MBG Part Two: Lecture 9 Flashcards
What are ribosomes composed of?
Proteins and several different rRNAs
What is a Ribosomes main purpose?
It is an “RNA machine” with key roles in protein synthesis, including the formation of peptide bonds between amino acids.
What is the Size of a Prokaryotic Ribosome?
70S
What is the Size of a Eukaryotic Ribosome
80S
What does each amino acid have?
Its own aminoacyl tRNA synthetase. This enzyme “charges” the tRNA with its specific amino acid.
What does the tRNA synthetase enzyme do?
Attaches its specific amino acid to the 3’ end of the tRNA.
How does the amino acid become attached to the appropriate tRNA by the amino acyl tRNA synthetase?
In a Two Step Reaction:
- Addition of AMP
- Addition of the tRNA
What is the first step of the tRNA Synthetase reaction?
The amino acid rects with ATP to produce aminoacyl-AMP and PPi
What is the Second Step of the tRNA Synthetase reaction?
The Amino Acid is transferred to the tRNA and the AMP is released
What is the Amino Acid covalently attached to?
The 3’ end of the tRNA.
What are tRNAs adapters between?
Amino Acids and the codons in mRNA.
What does the anticodon of the tRNA base pair with?
The codon of the mRNA
What happens during initiation of Translation?
The components of the translational apparatus come together with an mRNA and a tRNA carrying the first amino acid (AA1) binds to the start codon (AUG)
What happens during elongation?
Amino Acids are Brough to the mRNA by tRNAs and are added, one by one, to a growing polypeptide chain
What happens during termination?
A stop codon in the mRNA is recognized by a protein release factor, and the translational apparatus comes apart, releasing a completed polypeptide.
What is required to form the initiation complex in prokaryotes?
mRNA, large and small ribosomal subunits, initiation factors (IF1-3) and GTP
What is the 16S rRNA a component of in prokaryotes?
The 30S small ribosomal subunit and contains the complement to the Shine-Dalgarno sequence in the mRNA.
What does pairing between the two sequences position the ribosome near in prokaryotes ?
Near the AUG start codon
What is IF-3 required to do?
Inhibit large (50S) subunit from binding (30S) small subunit.
What does IF-1 and IF-2 position formylate?
(f)Met-tRNA over the start codon
What makes the 30S initiation complex?
Fmet-tRNA + 30S ribosome + IF2-GTP+IF-3+IF-1
What happens when GTP is hydrolyzed to GDP?
The remaining IF factors dissociate
What does IF-3 dissociating allow for?
The large 50S ribosomal unit to bind.
What is the complete complex called?
70S initiation complex
What does fMet=tRNA fmet occupy?
The P site of the ribosome
What is methionine in eukaryotes?
Not formulated
What is the Kozak Sequence?
Only found in eukaryotes and is 5’ C[A/G]NCAUGG 3’
What does the Kozak Sequence do?
Influences the efficiency of which AUG in the vicinity is used to start translation.
What does the poly(A) tail of the eukaryotic mRNA interact with?
The mRNA 5’ 7-MG cap structure via a cap-binding protein complex (CBC) to promote translation initiation,
What happens during elongation?
1.) A charge tRNA binds to the A site of the ribosome (requires complex formation between the amino-acyl tRNA, elongation factor Tu plus GTP)
2.) Once in the A site, GTP is hydrolyzed to GDP and is released along with EF-Tu.
3.) rRNA in large subunits catalyzes the formation of the peptide bond between the two amino acids.
4.) Polypeptide chain is transferred to the aa in the A-site
What is formed between the amino acids in the P and A sites?
A peptide bond and the tRNA in the P site releases its amino acid.
What does the ribosome require too move down the mRNA to the next codon (translocation)?
EF-G and GTP
What happens to the tRNA that was in the P site?
Enters the E site, from which it moves into the cytoplasm
What happens to the tRNA that occupied the A site?
It is now in the P site and the A site is open and ready to receive another tRNA.
How do all Amino Acids (other then f-Met or Met) move?
From the cytoplasm to the A site to the P site to the E site bak to the cytoplasm.
Where does F-Met or the first Met in Eukaryotes start?
P Site
How does ribosomes move down the mRNA
5’ to 3’
What way do polypeptide chains grow?
N terminal to C terminal
What site contains the growing polypeptide chain?
P Site
How does termination occur?
When a stop codon enters the A site of the ribosome
Are there tRNAs that bind to stop codons?
No.
What happens when a stop codon is encountered?
A release factor (RF) binds to the A site.
What does Release Factor 1 (RF-1) recognize?
UAG and UAA stop codons
What does Release Factor 2 (RF-2) recognize?
Release factor 2 (RF-2) recognizes UAA and UGA stop
What does the RF-1 and RF-2 binding promote?
Promotes the cleavage of the polypeptide chain from the tRNA, releasing the chain and leading to termination.
What does the binding of RF-3 and GTP to the ribosome assist?
In the dismantling of the entire complex.
What are the Six Antibiotics that inhibit Bacterial Protein Synthesis?
Chloromycetin
Erythromycin
Neomycin
Streptomycin
Tetracycline
Paromomycin
What does chloromycetin inhibit?
Formation of peptide bonds
What does erythromycin inhibit?
Translocation of mRNA along ribosome
What does Neomycin inhibit?
Interactions between tRNA and mRNA
What does Streptomycin inhibit?
Initiation of translation
What does Tetracycline inhibit?
Binding of tRNA to ribosome
What does Paromocycin inhibit?
Validation of mRNA-tRNA match
