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Questions and Answers
What is the total S value for eukaryotic ribosomes?
What is the total S value for eukaryotic ribosomes?
- 80 S (correct)
- 60 S
- 90 S
- 70 S
Which of the following statements is true regarding the rRNA component of ribosomes?
Which of the following statements is true regarding the rRNA component of ribosomes?
- Prokaryotic ribosomes do not have any RNA.
- Small ribosomal subunits do not contribute to peptide bond formation.
- Eukaryotic ribosomes contain 28 S rRNA. (correct)
- Ribosomes are solely composed of proteins.
Which type of mutation results in the introduction of a premature stop codon?
Which type of mutation results in the introduction of a premature stop codon?
- Nonsense mutation (correct)
- Frameshift mutation
- Missense mutation
- Silent mutation
What allows for the encoding of more than one amino acid by multiple codons?
What allows for the encoding of more than one amino acid by multiple codons?
During translation, which sequence determines the reading frame of mRNA?
During translation, which sequence determines the reading frame of mRNA?
What is the primary function of ribosomes in protein synthesis?
What is the primary function of ribosomes in protein synthesis?
Which component of tRNA is responsible for matching with codons on mRNA?
Which component of tRNA is responsible for matching with codons on mRNA?
During translation, where are new amino acids attached in the growing polypeptide chain?
During translation, where are new amino acids attached in the growing polypeptide chain?
What is the role of the initiator tRNA during protein synthesis?
What is the role of the initiator tRNA during protein synthesis?
What structural feature characterizes tRNA molecules?
What structural feature characterizes tRNA molecules?
What is the role of amino acyl tRNA synthetase in protein synthesis?
What is the role of amino acyl tRNA synthetase in protein synthesis?
What initiates the translation process in eukaryotic cells?
What initiates the translation process in eukaryotic cells?
What can occur when multiple ribosomes translate the same mRNA simultaneously?
What can occur when multiple ribosomes translate the same mRNA simultaneously?
In eukaryotes, why can transcription and translation not occur simultaneously?
In eukaryotes, why can transcription and translation not occur simultaneously?
Flashcards
What is the S value in ribosomes?
What is the S value in ribosomes?
The S value reflects a ribosome's sedimentation rate during centrifugation. It is not a linear measurement but rather a value related to the ribosome's mass and shape under centrifugal force.
What are ribosomes composed of?
What are ribosomes composed of?
Ribosomes are comprised of ribosomal RNA (rRNA) and proteins. The proteins primarily have structural roles and do not directly catalyze reactions. rRNA, particularly the 23S rRNA in prokaryotes and the 28S rRNA in eukaryotes, possesses catalytic activity, making it a ribozyme.
What is a reading frame in the context of translation?
What is a reading frame in the context of translation?
Ribosomes read mRNA in triplets called codons to translate genetic information into proteins. Each codon specifies a specific amino acid. If the reading frame is shifted, the wrong amino acids will likely be incorporated, leading to an altered protein.
Explain a missense mutation.
Explain a missense mutation.
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What is a nonsense mutation?
What is a nonsense mutation?
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Translation
Translation
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Amino Acids
Amino Acids
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Messenger RNA (mRNA)
Messenger RNA (mRNA)
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Ribosomes
Ribosomes
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Transfer RNA (tRNA)
Transfer RNA (tRNA)
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Codon
Codon
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Translocation
Translocation
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Polyribosome
Polyribosome
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C-terminus
C-terminus
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Aminoacyl tRNA Synthetases
Aminoacyl tRNA Synthetases
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Study Notes
Bacterial Ribosomes
- Composed of two subunits: large and small
- Measured in Svedberg units (S)
- Large subunit is 50S, small subunit is 30S, together forming 70S
- S units are not linear but related to mass under centrifugal force
- Eukaryotic ribosomes are 60S + 40S = 80S
Ribosome Composition
- Composed of rRNA and proteins
- Proteins are structural; rRNA catalyzes reactions
- Prokaryotic ribosomes contain 23S rRNA
- Eukaryotic ribosomes contain 28S, 5.8S, 5S rRNAs
Ribosomal Function
- Ribosomes are protein synthesis factories
- mRNA carries genetic information in codons
- tRNA acts as an adaptor between codons and amino acids
- rRNA associates with proteins to form the ribosome
- Peptide bonds form between amino acids on the ribosome
Ribosome and mRNA Reading Frames
- mRNA is read in triplets (codons)
- Reading frame determines which amino acids are incorporated
- Frameshift mutations occur when the reading frame is altered
- This changes the amino acid sequence and protein function
Types of Point Mutations
- Missense mutation: One nucleotide substitution leads to a different amino acid
- Nonsense mutation: One nucleotide substitution creates a stop codon, prematurely terminating protein synthesis
- Frameshift mutation: Insertion or deletion of nucleotides shifts the reading frame, drastically altering the amino acid sequence
The Genetic Code
- 64 possible codons in the genetic code
- 61 of these specify amino acids, the other 3 are stop codons
- Some amino acids are specified by multiple codons (degeneracy)
- The genetic code is universal, but codon usage can vary
Amino Acids
- There are 20 proteinogenic amino acids
- Each amino acid has a specific three-letter and one-letter abbreviation
Ribosomal Activity During Translation
- First amino acid undergoes a nucleophilic attack from the next
- Peptide bond forms between amino acids
- Ribosome moves along the mRNA
- New amino acids are attached to the growing polypeptide chain (C-terminus)
- The ribosome exists as separate large and small subunits during the translation processes
Multiple Ribosomes
- Multiple ribosomes can translate the same mRNA molecule
- These are called polyribosomes or polysomes
- This process is possible in both prokaryotes and eukaryotes, but cotranscriptional translation does not occur in eukaryotes (as transcription occurs in the nucleus)
tRNA
- Single-stranded RNA molecule
- Adapters between codons and amino acids
- tRNA is encoded by clusters of genes
- Undergoes processing, which can include unusual base formation
- Has a cloverleaf structure
- Contains an anticodon that is complementary to a codon in mRNA
- Amino acid attached at the 3' end
Wobble Phenomenon
- Third position of codon in mRNA and first position in tRNA are relaxed in base pairing
- This allows one tRNA to recognize more than one codon
- Uses inosine as a base in the wobble position
Shine-Dalgarno Sequence
- Consensus sequence (5'-UAAGGAGGU-3')
- Used in prokaryotes to initiate translation by ribosomes recognising start codon in mRNA
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