Podcast
Questions and Answers
What is the starting material for transcription?
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?
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?
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?
Which component is NOT required for translation?
What is the initial precursor RNA in transcription called?
What is the initial precursor RNA in transcription called?
What processing events occur during pre-mRNA processing?
What processing events occur during pre-mRNA processing?
Which component acts as a scaffold for organizing factors involved in pre-mRNA processing?
Which component acts as a scaffold for organizing factors involved in pre-mRNA processing?
What is the primary function of tRNA in translation?
What is the primary function of tRNA in translation?
Which polymerase is responsible for transcribing tRNA genes?
Which polymerase is responsible for transcribing tRNA genes?
What term describes the phenomenon where multiple codons can code for the same amino acid?
What term describes the phenomenon where multiple codons can code for the same amino acid?
What separates transcription units in a tDNA gene cluster?
What separates transcription units in a tDNA gene cluster?
How many tRNA species do cells typically have despite there being 61 codons?
How many tRNA species do cells typically have despite there being 61 codons?
What is the estimated percentage of human genes that undergo alternative splicing?
What is the estimated percentage of human genes that undergo alternative splicing?
Which type of RNA catalyzes the covalent linking of amino acids during translation?
Which type of RNA catalyzes the covalent linking of amino acids during translation?
What is the sedimentation coefficient (S) a measure of?
What is the sedimentation coefficient (S) a measure of?
Which RNA polymerase is responsible for transcribing the 5S rRNA?
Which RNA polymerase is responsible for transcribing the 5S rRNA?
Where does rRNA processing primarily occur?
Where does rRNA processing primarily occur?
How are rRNA genes organized in humans?
How are rRNA genes organized in humans?
What pattern characterizes rRNA transcription?
What pattern characterizes rRNA transcription?
What percentage of a cell's RNA is typically rRNA?
What percentage of a cell's RNA is typically rRNA?
Which subunit contains the 28S rRNA?
Which subunit contains the 28S rRNA?
What does the fibrillar center (fc) primarily contain?
What does the fibrillar center (fc) primarily contain?
What role do box H/ACA snoRNAs play in the nucleolus?
What role do box H/ACA snoRNAs play in the nucleolus?
Where are the small nucleolar ribonucleoproteins (snoRNPs) found?
Where are the small nucleolar ribonucleoproteins (snoRNPs) found?
What do the granule components of the nucleolus primarily contain?
What do the granule components of the nucleolus primarily contain?
What is the average length of tRNA molecules?
What is the average length of tRNA molecules?
Which structure is responsible for the export of assembled ribosomes to the cytoplasm?
Which structure is responsible for the export of assembled ribosomes to the cytoplasm?
What percentage of tRNA genes are found in humans?
What percentage of tRNA genes are found in humans?
Which component of the nucleolus contains pre-rRNA transcripts?
Which component of the nucleolus contains pre-rRNA transcripts?
What type of structure do small nucleolar RNAs (snoRNAs) form when associated with proteins?
What type of structure do small nucleolar RNAs (snoRNAs) form when associated with proteins?
What is a function of the dense fibrillar component (dfc) in the nucleolus?
What is a function of the dense fibrillar component (dfc) in the nucleolus?
What are exons?
What are exons?
What percentage of the pre-RNA does intron comprise?
What percentage of the pre-RNA does intron comprise?
What are splice sites associated with?
What are splice sites associated with?
Which sequence is typically found at the 5’ splice site?
Which sequence is typically found at the 5’ splice site?
What role do exonic splicing enhancers (ESE) play?
What role do exonic splicing enhancers (ESE) play?
What is a common consequence of errors in RNA splicing?
What is a common consequence of errors in RNA splicing?
What are small nuclear ribonuclear proteins (snRNPs) primarily composed of?
What are small nuclear ribonuclear proteins (snRNPs) primarily composed of?
Which of the following is NOT a component of the spliceosome?
Which of the following is NOT a component of the spliceosome?
What's the purpose of the branch point sequence in RNA splicing?
What's the purpose of the branch point sequence in RNA splicing?
What role does eIF4G play in the initiation of translation?
What role does eIF4G play in the initiation of translation?
During elongation, what triggers the conformational changes within the ribosome?
During elongation, what triggers the conformational changes within the ribosome?
Which site in the ribosome is responsible for holding the tRNA that carries the growing polypeptide chain?
Which site in the ribosome is responsible for holding the tRNA that carries the growing polypeptide chain?
What is the primary function of the peptidyl transferase activity during translation?
What is the primary function of the peptidyl transferase activity during translation?
What happens during the translocation step in ribosomal translation?
What happens during the translocation step in ribosomal translation?
What is the primary role of rRNA during translation?
What is the primary role of rRNA during translation?
What is the significance of the 'Christmas tree' pattern in rRNA transcription?
What is the significance of the 'Christmas tree' pattern in rRNA transcription?
What is the function of nontranscribed spacers (NTS) in rRNA gene clusters?
What is the function of nontranscribed spacers (NTS) in rRNA gene clusters?
How many distinct types of ribosomal RNA (rRNA) are present within a eukaryotic ribosome?
How many distinct types of ribosomal RNA (rRNA) are present within a eukaryotic ribosome?
Where does the assembly of rRNAs with corresponding r-proteins primarily occur?
Where does the assembly of rRNAs with corresponding r-proteins primarily occur?
What percentage of a eukaryotic cell's RNA is typically composed of rRNA?
What percentage of a eukaryotic cell's RNA is typically composed of rRNA?
What is the primary reason for the clustering of rDNA genes on different chromosomes?
What is the primary reason for the clustering of rDNA genes on different chromosomes?
What defines a splice variant in the context of gene expression?
What defines a splice variant in the context of gene expression?
What is a characteristic feature of tRNA genes in the genome?
What is a characteristic feature of tRNA genes in the genome?
What role does the nontranscribed spacer (NTS) play in a tDNA gene cluster?
What role does the nontranscribed spacer (NTS) play in a tDNA gene cluster?
What is the function of codon degeneracy in the genetic code?
What is the function of codon degeneracy in the genetic code?
How many tRNA isoacceptors typically bind to the same amino acid?
How many tRNA isoacceptors typically bind to the same amino acid?
What is the primary product of the transcription process?
What is the primary product of the transcription process?
Which statement best describes the role of anticodons in tRNA?
Which statement best describes the role of anticodons in tRNA?
What are the primary components required for translation?
What are the primary components required for translation?
In the context of tRNA processing, what happens to the precursor tRNA?
In the context of tRNA processing, what happens to the precursor tRNA?
What is the significance of wobble pairing in translation?
What is the significance of wobble pairing in translation?
What is the function of wobble pairing in codon-anticodon interactions?
What is the function of wobble pairing in codon-anticodon interactions?
What role do aminoacyl-tRNA synthetases (AARS) play in translation?
What role do aminoacyl-tRNA synthetases (AARS) play in translation?
Why is the third position of codon pairing considered less critical?
Why is the third position of codon pairing considered less critical?
Which step in the initiation process involves the binding of eukaryotic initiation factors to the ribosomal subunit?
Which step in the initiation process involves the binding of eukaryotic initiation factors to the ribosomal subunit?
What is required for the activation of amino acids during tRNA formation?
What is required for the activation of amino acids during tRNA formation?
What does codon degeneracy mean in the context of DNA mutations?
What does codon degeneracy mean in the context of DNA mutations?
What happens during the process of amino acid activation?
What happens during the process of amino acid activation?
Which of the following is true regarding initiation in prokaryotic versus eukaryotic cells?
Which of the following is true regarding initiation in prokaryotic versus eukaryotic cells?
What is the main function of exons in the gene structure?
What is the main function of exons in the gene structure?
What sequence is typically found at the 3' splice site in RNA?
What sequence is typically found at the 3' splice site in RNA?
What is the first molecule that enters the P site during the initiation process?
What is the first molecule that enters the P site during the initiation process?
Which component is included in the composition of small nuclear ribonuclear proteins (snRNPs)?
Which component is included in the composition of small nuclear ribonuclear proteins (snRNPs)?
What can a double-ringed G pair with and why is this relevant?
What can a double-ringed G pair with and why is this relevant?
What role do exonic splicing enhancers (ESE) play during RNA processing?
What role do exonic splicing enhancers (ESE) play during RNA processing?
Approximately what percentage of human inherited diseases are estimated to arise from errors in RNA splicing?
Approximately what percentage of human inherited diseases are estimated to arise from errors in RNA splicing?
What is the primary type of sequence found at the exon-intron boundary?
What is the primary type of sequence found at the exon-intron boundary?
What are the primary functions of the branch point sequence in splicing?
What are the primary functions of the branch point sequence in splicing?
What are the consequences of codon degeneracy on the impact of DNA mutations?
What are the consequences of codon degeneracy on the impact of DNA mutations?
Which mechanism primarily facilitates the activation of amino acids to form aminoacyl-tRNA?
Which mechanism primarily facilitates the activation of amino acids to form aminoacyl-tRNA?
During the initiation of translation, what role do eukaryotic initiation factors (eIFs) play?
During the initiation of translation, what role do eukaryotic initiation factors (eIFs) play?
What distinguishes the wobble pairing mechanism from traditional base pairing in codon-anticodon interactions?
What distinguishes the wobble pairing mechanism from traditional base pairing in codon-anticodon interactions?
What is a primary function of the initiator tRNA-Met during translation initiation?
What is a primary function of the initiator tRNA-Met during translation initiation?
Which snRNP complex is responsible for attaching to the 5' splice site during RNA splicing?
Which snRNP complex is responsible for attaching to the 5' splice site during RNA splicing?
What effect does U2 snRNP binding have on the intron during splicing?
What effect does U2 snRNP binding have on the intron during splicing?
What is the main role of U6 snRNA during the splicing process?
What is the main role of U6 snRNA during the splicing process?
Which of the following is released after the second cleavage reaction during splicing?
Which of the following is released after the second cleavage reaction during splicing?
What is one significant outcome of alternative splicing?
What is one significant outcome of alternative splicing?
Which protein aids the U2 snRNP in recognizing the 3' splice site during splicing?
Which protein aids the U2 snRNP in recognizing the 3' splice site during splicing?
What characterizes the macromolecular machine known as the spliceosome?
What characterizes the macromolecular machine known as the spliceosome?
Which splicing reaction occurs first during RNA splicing?
Which splicing reaction occurs first during RNA splicing?
What type of sequence does U1 snRNA complementarily bind to during splicing?
What type of sequence does U1 snRNA complementarily bind to during splicing?
How many distinct ribosomal RNAs are synthesized by RNA Polymerase I in eukaryotic cells?
How many distinct ribosomal RNAs are synthesized by RNA Polymerase I in eukaryotic cells?
Which statement accurately describes the role of nontranscribed spacers in ribosomal gene clusters?
Which statement accurately describes the role of nontranscribed spacers in ribosomal gene clusters?
What subunit of ribosomes includes the 5S rRNA?
What subunit of ribosomes includes the 5S rRNA?
Which of the following correctly identifies the transcriptional enzyme responsible for 5S rRNA synthesis?
Which of the following correctly identifies the transcriptional enzyme responsible for 5S rRNA synthesis?
What is a defining characteristic of the 'Christmas tree' pattern observed in rRNA transcription?
What is a defining characteristic of the 'Christmas tree' pattern observed in rRNA transcription?
Which ribosomal RNA comprises the smaller ribosomal subunit in eukaryotes?
Which ribosomal RNA comprises the smaller ribosomal subunit in eukaryotes?
What proportion of the total RNA in eukaryotic cells is commonly represented by rRNA?
What proportion of the total RNA in eukaryotic cells is commonly represented by rRNA?
In the context of ribosomal biogenesis, where does the assembly of rRNAs with corresponding r-proteins primarily occur?
In the context of ribosomal biogenesis, where does the assembly of rRNAs with corresponding r-proteins primarily occur?
What best describes the structural arrangement of ribosomal RNA genes within the nucleolar framework?
What best describes the structural arrangement of ribosomal RNA genes within the nucleolar framework?
What is the main function of rRNA in the ribosome during translation?
What is the main function of rRNA in the ribosome during translation?
What is a key feature of tRNA genes in the genome?
What is a key feature of tRNA genes in the genome?
Which of the following statements about tRNA precursor is true?
Which of the following statements about tRNA precursor is true?
What allows for codon degeneracy in the genetic code?
What allows for codon degeneracy in the genetic code?
Which component of the translation process is essential for matching mRNA codons to amino acids?
Which component of the translation process is essential for matching mRNA codons to amino acids?
What specifically characterizes the process of transcription?
What specifically characterizes the process of transcription?
Which of the following best describes the role of nontranscribed spacers (NTS) in tDNA gene clusters?
Which of the following best describes the role of nontranscribed spacers (NTS) in tDNA gene clusters?
Why do cells have approximately 50 tRNA species despite having 61 codons?
Why do cells have approximately 50 tRNA species despite having 61 codons?
What is the main purpose of anticodons on tRNA?
What is the main purpose of anticodons on tRNA?
Which statement accurately reflects the relationship between codons and amino acids?
Which statement accurately reflects the relationship between codons and amino acids?
What piece of machinery is primarily required for translation?
What piece of machinery is primarily required for translation?
What is the purpose of exonic splicing enhancers (ESE) in RNA splicing?
What is the purpose of exonic splicing enhancers (ESE) in RNA splicing?
Which sequences are highly conserved at the exon-intron boundaries?
Which sequences are highly conserved at the exon-intron boundaries?
What percentage of the human genes is estimated to be affected by errors in splicing?
What percentage of the human genes is estimated to be affected by errors in splicing?
What function do splice sites serve in RNA splicing?
What function do splice sites serve in RNA splicing?
How much of the pre-RNA does intron comprise in terms of percentage?
How much of the pre-RNA does intron comprise in terms of percentage?
What is generally found at the branch point sequence in RNA splicing?
What is generally found at the branch point sequence in RNA splicing?
Flashcards
Gene transcription
Gene transcription
The process of creating RNA from a DNA template in the cell nucleus.
Translation
Translation
The process of building a protein from an mRNA molecule in the cytoplasm.
Pre-mRNA
Pre-mRNA
An initial RNA molecule that needs processing before becoming a functional mRNA.
hnRNA
hnRNA
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RNA polymerase II
RNA polymerase II
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mRNA processing
mRNA processing
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RNA Polymerase II CTD
RNA Polymerase II CTD
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Exon
Exon
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Intron
Intron
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RNA Splicing
RNA Splicing
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5' splice site
5' splice site
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3' splice site
3' splice site
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Spliceosome
Spliceosome
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snRNPs (snurps)
snRNPs (snurps)
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Exon-intron boundary
Exon-intron boundary
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tDNA gene cluster
tDNA gene cluster
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Non-Transcribed Spacer (NTS)
Non-Transcribed Spacer (NTS)
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tRNA Promoter
tRNA Promoter
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tRNA Precursor
tRNA Precursor
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tRNA Modification
tRNA Modification
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Alternative Splicing
Alternative Splicing
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Splice Variants
Splice Variants
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rRNA Function
rRNA Function
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tRNA Function
tRNA Function
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Ribosome Subunits
Ribosome Subunits
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Svedberg Unit
Svedberg Unit
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rDNA Clusters
rDNA Clusters
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Nucleolus Function
Nucleolus Function
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RNA Polymerase I & III
RNA Polymerase I & III
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Nucleolus: fibrillar center (fc)
Nucleolus: fibrillar center (fc)
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Nucleolus: dense fibrillar component (dfc)
Nucleolus: dense fibrillar component (dfc)
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Nucleolus: granular component (gc)
Nucleolus: granular component (gc)
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Pre-rRNA modifications
Pre-rRNA modifications
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60S and 40S ribosomes
60S and 40S ribosomes
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Ribosome export
Ribosome export
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Rough endoplasmic reticulum (RER)
Rough endoplasmic reticulum (RER)
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Small nucleolar ribonucleoproteins (snoRNPs)
Small nucleolar ribonucleoproteins (snoRNPs)
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Box H/ACA snoRNAs
Box H/ACA snoRNAs
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Box C/D snoRNAs
Box C/D snoRNAs
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Ribosomal RNA (rRNA)
Ribosomal RNA (rRNA)
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Transfer RNA (tRNA)
Transfer RNA (tRNA)
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Nucleolus
Nucleolus
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Christmas Tree Pattern
Christmas Tree Pattern
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Splice Sites
Splice Sites
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What is the polypyrimidine tract?
What is the polypyrimidine tract?
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What is the branch point sequence?
What is the branch point sequence?
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What are snRNPs?
What are snRNPs?
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Exonic splicing enhancers (ESE)
Exonic splicing enhancers (ESE)
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m⁷G cap
m⁷G cap
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eIF4E
eIF4E
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eIF4A
eIF4A
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eIF4G
eIF4G
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Kozak sequence
Kozak sequence
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Wobble Pairing
Wobble Pairing
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Codon Degeneracy
Codon Degeneracy
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Aminoacyl-tRNA Synthetase (AARS)
Aminoacyl-tRNA Synthetase (AARS)
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Initiation Process
Initiation Process
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Eukaryotic Initiation Factors (eIFs)
Eukaryotic Initiation Factors (eIFs)
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Small Ribosomal Subunit (40S)
Small Ribosomal Subunit (40S)
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Peptidyl Site (P site)
Peptidyl Site (P site)
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Initiator tRNA-Met
Initiator tRNA-Met
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eIF2-GTP
eIF2-GTP
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43S Complex
43S Complex
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A Site (Aminoacyl Site)
A Site (Aminoacyl Site)
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tRNA Trimming
tRNA Trimming
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Base Modification
Base Modification
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What is the tRNA anticodon?
What is the tRNA anticodon?
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What is codon degeneracy?
What is codon degeneracy?
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What is wobble pairing?
What is wobble pairing?
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What are Introns?
What are Introns?
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What are Exons?
What are Exons?
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What is RNA Splicing?
What is RNA Splicing?
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What is the Spliceosome?
What is the Spliceosome?
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U1 snRNP
U1 snRNP
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U2AF
U2AF
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U2 snRNP
U2 snRNP
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U4/U6 and U5 snRNPs
U4/U6 and U5 snRNPs
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Lariat
Lariat
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Ribozyme
Ribozyme
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Aminoacyl-tRNA Synthetase
Aminoacyl-tRNA Synthetase
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Eukaryotic Initiation Factors
Eukaryotic Initiation Factors
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tRNA isoacceptors
tRNA isoacceptors
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mRNA Isoforms
mRNA Isoforms
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Svedberg Unit (S)
Svedberg Unit (S)
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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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Test your knowledge on the transcription process in molecular biology. This quiz covers essential concepts including starting materials, enzymes involved, and RNA processing events. Ideal for students studying molecular biology or genetics.
