This set of Basic Molecular Biology Questions and Answers focuses on “The tRNA Occupies Three Sites During Elongation of the Polypeptide”.
1. For the correct addition of amino acid to the growing polypeptide chain which of the following does not play any role?
a) Loading of initiator tRNA to the P site
b) Loading of correct aminoacyl tRNA to the A site
c) Formation of peptide bond between the existing amino acid and the incoming amino acid
d) The formed tRNA with the growing polypeptide is translocated from the A site to the P site
Explanation: The correct addition of amino acid involves only three steps:
i) Loading of correct aminoacyl tRNA to the A site
ii) Formation of peptide bond between the existing amino acid and the incoming amino acid
iii) The formed tRNA with the growing polypeptide is translocated from the A site to the P site
Loading of the initiator tRNA is the part of initiation of translation process.
2. The events controlling the correct addition of amino acid is controlled by ____________ proteins.
Explanation: The events controlling the correct addition of amino acid are controlled by 3 proteins known as elongation factors. They are IF1, IF2 and IF3.
3. The auxillary protein required for the elongation of polypeptide is energy independent.
Explanation: The auxillary protein required for the elongation of polypeptide is energy dependent processes. All these factors use energy of the GTP binding and hydrolysis to enhance the rate and accuracy of ribosome function.
4. Aminoacyl tRNA needs an escort to carry them to the ribosome bound mRNA.
Explanation: Aminoacyl-tRNAs do not bind to the ribosome on their own. Instead, they are escorted to the ribosome by the elongation factor EF-Tu.
5. Once the tRNA is aminoacylated, EF-Tu binds to the tRNA at the ____________
a) 5’ end of the tRNA
b) 3’ end of the tRNA
c) Amino acid
d) Variable loop of tRNA
Explanation: Once the tRNA is aminoacylated, EF-Tu binds to the tRNA at the3’ end of the tRNA, masking the coupled amino acid. This interaction prevents the bound aminoacyl-tRNA from participating in peptide bond formation until it is released from EF-Tu.
6. The activation of GTPase activity of the enzyme EF-Tu occurs after its binding to the aminoacylated tRNA.
Explanation: The trigger h the factor binding center after the tRNA is loaded into that activates the EF-Tu GTPase id known as factor binding center. EF-Tu only interacts with the factor binding center after the tRNA is loaded into the A site and a correct codon-anticodon match is made. At this point, EF-Tu hydrolyzed its bound GTP and is released from the ribosome.
7. The only fidelity testing of the translation is by observing the energy difference involved in the correct and incorrect base pairing.
Explanation: The energy difference between a correctly formed codon-anticodon pair and that of a near match is not equal but still cannot account for the high level of accuracy required for the translation process. Thus there are three more mechanisms that contribute to this specificity. In each case, these mechanisms select against incorrect codon-anticodon pairings.
8. How many mechanisms are involved in the maintenance of the fidelity of translation?
Explanation: A total of 4 mechanisms are involved in the maintenance of the fidelity of translation. In addition to the codon-anticodon interactions, the ribosome exploits minor grove interactions and two phases of proof reading to ensure that a correct aminoacyl-tRNA binds in the A site.
9. Which part of the large subunit helps in the formation of the peptide bond?
a) 5S rRNA
c) 23S rRNA
d) 18S rRNA
Explanation: Once the correctly charged tRNA has been placed in the A site and has rotated into the peptidyl transferase center, peptide bond formation takes place. This reaction is catalyzed by RNA, specifically the 23S rRNA component of the large subunit.
10. Which of the following steps is not required for the process of translocation?
a) Frame shift in hybrid states
b) Movement of P site tRNA to E site
c) Movement of A site tRNA to P site
d) Movement of ribosome by three nucleotides
Explanation: For a new round of peptide chain elongation to occur, the P site tRNA must move to the E site and the A site tRNA must move to the P site. At the same time the mRNA must move by 3 nucleotides to expose the next codon. These movements are coordinated within the ribosome and are collectively referred to as translocation.
11. The completion of translocation requires the action of the factor _________________
Explanation: The completion of translocation requires the action of a second elongation factor called EF-G. EF-G can only bind to the ribosome when associated with GTP. After the peptidyl transferase reaction, the shift in the location of the A site tRNA uncovers a binding site for EF-G in the large subunit portion of the A site.
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