This set of Molecular Biology Multiple Choice Questions & Answers (MCQs) focuses on “Eukaryotic Messenger RNA Contains a Cap and a Tail”.
1. The transcription starts with a purine that ends with a __________
Explanation: Transcription starts with a nucleoside triphosphate (A/G) and a 5’ – triphosphate groups is retained at the first position. The transcript thus ends with the triphosphate of the nucleotide and is represented as 5’….pppApNpNp…….3’.
2. The enzyme used for capping mRNA is a __________
Explanation: To the 5’ end is added a terminal G with the help of guanyl transferase. It is represented as follows:
5’Gppp + 5’….pppApNpNp….3’ → GpppApNpNp….3’
3. The guanyl residue added during capping is added in the same direction as the other nucleotides
Explanation: The guanyl transferase adds a guanine cap to the 5’ end of the transcript. The guanosine triphosphate thus added is in the opposite direction that is the triphosphate pointing inward rather than outward. The guanine cap also undergoes methylation.
4. How many methylated nucleotides are observed during capping?
Explanation: The eukaryotic mRNAs are capped with a Guanosine triphosphate residue which is methylated and is denoted as Cap 0. But most of the mRNAs undergo a second methylation at the penultimate base and is termed as Cap 1.
5. During capping the methylated penultimate base is known as the __________
a) Cap 1
b) Cap 2
c) Cap G
d) Cap 0
Explanation: The cap with a single methyl group is added at the 5’ terminal which is found in eukaryotic cells and is describes as Cap 0. But most eukaryotes methyl group may also be present on the penultimate base at 2’ position of the sugar moiety, so that with methyl groups at two terminal nucleotides, it is now described as Cap 1.
6. The specific sequence present in the DNA for the triggering of polyadenylation is __________
Explanation: The polyadenylation reaction requires a specific sequence in the DNA strand and thus in the pre – mRNA transcript as well. This sequence is known as the polyadenylation signal in the mRNA transcript and is a consensus sequence of 5’….AAUAAA….3’. Thus the DNA complement of this sequence will be 3’….TTATTT….5’.
7. The poly A adding site is 5’….GA….3’.
Explanation: The polyadenylation site is 5’….NA….3’which is followed by a GU rich region and preceded by the polyadenylation signal. The “N” in the polyadenylation site is always a pyrimidine, C or T.
8. The requirements of polyadenylation site does not include __________
a) 5’….AATAAA….3’ sequence
b) GU rich region downstream
c) Cleavage site of “C/TA”
d) Polyadenylation after GU rich region at cleavage site
Explanation: The requirements of polyadenylation site include a 5’….AATAAA….3’ polyadenylation signal sequence, a cleavage site of “C/TA” and a GU rich region downstream. The polyadenylation occurs at the cleavage site after the adenine residue and the polyadenylation site is in between the polyadenylation signal sequence and the GU rich region.
Sanfoundry Global Education & Learning Series – Molecular Biology.
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