This set of Molecular Biology Multiple Choice Questions & Answers (MCQs) focuses on “Ribosomes Read the Genetic Code”.
1. With respect to polymerization of amino acids which of the following statements is wrong?
a) The polymerization machinery consists of fifty proteins and two RNA molecules
b) Overall molecular mass 2.5 megadalton
c) Occurs at a slow pace of only 2 – 20 amino acids per second
d) In eukaryotes the process occurs in the cytoplasm
Explanation: Though the polymerizing machinery consists of 50 proteins, it atleast should consist of 3 RNA molecules. The molecules that take part in translation are mRNA, tRNA and rRNA. Messenger or mRNA is the transcript, transfer or tRNA is the carrier of amino acid and the ribosome contains rRNA or ribosomal RNA which decodes the transcript codon.
2. Ribosome begins its translation at the 3’ end of the mRNA in prokaryotes.
Explanation: In prokaryotes the transcription and translation occurs simultaneously. Thus, the end of mRNA which emerges from transcription is translated first. As the RNA polymerase reads the frame in 5’ to 3’ direction thus the 5’ end of mRNA is synthesized first thus, it is translated first.
3. Check which one of the following mechanisms is wrongly paired with its location of occurrence.
a) Transcription – nucleus
b) Post transcriptional mechanism – ER
c) Translation – cytoplasm
d) Post translational mechanism – ER
Explanation: Post transcriptional mechanisms are splicing, capping and tailing. These three mechanisms occurs in the nucleus as if the nascent mRNA (hnRNA) leaves the nucleus without the modification ribosome would not be able to read it and also it would not be able to read it and also it would be hydrolysed by the enzyme RNase present in the cell cytoplasm.
4. With respect to the subunits of ribosome which of the following is wrongly paired?
a) Ribosome = rRNA + protein
b) Large subunit = decoding center
c) Small subunit = decoding center
d) Subunit sedimentation unit = Svedberg
Explanation: The large subunit contains the three sites A, P and E. These are the centers where the amino acyl tRNA brings the amino acid peptide linkage is formed and the tRNA exits respectively. Thus the subunit is known as peptidyl transferase center.
5. An mRNA bearing multiple ribosomes is known as______________
a) Small subunit-mRNA-initiator tRNA complex
b) mRNA ribosome complex
c) Polyamine-ribosome complex
Explanation: A single ribosome contacts around so nucleotides on mRNA whereas the due to the large density of ribosome it allows one ribosome per 80 nucleotides. Thus a typical mRNA could be very long (example: 1000 nucleotides). Therefore more than one ribosome can attach to the mRNA forming a structure known as polyribosome or polysome.
6. What is the actual substrate for each round of amino acid addition?
b) Amino acid
Explanation: The actual substrate for the addition of amino acid to the growing polypeptide chain in each round are two charged species of t RNAs. They are an aminoacyl tRNA which is the carrier of amino acid to the ribosome-mRNA complex and the peptidyl tRNA which forms the peptide linkages between the amino acids.
7. The reaction to form a new peptide bond is known as the peptidyl transferase reaction
Explanation: The mechanism for the formation of new peptide bond first requires the synthesis of amino terminus before carboxyl terminus. The second step of the mechanism is the transfer of growing polypeptide chain from the peptidyl tRNA to the amino acyl tRNA. Thus reaction is called peptidyl transferase reaction.
8. Where does ribosome gets the energy for the formation of peptide bond?
a) Breaking of the keto bond
b) Breaking of acyl bond
c) Simultaneous hydrolysis of ATP
d) Energy released during peptidyl transferase reaction
Explanation: The energy provided for the formation of peptide bond is provided by the breaking of high energy acyl bonds. This is because the charging reaction of the tRNA involves the hydrolysis of a molecule of ATP. Thus, the energy of peptide bond formation originates from the molecule of ATP that was hydrolysed during tRNA charging reaction but not by simultaneous hydrolysis.
9. With respect to the mRNA – ribosome complex for translation mechanism which of the following pair is wrong?
a) Small subunit – decoding center
b) A site – aminoacylated tRNA binding site
c) P site – peptidyl tRNA binding site
d) E site – polypeptide exit site
Explanation: E site is the binding site for the tRNA that is released after the growing polypeptide chain has been transferred to the aminoacyl tRNA. Thus E site is the peptidyl tRNA exit site.
10. Only a single stranded RNA can pass through the ribosome for decoding.
Explanation: The mRNA enters the decoding center through two narrow channels in the small subunit. The entry channel is only wide enough for unpaired RNA to pass through. This features ensures that mRNA is in a single stranded from as it enters the decoding center by removing any intermolecular base pairing interactions that might have been formed in the mRNA.
11. How many channels are present in the ribosome?
Explanation: There are three channels present in the smaller subunit of ribosome which facilitates the entering and exit of the mRNA. The third channel is for the exit of the growing polypeptide chain.
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