryotes?

It primarily occurs in the mitochondria and chloroplasts Signup and view all the answers

What distinguishes ribozymes from typical RNA molecules?

They have catalytic activity Signup and view all the answers

In the context of the fibronectin gene, what variation is observed due to alternative splicing?

Different proteins produced from varying exon combinations based on cell type Signup and view all the answers

Which of the following RNA types is NOT typically involved in ribozyme activity?

mRNA Signup and view all the answers

What role do RNA-binding proteins play in alternative splicing?

They regulate the joining of exons by binding specific sequences Signup and view all the answers

Which of the following statements about transport across the nuclear membrane is true?

The nuclear membrane consists of two phospholipid bilayers Signup and view all the answers

What is the primary purpose of adding a 7-methylguanylate cap to the 5’ end of pre-mRNA?

To protect the 5’ end from enzymatic degradation Signup and view all the answers

Which enzyme is responsible for adding adenine residues to the 3’ end of mRNA during polyadenylation?

Poly(A) polymerase Signup and view all the answers

During RNA splicing, what ensures that introns are accurately removed at the correct sites?

Conserved sequences at exon-intron junctions Signup and view all the answers

What component of the spliceosome is crucial for the initial recognition of the 5’ splice site?

U1 snRNA Signup and view all the answers

What occurs after the circular lariat intron is formed during splicing?

Exons are ligated together Signup and view all the answers

Which nucleotide is invariant at the branch point during splicing?

A Signup and view all the answers

What type of RNA is involved in the formation of the spliceosome?

snRNA Signup and view all the answers

What roles do Nuclear Pore Complexes (NPCs) play in cellular transport?

They enable bidirectional transport of molecules. Signup and view all the answers

How does polyadenylation help in mRNA stability?

By attaching a long chain of nucleotides to the 3’ end Signup and view all the answers

What is the composition of the Nuclear Pore Complex?

It contains nucleoporins, a nuclear basket, and a central transporter. Signup and view all the answers

Which molecules are allowed passage through the Nuclear Pore Complex selectively?

H2O-soluble molecules pass through while macromolecules do not. Signup and view all the answers

Which statement about RNA polymerase II is accurate regarding RNA capping?

It initiates transcription before the capping process begins Signup and view all the answers

What is the role of the debranching enzyme during splicing?

To convert the lariat intron into linear RNA Signup and view all the answers

What is the function of nuclear-localization signals (NLS)?

They direct the transport of cytosolic proteins into the nucleus. Signup and view all the answers

How do importins function in the nuclear transport process?

They transport proteins containing NLS into the nucleus. Signup and view all the answers

What type of regions do nuclear transporters possess that allow them to interact with FG-nucleoporins?

Hydrophobic regions that bind reversibly. Signup and view all the answers

What effect does fusing a nuclear-localization signal (NLS) to a cytoplasmic protein have?

It enables the protein to enter the nucleus. Signup and view all the answers

What is the primary role of FG nucleoporins in the nuclear pore complex?

They form a molecular sieve that allows selective transport. Signup and view all the answers

What is the role of importin in the nuclear import process?

It binds to nuclear-localization signals of cargo proteins. Signup and view all the answers

How does exportin facilitate the export of proteins out of the nucleus?

By forming a complex with Ran-GTP and NES. Signup and view all the answers

What is the necessary state of Ran for cargo proteins to be released from importin in the nucleus?

Ran-GTP Signup and view all the answers

Which of the following proteins are involved in switching Ran from an active to an inactive state?

GAPs Signup and view all the answers

Which mechanism is employed for the transport of mRNPs out of the nucleus?

Transient interactions with FG-nucleoporins. Signup and view all the answers

Which part of the large subunit of the mRNA exporter directly binds to mRNP?

N domain Signup and view all the answers

What type of signals does cargo protein need to exhibit in order to shuttle between the nucleus and cytoplasm?

Both nuclear-export and nuclear-localization signals. Signup and view all the answers

What are the primary roles of GEF in the context of GTP switch proteins?

Stimulates the binding of GTP to the protein. Signup and view all the answers

What is the primary function of nucleoporins in the nuclear transport mechanism?

They form channels for the diffusion of proteins through the NPC. Signup and view all the answers

What is the role of GDP in the function of GTP-binding proteins?

Represents the inactive state of the protein. Signup and view all the answers

Study Notes

### Pre-mRNA Processing

Occurs in the nucleus
Includes 5' capping, 3' polyadenylation, intron removal & exon splicing
Pre-mRNA transcripts are synthesized by RNA polymerase II

hnRNP

Pre-mRNA is always associated with proteins
hnRNP is a complex of hnRNA and proteins
hnRNP proteins help prevent pre-mRNA folding to ensure proper processing and transport
hnRNP also assists in transporting mRNA out of the nucleus

5’ Capping

Occurs shortly after transcription initiation
A 7-methylguanylate cap is added to the 5' end of pre-mRNA
Capping enzyme associates with the CTD of RNA polymerase II
Protects the 5' end from degradation and assists with export to the cytosol

Polyadenylation

A multi-protein complex forms at the poly(A) signal in pre-mRNA
The pre-mRNA is cleaved at a site generating a free 3' OH
Poly(A) polymerase adds A residues to the 3’end
This protects the mRNA from degradation in the cytoplasm

Intron Removal & Exon Splicing

Introns are removed and exons are spliced together at every exon/intron junction
Splicing requires conserved sequences at each end of every intron
Invariant nucleotides include G-U at the 5’ splice site, A at the branch point, and A-G at the 3’ splice site

RNA Splicing Mechanism

Splicing is carried out by a large ribonucleoprotein complex called the spliceosome
The spliceosome contains snRNPs which are made of snRNA associated with proteins
U1 snRNA binds to the 5’ splice site, U2 to the branch point, and U4/U6/U5 associate
Two transesterification reactions lead to the removal of the lariat intron and ligation of exons
The lariat intron is converted to linear RNA and degraded by nuclear exonucleases

Trans-Splicing

Involves splicing together exons of separate RNA molecules, forming a single mature mRNA
Occurs less commonly than cis-splicing
Found in organisms like C. elegans, Trypanosomes, and Euglenoids

Alternative Splicing

Different combinations of exons from the same pre-mRNA can be spliced together
Results in different mature mRNAs (isoforms)
Regulated by RNA-binding proteins

RNA Editing

Exon nucleotides are altered in the nucleus prior to mRNA maturation
The sequence of the mature mRNA differs from the genomic DNA
Occurs rarely in higher eukaryotes, but common in mitochondria of plants and protozoans

Ribozymes

RNA molecules with catalytic activity
23S and 28S rRNA have peptidyl transferase activity
Group I and II self-splicing introns are found in various organisms

Nuclear Transport

The nucleus is surrounded by two membranes with nuclear pore complexes (NPCs) for transport
NPCs allow passive diffusion of small molecules and active transport of larger molecules
FG nucleoporins form a sieve that selectively allows for macromolecule transport

Import into the Nucleus

Proteins meant for use in the nucleus are synthesized in the cytosol
These proteins contain a nuclear localization signal (NLS)
Importins bind to the NLS and transport the cargo into the nucleus
Ran-GTP mediates the release of the cargo from the importin in the nucleoplasm

Export out of the Nucleus

Exportin transports cargo proteins containing nuclear-export signals (NES) out of the nucleus
NES cargo binds to exportin and Ran-GTP
Complex diffuses through the NPC
Hydrolysis of Ran-GTP causes dissociation of the complex and exportin recycling

mRNP Transport out of Nucleus

mRNA exporter protein directs most mRNPs through the nuclear pores
This protein interacts with FG-nucleoporins as it progresses through the pore

GTP Switch Proteins

Guanine nucleotide-binding proteins switch between active (GTP-bound) and inactive (GDP-bound) states
GEF promotes GTP binding and GAP accelerates GTP hydrolysis

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Description

This quiz covers the essential processes of pre-mRNA processing, including 5' capping, 3' polyadenylation, intron removal, and exon splicing. Understand the roles of hnRNP proteins and the significance of each step in mRNA maturation. Test your knowledge on how these processes facilitate proper gene expression.