Post-transcriptional gene control
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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?

    <p>hnRNP</p> Signup and view all the answers

    What is one of the primary functions of hnRNA?

    <p>To be associated with nuclear proteins</p> Signup and view all the answers

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

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

    Which of the following best describes alternative splicing?

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

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

    <p>It primarily occurs in the mitochondria and chloroplasts</p> Signup and view all the answers

    What distinguishes ribozymes from typical RNA molecules?

    <p>They have catalytic activity</p> Signup and view all the answers

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

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

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

    <p>mRNA</p> Signup and view all the answers

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

    <p>They regulate the joining of exons by binding specific sequences</p> Signup and view all the answers

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

    <p>The nuclear membrane consists of two phospholipid bilayers</p> Signup and view all the answers

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

    <p>To protect the 5’ end from enzymatic degradation</p> Signup and view all the answers

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

    <p>Poly(A) polymerase</p> Signup and view all the answers

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

    <p>Conserved sequences at exon-intron junctions</p> Signup and view all the answers

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

    <p>U1 snRNA</p> Signup and view all the answers

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

    <p>Exons are ligated together</p> Signup and view all the answers

    Which nucleotide is invariant at the branch point during splicing?

    <p>A</p> Signup and view all the answers

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

    <p>snRNA</p> Signup and view all the answers

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

    <p>They enable bidirectional transport of molecules.</p> Signup and view all the answers

    How does polyadenylation help in mRNA stability?

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

    What is the composition of the Nuclear Pore Complex?

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

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

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

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

    <p>It initiates transcription before the capping process begins</p> Signup and view all the answers

    What is the role of the debranching enzyme during splicing?

    <p>To convert the lariat intron into linear RNA</p> Signup and view all the answers

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

    <p>They direct the transport of cytosolic proteins into the nucleus.</p> Signup and view all the answers

    How do importins function in the nuclear transport process?

    <p>They transport proteins containing NLS into the nucleus.</p> Signup and view all the answers

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

    <p>Hydrophobic regions that bind reversibly.</p> Signup and view all the answers

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

    <p>It enables the protein to enter the nucleus.</p> Signup and view all the answers

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

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

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

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

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

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

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

    <p>Ran-GTP</p> Signup and view all the answers

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

    <p>GAPs</p> Signup and view all the answers

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

    <p>Transient interactions with FG-nucleoporins.</p> Signup and view all the answers

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

    <p>N domain</p> 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?

    <p>Both nuclear-export and nuclear-localization signals.</p> Signup and view all the answers

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

    <p>Stimulates the binding of GTP to the protein.</p> Signup and view all the answers

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

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

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

    <p>Represents the inactive state of the protein.</p> 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.

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