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Questions and Answers

What is the starting material for transcription?

  • Messenger RNA (mRNA)
  • Polypeptide
  • Ribosomal RNA (rRNA)
  • DNA (correct)

    • Which enzyme is essential for the process of transcription?

  • Aminoacyl-tRNA synthetase
  • Ribosome
  • RNA polymerase II (correct)
  • DNA polymerase

    • What is the end product of transcription?

  • Messenger RNA (mRNA) (correct)
  • Transfer RNA (tRNA)
  • Ribosomal RNA (rRNA)
  • Protein

    • Which component is NOT required for translation?

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

    What is the initial precursor RNA in transcription called?

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

    What processing events occur during pre-mRNA processing?

    <p>Capping, polyadenylation, and intron removal</p> Signup and view all the answers

    Which component acts as a scaffold for organizing factors involved in pre-mRNA processing?

    <p>C-terminal domain (CTD) of RNA polymerase</p> Signup and view all the answers

    What is the primary function of tRNA in translation?

    <p>To transport amino acids to the ribosome</p> Signup and view all the answers

    Which polymerase is responsible for transcribing tRNA genes?

    <p>RNA polymerase III</p> Signup and view all the answers

    What term describes the phenomenon where multiple codons can code for the same amino acid?

    <p>Codon degeneracy</p> Signup and view all the answers

    What separates transcription units in a tDNA gene cluster?

    <p>Nontranscribed spacer</p> Signup and view all the answers

    How many tRNA species do cells typically have despite there being 61 codons?

    <p>50</p> Signup and view all the answers

    What is the estimated percentage of human genes that undergo alternative splicing?

    <p>50-75%</p> Signup and view all the answers

    Which type of RNA catalyzes the covalent linking of amino acids during translation?

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

    What is the sedimentation coefficient (S) a measure of?

    <p>Particle settling speed in a centrifuge</p> Signup and view all the answers

    Which RNA polymerase is responsible for transcribing the 5S rRNA?

    <p>RNA Pol III</p> Signup and view all the answers

    Where does rRNA processing primarily occur?

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

    How are rRNA genes organized in humans?

    <p>In tandem clusters across multiple chromosomes</p> Signup and view all the answers

    What pattern characterizes rRNA transcription?

    <p>Christmas tree pattern</p> Signup and view all the answers

    What percentage of a cell's RNA is typically rRNA?

    <p>More than 80%</p> Signup and view all the answers

    Which subunit contains the 28S rRNA?

    <p>Large ribosomal subunit (60S)</p> Signup and view all the answers

    What does the fibrillar center (fc) primarily contain?

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

    What role do box H/ACA snoRNAs play in the nucleolus?

    <p>Convert uridines to pseudouridines</p> Signup and view all the answers

    Where are the small nucleolar ribonucleoproteins (snoRNPs) found?

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

    What do the granule components of the nucleolus primarily contain?

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

    What is the average length of tRNA molecules?

    <p>73-93 nucleotides</p> Signup and view all the answers

    Which structure is responsible for the export of assembled ribosomes to the cytoplasm?

    <p>Nuclear pore complexes</p> Signup and view all the answers

    What percentage of tRNA genes are found in humans?

    <p>Approximately 500</p> Signup and view all the answers

    Which component of the nucleolus contains pre-rRNA transcripts?

    <p>Dense fibrillar component</p> Signup and view all the answers

    What type of structure do small nucleolar RNAs (snoRNAs) form when associated with proteins?

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

    What is a function of the dense fibrillar component (dfc) in the nucleolus?

    <p>Contain pre-rRNA transcripts</p> Signup and view all the answers

    What are exons?

    <p>Parts of the gene that contribute to mature RNA product</p> Signup and view all the answers

    What percentage of the pre-RNA does intron comprise?

    <p>95%</p> Signup and view all the answers

    What are splice sites associated with?

    <p>Breaks at the 5’ and 3’ ends of exons</p> Signup and view all the answers

    Which sequence is typically found at the 5’ splice site?

    <p>G/GU</p> Signup and view all the answers

    What role do exonic splicing enhancers (ESE) play?

    <p>They help recruit small nuclear ribonuclear proteins (snRNPs)</p> Signup and view all the answers

    What is a common consequence of errors in RNA splicing?

    <p>Development of inherited diseases</p> Signup and view all the answers

    What are small nuclear ribonuclear proteins (snRNPs) primarily composed of?

    <p>RNA and specific proteins</p> Signup and view all the answers

    Which of the following is NOT a component of the spliceosome?

    <p>RNA polymerase</p> Signup and view all the answers

    What's the purpose of the branch point sequence in RNA splicing?

    <p>It aids in the recognition of splice sites</p> Signup and view all the answers

    What role does eIF4G play in the initiation of translation?

    <p>It links the 5<code> cap of mRNA to the 3</code> poly(A) tail.</p> Signup and view all the answers

    During elongation, what triggers the conformational changes within the ribosome?

    <p>The binding of aminoacyl-tRNA with the correct anticodon.</p> Signup and view all the answers

    Which site in the ribosome is responsible for holding the tRNA that carries the growing polypeptide chain?

    <p>P site</p> Signup and view all the answers

    What is the primary function of the peptidyl transferase activity during translation?

    <p>To catalyze the formation of peptide bonds.</p> Signup and view all the answers

    What happens during the translocation step in ribosomal translation?

    <p>The ribosome moves three nucleotides along the mRNA.</p> Signup and view all the answers

    What is the primary role of rRNA during translation?

    <p>It catalyzes the formation of covalent bonds between amino acids.</p> Signup and view all the answers

    What is the significance of the 'Christmas tree' pattern in rRNA transcription?

    <p>It indicates multiple rRNA genes are being transcribed simultaneously.</p> Signup and view all the answers

    What is the function of nontranscribed spacers (NTS) in rRNA gene clusters?

    <p>They separate different transcription units.</p> Signup and view all the answers

    How many distinct types of ribosomal RNA (rRNA) are present within a eukaryotic ribosome?

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

    Where does the assembly of rRNAs with corresponding r-proteins primarily occur?

    <p>In the nucleoli</p> Signup and view all the answers

    What percentage of a eukaryotic cell's RNA is typically composed of rRNA?

    <p>Approximately 80%</p> Signup and view all the answers

    What is the primary reason for the clustering of rDNA genes on different chromosomes?

    <p>To increase the efficiency of ribosome production.</p> Signup and view all the answers

    What defines a splice variant in the context of gene expression?

    <p>Various mRNA isoforms produced from a single gene.</p> Signup and view all the answers

    What is a characteristic feature of tRNA genes in the genome?

    <p>They exist in small clusters scattered around the genome.</p> Signup and view all the answers

    What role does the nontranscribed spacer (NTS) play in a tDNA gene cluster?

    <p>It separates transcription units.</p> Signup and view all the answers

    What is the function of codon degeneracy in the genetic code?

    <p>To minimize the chances of errors during protein synthesis.</p> Signup and view all the answers

    How many tRNA isoacceptors typically bind to the same amino acid?

    <p>Three or more for each codon.</p> Signup and view all the answers

    What is the primary product of the transcription process?

    <p>messenger RNA (mRNA).</p> Signup and view all the answers

    Which statement best describes the role of anticodons in tRNA?

    <p>They are sequences that are complementary to mRNA codons.</p> Signup and view all the answers

    What are the primary components required for translation?

    <p>mRNA, rRNA, and tRNA.</p> Signup and view all the answers

    In the context of tRNA processing, what happens to the precursor tRNA?

    <p>It undergoes trimming and base modifications.</p> Signup and view all the answers

    What is the significance of wobble pairing in translation?

    <p>It enables multiple codons to code for a single amino acid.</p> Signup and view all the answers

    What is the function of wobble pairing in codon-anticodon interactions?

    <p>Allows flexibility in the base pairing of the third codon position</p> Signup and view all the answers

    What role do aminoacyl-tRNA synthetases (AARS) play in translation?

    <p>They covalently link amino acids to their respective tRNA</p> Signup and view all the answers

    Why is the third position of codon pairing considered less critical?

    <p>It allows for alternative bases to pair without affecting the overall function</p> Signup and view all the answers

    Which step in the initiation process involves the binding of eukaryotic initiation factors to the ribosomal subunit?

    <p>Step 1</p> Signup and view all the answers

    What is required for the activation of amino acids during tRNA formation?

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

    What does codon degeneracy mean in the context of DNA mutations?

    <p>Multiple codons can specify the same amino acid, reducing the mutation impact</p> Signup and view all the answers

    What happens during the process of amino acid activation?

    <p>Amino acids are linked to AMP before being transferred to tRNA</p> Signup and view all the answers

    Which of the following is true regarding initiation in prokaryotic versus eukaryotic cells?

    <p>The overall sequence of steps is different between eukaryotes and prokaryotes</p> Signup and view all the answers

    What is the main function of exons in the gene structure?

    <p>They contribute to the mature RNA product.</p> Signup and view all the answers

    What sequence is typically found at the 3' splice site in RNA?

    <p>AG/G</p> Signup and view all the answers

    What is the first molecule that enters the P site during the initiation process?

    <p>Initiator tRNA-Met</p> Signup and view all the answers

    Which component is included in the composition of small nuclear ribonuclear proteins (snRNPs)?

    <p>Small nuclear RNAs</p> Signup and view all the answers

    What can a double-ringed G pair with and why is this relevant?

    <p>With either U or C, which demonstrates flexibility in genetic coding</p> Signup and view all the answers

    What role do exonic splicing enhancers (ESE) play during RNA processing?

    <p>They help recruit splicing factors.</p> Signup and view all the answers

    Approximately what percentage of human inherited diseases are estimated to arise from errors in RNA splicing?

    <p>10-20%</p> Signup and view all the answers

    What is the primary type of sequence found at the exon-intron boundary?

    <p>Conserved splice sites</p> Signup and view all the answers

    What are the primary functions of the branch point sequence in splicing?

    <p>It provides signals for splicing machinery.</p> Signup and view all the answers

    What are the consequences of codon degeneracy on the impact of DNA mutations?

    <p>It reduces the effects of mutations on the final amino acid sequence.</p> Signup and view all the answers

    Which mechanism primarily facilitates the activation of amino acids to form aminoacyl-tRNA?

    <p>Aminoacyl-tRNA synthetases using ATP</p> Signup and view all the answers

    During the initiation of translation, what role do eukaryotic initiation factors (eIFs) play?

    <p>They assist in the binding of the small ribosomal subunit.</p> Signup and view all the answers

    What distinguishes the wobble pairing mechanism from traditional base pairing in codon-anticodon interactions?

    <p>It allows non-standard base pairing at the third position.</p> Signup and view all the answers

    What is a primary function of the initiator tRNA-Met during translation initiation?

    <p>It carries the first amino acid to the P site of the ribosome.</p> Signup and view all the answers

    Which snRNP complex is responsible for attaching to the 5' splice site during RNA splicing?

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

    What effect does U2 snRNP binding have on the intron during splicing?

    <p>It causes an adenosine residue to bulge out.</p> Signup and view all the answers

    What is the main role of U6 snRNA during the splicing process?

    <p>It acts as a catalyst for the cleavage of the 5' splice site.</p> Signup and view all the answers

    Which of the following is released after the second cleavage reaction during splicing?

    <p>Lariat intron</p> Signup and view all the answers

    What is one significant outcome of alternative splicing?

    <p>It allows a gene to code for multiple polypeptides.</p> Signup and view all the answers

    Which protein aids the U2 snRNP in recognizing the 3' splice site during splicing?

    <p>U2AF</p> Signup and view all the answers

    What characterizes the macromolecular machine known as the spliceosome?

    <p>It is made up of snRNPs and associated proteins.</p> Signup and view all the answers

    Which splicing reaction occurs first during RNA splicing?

    <p>Cleavage of the 5' splice site</p> Signup and view all the answers

    What type of sequence does U1 snRNA complementarily bind to during splicing?

    <p>5' splice site</p> Signup and view all the answers

    How many distinct ribosomal RNAs are synthesized by RNA Polymerase I in eukaryotic cells?

    <p>4</p> Signup and view all the answers

    Which statement accurately describes the role of nontranscribed spacers in ribosomal gene clusters?

    <p>They separate transcription units within a rRNA gene cluster.</p> Signup and view all the answers

    What subunit of ribosomes includes the 5S rRNA?

    <p>Large ribosomal subunit (60S)</p> Signup and view all the answers

    Which of the following correctly identifies the transcriptional enzyme responsible for 5S rRNA synthesis?

    <p>RNA Polymerase III</p> Signup and view all the answers

    What is a defining characteristic of the 'Christmas tree' pattern observed in rRNA transcription?

    <p>It shows branches of active transcription sites.</p> Signup and view all the answers

    Which ribosomal RNA comprises the smaller ribosomal subunit in eukaryotes?

    <p>18S rRNA</p> Signup and view all the answers

    What proportion of the total RNA in eukaryotic cells is commonly represented by rRNA?

    <blockquote> <p>80%</p> </blockquote> Signup and view all the answers

    In the context of ribosomal biogenesis, where does the assembly of rRNAs with corresponding r-proteins primarily occur?

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

    What best describes the structural arrangement of ribosomal RNA genes within the nucleolar framework?

    <p>Grouped in clusters on different chromosomes.</p> Signup and view all the answers

    What is the main function of rRNA in the ribosome during translation?

    <p>Catalyzing the formation of peptide bonds.</p> Signup and view all the answers

    What is a key feature of tRNA genes in the genome?

    <p>They are arranged in tandem and scattered.</p> Signup and view all the answers

    Which of the following statements about tRNA precursor is true?

    <p>It undergoes trimming and base modifications.</p> Signup and view all the answers

    What allows for codon degeneracy in the genetic code?

    <p>Multiple codons coding for the same amino acid.</p> Signup and view all the answers

    Which component of the translation process is essential for matching mRNA codons to amino acids?

    <p>Transfer RNA (tRNA).</p> Signup and view all the answers

    What specifically characterizes the process of transcription?

    <p>It synthesizes messenger RNA from a DNA template.</p> Signup and view all the answers

    Which of the following best describes the role of nontranscribed spacers (NTS) in tDNA gene clusters?

    <p>They separate transcription units in tDNA gene clusters.</p> Signup and view all the answers

    Why do cells have approximately 50 tRNA species despite having 61 codons?

    <p>tRNAs can bind to multiple codons due to wobble pairing.</p> Signup and view all the answers

    What is the main purpose of anticodons on tRNA?

    <p>To ensure proper amino acid sequence during translation.</p> Signup and view all the answers

    Which statement accurately reflects the relationship between codons and amino acids?

    <p>Some amino acids are coded by multiple codons.</p> Signup and view all the answers

    What piece of machinery is primarily required for translation?

    <p>Transfer RNA (tRNA) and ribosomal RNA (rRNA).</p> Signup and view all the answers

    What is the purpose of exonic splicing enhancers (ESE) in RNA splicing?

    <p>To facilitate the binding of snRNPs to splice sites</p> Signup and view all the answers

    Which sequences are highly conserved at the exon-intron boundaries?

    <p>5' splice site - G/GU and 3' splice site - AG/G</p> Signup and view all the answers

    What percentage of the human genes is estimated to be affected by errors in splicing?

    <p>About 15-30%</p> Signup and view all the answers

    What function do splice sites serve in RNA splicing?

    <p>They provide the signals for the removal of introns</p> Signup and view all the answers

    How much of the pre-RNA does intron comprise in terms of percentage?

    <p>Around 95%</p> Signup and view all the answers

    What is generally found at the branch point sequence in RNA splicing?

    <p>A/U-rich region approximately 30 bases upstream of the 3' splice site</p> Signup and view all the answers

    Study Notes

    Molecular Mechanisms of Disease

    • The lecture covered molecular mechanisms of disease, including topics on transcription and translation from genes to proteins.

    Today's Outline

    • Announcements
    • Gene Transcription and Translation

    Transcription

    • Synthesis of complementary RNA from DNA
    • Performed in the cell nucleus.
    • Starting material is DNA.
    • Required machinery: RNA polymerase II and transcription factors.
    • End product: messenger RNA (mRNA).

    Translation

    • Synthesis of proteins in the cytoplasm using information encoded by mRNA.
    • Starting material is mRNA.
    • Required machinery is Ribosomal RNA (rRNA) and ribosomal proteins, and Transfer RNA (tRNAs).
    • End product: polypeptide.

    From Genes to Proteins (Complex)

    • Transcription involves pre-mRNA formation, intron removal and post-transcriptional modifications.
    • The process includes adding a cap (5') and tail (3') to the mRNA.
    • The process involves pre-mRNA, intron removal, post-transcriptional modifications and 5' and 3' end modification.

    Transcription: mRNA Processing

    • 5' Caps: Capping enzymes are recruited by phosphorylated CTD. They prevent digestion from exonucleases and aid in mRNA transport out of the nucleus and play a role in initiation of translation.
    • 3' poly(A) tail: Poly(A) tail enzymes are recruited by phosphorylated CTD. The tail protects mRNA from premature degradation.
    • RNA splicing: Splice sites occur at the 5' and 3' ends of exons and introns. Exons are parts of the gene that contribute to the mature RNA product while introns are non-coding sequences and are removed.

    Transcription: mRNA Processing Coordination

    • C-terminal domain acts as a scaffold to organize factors for pre-mRNA processing.
    • Processing includes capping, polyadenylation, and intron removal.
    • Machinery for mRNA processing and travels with the polymerase as a part of a giant mRNA factory.

    Transcription: Processing mRNA (5'Caps)

    • RNA triphosphatase removes the last 5' Pi.
    • Guanylyltransferase adds GMP in inverted orientation.
    • RNA Methyltransferases add methyl groups.
    • The 5' cap protects from digestion of the 5' end from exonucleases, aids in export from the nucleus and plays a role in initiation of translation.

    Transcription: Processing mRNA (7' Methylguanosine cap)

    • RNA triphosphatase removes the last 5' Pi.
    • Guanylyltransferase adds GMP in inverted orientation.
    • RNA Methyltransferases add methyl groups to the terminal guanosine cap and the ribose nucleotides.

    Transcription: Processing mRNA (3' poly(A) tail)

    • Poly(A) tail enzymes recruited by phosphorylated CTD
    • Protects mRNA from premature degradation by exonucleases.

    RNA Splicing

    • Exons are parts of the gene that contribute to the mature RNA product.
    • Introns are noncoding sequences that are removed.
    • Splice sites: (5`)G/GU and (3')AG/G.

    RNA Splicing: Coordinated Events

    • Splice sites: breaks at the 5' and 3' ends of exons/introns boundary sequences are highly conserved.
    • Exon-intron boundary sequences are highly conserved: 5' splice site - G/GU, 3' splice site - AG/G, polypyrimidine tract, 10-20 YN, branch point sequence (YUNAY) ~30 bases upstream of 3` .
    • Exonic splicing enhancers or ESEs are found within exons. They help recruit small nuclear ribonuclear proteins, called snRNPs, composed of small nuclear RNAs bound to proteins.

    RNA Splicing: Small Nuclear Ribonuclear Proteins (snRNPs)

    • Consist of small nuclear RNAs (150 nucleotides) and associated proteins (Ex. U1, U2, U4, U5, U6 snRNAs are examples)
    • Involved in splicing pre-mRNA to the final mRNA product.
    • Have small nuclear ribonuclear proteins (snRNPs) with spliceosomal cavity.

    Alternative Splicing

    • Same gene can code for more than one polypeptide.
    • Important in health and disease.
    • Estimated that 50-75% of human genes undergo alternative splicing.
    • Results in multiple mRNA isoforms which leads to a greater diversity of proteins than would be expected based on the number of genes alone.
    • The process involves different splicing enhancers/silencers which determines types of mRNA produced from one gene.

    rRNA and tRNA

    • DNA results in different types of transcripts.
    • rRNA are ribosomal RNA.
    • rRNA catalyzes amino acid covalent linking during translation.
    • rRNA provides structural support.
    • tRNA = transfer RNA.
    • tRNA matches mRNA code to amino acids during polypeptide translation.
    • RNA complex secondary and tertiary Structures.

    rRNA

    • Eukaryotic cells contain millions of ribosomes.
    • 80% of cells’ RNA is rRNA.
    • rRNA is divided into 4 distinct ribosomal RNAs.
    • Large subunit: 60S composed of 5S, 5.8S, 28S rRNAs.
    • Small subunit: 40S composed of the 18S rRNA.

    rRNA transcription

    • Clusters of rRNA genes.
    • Non-transcribed spacers (NTS).
    • Separates transcription units in a ribosomal gene cluster.
    • rRNA transcribed by:
      • RNA Pol I (28S, 18S, 5.8S).
      • RNA Pol III (5S).
    • rRNA transcription has a "Christmas tree" pattern.

    rRNA Processing

    • rRNA processing takes place in nucleoli (plural):
    • Assembly of rRNAs occurs along with corresponding r-proteins.
    • Fibrillar center (fc) contains rDNA.
    • The 5S rRNA genes are outside the nucleoli, transported to the nucleoli.
    • Dense fibrillar component (dfc) contains pre-rRNA transcripts, granular component (gc) contains ribosomes at various levels of assembly.
    • Pre-rRNA contains methylated nucleotides and pseudouridine residues impacting structural stability and function.
    • Assembled 60S or 40S ribosomes are transported through the nuclear pore complex, remains free or attach to the rough endoplasmic reticulum.

    Small Nucleolar Ribonucleoproteins (snoRNPs)

    • These are particles that consist of small nucleolar RNAs and associated proteins that are involved in processing pre-rRNA in nucleolus for rRNA stability and function.

    • 200 box H/ACA snoRNAs determine which uridines convert to pseudouridines which are in the nucleoside composed of the Uricil base and ribose sugar.

    • Box C/D snoRNAs determine which nucleotides have a ribose methylation in pre-rRNA.

    Translation

    • tRNA is capable of binding to specific amino acids.
    • tRNA is between 73-93 nucleotides in length for most.
    • There are ~500 tRNA genes in humans.
    • tRNA genes are arranged in small clusters throughout the genome.
    • Non-transcribed spacers (NTS) separate transcription units in tDNA.
    • tRNAs have promoter sequences within the coding region of the gene.
    • Transcribed by RNA pol III (like 5S rRNA)
    • tRNA precursors are modified.

    Translation Genetic Code Mutations

    • 20 amino acids, but 61 sense codons.
    • Multiple codons for most amino acids.
    • Redundancy minimizes the chance of wrong amino acid mutations.
    • tRNA isoacceptors have unique anticodons, binding the same amino acids.
    • Wobble pairing: the pairing of the third position of the codon is not as exact as the first two.

    Translation Initiation

    • Different processes in prokaryotic and eukaryotic cells.
    • Step 1: ~12 eukaryotic initiation factors (eIFs) bind to the small ribosomal subunit (40S).
    • Step 2: initiator tRNA-Met enters the P site (peptidyl site) of the 40S rRNA in association with eIF2-GTP.
    • Step 3: The 43S complex binds to the 5' cap (7-methylguanosine) of the mRNA.
    • Step 4: The 43S complex scans the mRNA for the initiation codon (AUG).
    • Step 5: GTP bound to eIF2 is hydrolyzed, the large 60S subunit associates with the complex and releases initiation factors.
    • Final Ribosome Assembled.

    Translation Elongation

    • Process of adding amino acids to the growing polypeptide chain.
    • Approximately ten amino acids added per second per ribosome.
    • Step 1: Second aminoacyl-tRNA combines with elongation factor eEF1a-GTP (places amino acid into the ribosome A site).
    • Step 2: Aminoacyl-tRNA is placed in the A site. GTP is hydrolyzed and eEF1a-GDP is released. Only tRNA with the correct anticodon will trigger conformational changes in the ribosome.
    • Step 3: Amine nitrogen of AA on the A site tRNA will attack the carbonyl carbon of the AA, this is performed by peptidyl transferase forming a peptide bond . No energy is required.

    Translation Elongation- cont'd

    • Step 4: Translocation. Small rRNA subunit ratchets relative to the large subunit. Movement moves the ribosome 3 nucleotides along the mRNA in the 5'-3' direction.
    • Dipeptidyl-tRNA moves into the P site. Deacylated tRNA moves into the E site. The process is driven by conformational changes in eEF2 following hydrolysis of bound GTP.
    • Step 5: Deacylated tRNA leaves the ribosome.
    • Step 6: Repeat of steps 1-5 (elongation)

    Translation Termination

    • Termination occurs at one of the three stop codons (UAA, UAG, UGA).
    • No tRNAs have anticodons complementary to stop codons.
    • Requires release factors (RFs).
    • Step 1: eRF1 and eRF3 work together to recognize all stop codons.
    • Step 2: eRF1 tri-peptide interacts with stop codon in the A site.
    • Step 3: Ester bond linking the nascent polypeptide to the tRNA is hydrolyzed.
    • Step 4: Hydrolysis of eRF3-GTP releases eRF1 from the A site.
    • Step 5: Release of deacylated tRNA form the P site, mRNA dissociation from the ribosome and disassembly of ribosomal subunits.
    • Requires several protein factors.

    Polyribosomes (polysomes)

    • A complex of multiple ribosomes (or polysomes) on mRNA, allowing simultaneous translation.
    • Increases the rate of protein synthesis.

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