This set of Molecular Biology Multiple Choice Questions & Answers (MCQs) focuses on “Processing of Ribosomal and Transfer RNA”.
1. How many pre – rRNA transcript in eukaryotes are used to produce all the different mature rRNAs?
Explanation: Eukaryotes have 4 species of ribosomal RNA molecules. Three of these rRNAs are derived by the cleavage of a single precursor whereas the last rRNA is synthesized as a single molecule.
2. In bacterial cell the 3 rRNAs 23S, 16S and 5S are derived from different transcripts.
Explanation: No in bacterial cell the 3 rRNAs 23S, 16S and 5S are not derived from different transcripts. As in eukaryotes, the bacterial rRNAs 23S, 16S and 5S are derived by the cleavage of one single pre – rRNA transcript.
3. Which one of the following rRNA undergoes least post – transcriptional processing?
Explanation: The rRNA undergoes least post – transcriptional processing is the 5S rRNA. This is because it is synthesized separately and thus do not undergo any kind of cleavage. The rest three are derived by the cleavage of one single pre – rRNA transcript.
4. The bacterial pre – rRNA undergo __________ cleavage.
Explanation: The bacterial pre – rRNA undergo 2 cleavages. As three rRNA molecules are flanked together the first cleavage separates the precursors of the three rRNAs. A further cleavage known as the secondary cleavage ensures the production of the mature rRNA molecules.
5. Which of the following types of processing is not used to prepare mature rRNA?
d) Conversion of uridine
Explanation: The preparation of mature rRNA does not include splicing in most of the cases. They only include methylation of specific bases, and some specific sugar moieties and conversion of uridines to pseudouridines.
6. Each tRNA is produced from different transcripts .
Explanation: Like rRNAs, tRNAs in both bacteria and eukaryotes are synthesized as a longer precursor molecule. Some of these pre – tRNAs contain several individual tRNA sequences. Also in bacteria some of the tRNAs are included in pre – rRNA transcripts.
7. The pre – tRNA involves cleavage by __________
a) tRNA itself
b) RNase H
c) RNase P
d) RNA polymerase
Explanation: The processing of the 5’ end of the pre – tRNAs involves cleavage by an enzyme called RNase P. RNase P consists of RNA and in molecules, both of which are required for maximal activity.
8. RNase P is a ribozyme.
Explanation: In 1983 Sidney Altman and his colleagues demonstrated that the isolated RNA component of the RNase P is itself capable of catalyzing pre – tRNA cleavage. These experiments established that RNase P is a ribozyme – an enzyme in which RNA rather than protein is responsible for catalytic activity.
9. The common sequence for all tRNAs at their 3’ end is __________
Explanation: All tRNAs have the common sequence CCA at their 3’ end. This sequence is the site of amino acid attachment, so it is required for tRNA function during protein synthesis.
10. The 3’ CCA terminus is exceptionally added by the RNA polymerase during transcription.
Explanation: The 3’ CCA terminus is encoded in the DNA of some tRNA genes, but in others it is not. Thus the 3’ CCA terminus is added during RNA processing by an enzyme which recognizes and adds CCA to the 3’ end of all tRNAs that lack this sequence.
11. Though unusual, pre – tRNA also undergo splicing. It is achieved by the help of __________
a) tRNA itself
c) RNA polymerase exonuclease activity
Explanation: Though unusual, pre – tRNA and pre – rRNAs also undergo splicing. In contrast to other splicing reactions, which involve the activity of catalytic RNAs, tRNA splicing is mediated by conventional protein enzymes. An endonuclease cleaves the pre – tRNA at the splice site to excise the intron, followed by joining of the exons to form a mature tRNA molecule.
Sanfoundry Global Education & Learning Series – Molecular Biology.
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