Flashcards in Transcription And Translation II Deck (61):
What does protein synthesis occur on?
In what direction is a protein synthesized?
In the amino-to-carboxyl direction by the sequential addition of amino acids to the carboxyl end of the growing peptide chain.
The sequence of nucleotides in the mRNA molecule is read consecutively in groups of ____.
What is the term for a group of 3 consecutive nucleotides in RNA?
What does each codon specify?
Either one amino acid or a stop to the translation process.
Why is the genetic code degenerate?
A total of 61 codons code for the 20 known amino acids.
What matches amino acids to codons in mRNA?
What is the adaptor hypothesis?
A hypothesis that postulates that the genetic code is read by molecules that can recognize a codon and carry the corresponding amino acid.
What is the function of tRNA?
It serves as an adaptor that binds to a specific codon and brings with it an amino acid for incorporation into the polypeptide chain.
What is the general structure of tRNA?
It has a cloverleaf secondary structure and contains 4 short double-helical segments.
What are two important segments of the tRNA?
An anticodon loop
A 3' CCA terminal region
What is the function of the codon loop in tRNA?
It pairs with a complementary codon in an mRNA molecule.
What is the function of the 3' CCA terminal region of tRNA?
It binds the amino acid that matches the corresponding codon.
True or false: there is more than one tRNA for most amino acids.
What is the wobble hypothesis?
It states that some tRNAs require accurate base pairing only at the first two positions of the codon and can tolerate a mismatch (or wobble) at the third position.
What does wobble base-pairing explain?
It explains why so many of the alternative codons for an amino acid differ only in their third nucleotide.
What enzyme is needed for the recognition and attachment of the correct amino acid to the corresponding tRNA?
What is the function of the aminoacyl-tRNA synthetases?
They covalently couple an amino acid to the 3' terminal ribose residue of its correpsonding tRNA to form an aminoacyl tRNA.
Does aminoacyl tRNA require energy?
True or false: most cells have a different synthetase enzyme for each amino acid.
In order to pair aminoacyl-tRNA synthetases with the correct amino acids, what is present in aminoacyl-tRNA synthetase?
A highly discriminating activation site.
The corret amino acid has the highest affinity for the active-site pocket of its synthetase and is therefore favored over the other 19.
Aminoacyl-tRNA synthetases also contain an editing site for proofreading.
What is the purpose of aminoacyl-tRNAase performing hydrolytic editing?
It cleaves activated species that are smaller than the correct one.
It raises the overall accuracy of tRNA charging to approximately one mistake in 40,000 couplings.
What is responsible for determining the oversall shape of the ribosome, its ability to position tRNAs on the mRNA and its catalytic activity in forming peptide bonds?
What is the function of the small subunits on a ribosome?
The provide a framework on which the tRNAs are accurately matched to the codons of the mRNA.
What is the function of the large subunit on the ribosome?
It catalyzes the formation of the peptide bonds that link the amino acids together into a polypeptide chain.
Where is protein synthesis performed?
On a ribosome
What do the two ribosomal subunits do when they are not synthesizing proteins?
When protein synthesis is initiated, what occurs to the 2 subunits of a ribosome?
They join together at the 5' end of an mRNA and initiate protein synthesis.
How is mRNA pulled through the ribosome?
In teh 5'-3' direction.
As the codons enter the core of the ribosome, the mRNA sequence is translated into amino acid sequence using ____ as adaptors.
When does a ribosome release a protein?
When it encounters a stop codon.
How many binding sites dos a ribosome contain for RNA?
A, P, E, and 1 for mRNA
What are the A, P and E sites on the ribosome formed by?
The rRNA of the large subunit.
What subunit is the mRNA bound to on the ribosome?
The small subunit
When are tRNA molecules held in the A and P sites?
Only if their anticodon forms base pairs with complementary codons in mRNA threaded through the ribosome.
Where are amino acids added to a polypeptide?
How is a protein synthesized in the ribosome?
From its N-terminal to C-terminal end
Once protein synthesis has been initiated, each new amino acid is added to the elongating chain in a cycle of reactions containing four steps. What are these steps?
Peptide bond formation
Large subunit translocation
Small subunit translocation
What are the three phases of translation?
What do all known mRNA molecules contain?
Signals that define the beginning of each encoded polypeptide chain.
What does translation begin with?
The initiation codon AUG.
A special initiator tRNA carries the amino acid methoionine.
Where is the initiator tRNA-methionine complex loaded onto?
The small subunit of ribosomes on the P-site.
After initiator tRNA is bound to the P-site, what does the small ribosomal subunit bind to?
The 5' end of mRNA; it recognizes this by the initiation factors eIF4E and eIF4G.
What is the recognition consensus site ahead of AUG?
What happens after translation initiation?
Eukaryotic initiation factors drop off, and the large subunit joints the small subunit and protein synthesis starts.
What makes translation more efficient and accurate?
They are called EF-Tu and EF-G in bacteria and EF1 and EF2 in eukaryotes.
What do elongation factos bind to?
GTP and the aminoacyl tRNA.
This escorts it to the ribosome (checks whether amino acid-tRNA match is correct).
What is the function of elongation factors?
They monitor the interaction between anticodon of incoming aminoacyl tRNA and the codon of mRNA.
What do the interactions of EF-Tu, tRNA and the ribosome do?
They introduce critical proof-reading steps into protein synthesis.
What reaction brings about conformational changes in ribosomes and increases efficiency greatly?
What are the stop codons?
They are not recognized by any tRNA and do not code for any amino acid.
What are stop codons recognized by?
The release factors, which are proteins that promote the release of the completed protein from the tRNA.
Where to release factors (RFs) bind?
They bind to the A site of a ribosome containing the stop codon and catalyze the addition of a water molecule instead of an amino acid.
The reaction frees the carboxyl group of the polypeptide from tRNA.
The completed protein is then released from the ribosome into the cytoplasm, where the protein folds into its 3D structure.
Many effective antibiotics are compounds made by fungi that inhibit bacterial protein synthesis. These compounds exploit structural and functional differences between bacterial and eukaryotic ribisomes. What is their mechanism of function?
Many of them bind to ribosomes and interfere with protein synthesis by blocking ribosomal function.
Tetracycline acts only on bacteria. What is its function?
It blocks the binding of aminoacyl-tRNA to the A site of the ribosome.
Streptomycin only works on bacteria. What is its function?
It prevents the transition from translation initiation to chain elongation and also causes miscoding.
Chloramphenicol acts only on bacteria. What is its function?
It blocks the peptidyl transferase reaction on ribisomes.
Erythromycin acts only on bacteria. What is its mechanism of function?
It binds in the exit channel of the ribosome and thereby inhibits elongation fo the peptide chain.
Puromycin acts on bacteria and eukaryoties. What is its function?
It causes the premature release of nascent polypeptide chains by its addition to the growing chain end.
Cylcoheximide acts on eukaryotes, but not bacteria. What is its mechanism of function?
It blocks the translocation reaction on ribosomes.