This set of Molecular Biology Multiple Choice Questions & Answers (MCQs) focuses on “Overview of Protein Synthesis – 1”.
1. Which of the following is the most energetically costly process among the following?
c) Post transcriptional processing
Explanation: The process of translation is the most conserved of all other cellular processes mentioned above. It is also the most energetically costly process costing over 80% of the cells energy in rapidly growing cells.
2. The accurate ordering of amino acids in a polypeptide is determined by the direct interactions between the mRNA template and the amino acid.
Explanation: It is unlikely that the accurate ordering of amino acids in a polypeptide is determined by the direct interactions between the mRNA template and the amino acid. This is because the side chains of amino acids have little or no affinity for nucleotides found in the RNAs.
3. Who was the first to propose indirect interaction between mRNA template and amino acids to produce accurate ordering of amino acid in a polypeptide chain?
a) Paul C. Zamecnic
b) James Watson
c) Francis H. Crick
d) Mahlon B. Hoagland
Explanation: Francis H. Crick was the first to propose indirect interaction between mRNA template and amino acids to produce accurate ordering of amino acid in a polypeptide chain in 1955. He suggested that certain adaptor molecules were used to interact with the codons of mRNA and respective amino acids itself acting as a bridging element. This was later demonstrated by Paul C. Zamecnic and Mahlon B. Hoagland that a special class of RNA, the tRNA, was the adaptor molecules as suggested by Crick and thus named transfer RNA.
4. How many major components are used for the process of translation?
Explanation: Four major components are used for the process of translation. They are mRNAs, tRNAs, aminoacyl tRNA synthetases and ribosome.
5. Which of the following is not a property of open reading frame?
b) Non overlapping
c) Encodes a single protein
d) Starts and ends at either end of the mRNA.
Explanation: The protein coding region of each mRNA is composed of a contiguous, non-overlapping string of codons called open reading frame. Each ORF specifies a single protein and starts and ends at internal sites within the mRNA and thus have distinct start and stop codons used for translation.
6. Both Prokaryote and Eukaryote has one start codon 5’-AUG-3’.
Explanation: Eukaryotes have one start codon that is 5’-AUG-3’. Prokaryotes, instead of one, have three start codons: 5’-AUG-3’, 5’-GUG-3’ and 5’-UUG-3’.
7. The start codon has a vital role to play in incorporating the specific amino acids in the peptide chains.
Explanation: The start codon plays two important roles in the synthesis of a protein. First, it specifies the first amino acid to be incorporated in the growing polypeptide chain. Second, it defines the reading frame for all the subsequent codons.
8. How many reading frames are applicable in case of translation?
Explanation: As the codons are immediately adjacent to each other and because the codons are three nucleotides long, any stretch of mRNA can be translated into three different reading frames. However once the translation starts the reading frame is fixed as the codons are subsequent to each other.
9. Both prokaryotes and Eukaryotes have one ORF.
Explanation: Eukaryotes have only one ORF and are thus known as monocistronic mRNAs. Prokaryotes may have more than one ORF and are thus known as polycistronic mRNAs.
10. Shine – Dalgarno sequence is also known as the __________
c) Stop codon
d) Start codon
Explanation: In prokaryotic mRNA a short sequence is found on the 5’ side where the ribosome binds to the mRNA. This sequence is known as the ribosome binding site (RBS) or the Shine – Dalgarno sequence after the scientist who discovered it.
11. Which component of the rRNA binds to the mRNA?
Explanation: The ribosome binding site is typically located 3 – 9 bp on the 5’ side of the start codon. This sequence is complementary to the sequence located near the 3’ end of the 16S rRNA. The core of the mRNA binding site of the 16S rRNA has a sequence of 5’ – CCUCCU – 3’ and not surprisingly prokaryotic RBS is often the subset of 5’ – AGGAGG – 3’.
12. The phenomenon of translating two a protein by two ribosomes is known as translational coupling.
Explanation: Some prokaryotic ORFs internal to a polycistronic message lack a strong RBS but are nonetheless translated. In such cases the start codon often overlaps with the 3’ end of the adjacent ORF. Thus, a ribosome that has just completed translating the upstream ORF is appropriately positioned to begin translating from the start codon for the downstream ORF. This process of translating a protein is known as translational coupling.
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