This set of Molecular Biology Multiple Choice Questions & Answers (MCQs) focuses on “Various Classes of RNA Have Different Functions – 1”.
1. Which of the following is a character of ORF?
b) 3 – nucleotide codons
d) Non – overlapping
Explanation: Exons are the part of nascent RNA that are connected together to form RNA after the removal of introns by the splicosome. The splicosome in the complex formed in the nucleus with nascent RNA being the substrate for the formation of mature RNA after its splicing and other post transcriptional mechanism.
2. With respect to the open reading frame (ORF) which of the following is wrong?
a) Starts at 5’ end
b) ‘AUG’ is the only start codon for both prokaryotes and eukaryotes
c) The start codon defines the reading frame for all the subsequent codons
d) The stop codon is distinct from the ends of the mRNA
Explanation: ‘AUG’ is the only start codon for eukaryotes. For prokaryotic system ‘AUG’, ‘GUG’ and sometimes even ‘UUG’ act as the start codon.
3. With respect to polycistronic mRNAs which of the following is wrong?
a) Multiple ORFs
b) Found in eukaryotes
c) Encodes proteins with related functions
d) Multiple polypeptide chain
Explanation: Eukaryotes only contain 1 ORF per mRNA and are thus monocistronic. Polycistronic mRNAs are generally found in prokaryotes with 2 or more ORFs.
4. What was the name of ribosome binding site?
c) A site
d) Shine – Dalgarno sequence
Explanation: Upstream the ORF a 3 – 9 base pair sequence on the 5’ side of the sequence is identified as the ribosome binding site (RBS). This element is referred to as the Shine – Dalgarno sequence, named after the scientists who discovered it by comparing the sequences of multiple mRNAs.
5. Which part of the ribosome identifies the Shine – Dalgarno sequence?
b) 16S rRNA
c) 23S rRNA
d) 5S rRNA
Explanation: The Shine – Dalgarno sequence is identified by the 16S rRNA. The core of the 16S rRNA has the sequence of 5’…..CCUCCU…..3’ and is located near the 3’ end of the rRNA. Not surprisingly the prokaryotic RBS are most often the subset of sequence 5’…..AGGAGG…..3’. Thus, 16S rRNA is the one that aligns the ribosome with the mRNA.
6. Eukaryotic mRNAs recruit ribosomes using the Shine – Dalgarno sequence.
Explanation: Eukaryotic mRNAs recruit ribosomes using specific chemical modifications called 5’ cap. The 5’ end of the mRNA is capped with methylated Guanine nucleotide to the mRNA via an unusual 5’ to 5’ linkage. To this methylated Guanine three phosphates are added. This cap binds to the ribosome which then slides along the mRNA length to find the ‘AUG’ for start of translation.
7. Eukaryotic mRNA is read in the 3’ to 5’ direction.
Explanation: The eukaryotic mRNA is read in the 5’ to 3’ direction. This is because capping of mRNA occurs at the 5’ end, thus the ribosome attaches to the 5’ end and starts translating in the 5’ to 3’ direction.
8. Which one of the following is the following is known as the Kozak sequence?
Explanation: The Kozak sequence is named after its identification by Marilyn Kozak. This sequence contains purines (A/G) three bases upstream of ‘AUG’ and a ‘G’ immediately following it. Thus the sequence stands up to 5’…..A/GNNAUGG…..3’. this sequence is known to increase translation efficiency.
9. With respect to the composition of ribosome which of the following is correct.
a) Ribosome is composed of 60S and 30S subunit
b) Eukaryotic ribosome small subunit contains only one 16S rRNA
c) 60S subunit consists of 5S rRNA and 23S rRNA
d) 60S and 40S makes up the 80S ribosome
Explanation: There is a discrepancy in the sedimentation velocity of the subunits separately and as a whole. This is because of the fact that the sedimentation velocity is determined both by shape and size and hence, it is not an exact measure of mass. Prokaryotic ribosome consists of 50S and 30S subunit and the 30S subunit contains only one 16S rRNA. The 50S subunit consists of 5S and 23S rRNA.
10. Which element of the ribosome plays the key role in mRNA translation?
a) rRNA of the large subunit
b) Proteins of the large subunit
c) rRNA of the small subunit
d) Proteins of the small subunit
Explanation: The anticodon loops of the charged tRNA and the codons of the mRNA contacts the 16S rRNA. This 16S rRNA is present in the small subunit and plays the key role in mRNA translation even though all the rest needed without this 16S rRNA no translation would occur.
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