This set of Molecular Biology Questions and Answers for Entrance exams focuses on “Introns are Removed from RNA by Splicing – 3”.
1. The binding of U2 snRNP is aided by BBP and thus promotes 5’ site splicing.
Explanation: U2 snRNP binds to the branch point site by the help of U2AF by displacing BBP. The base pairing between the U2 snRNA and the branch point site is such that the branch site A is extruded from the resulting stretch of the double helical RNA as a single nucleotide bulge. This A residue is thus unpaired and available to react with the 5’ splice site.
2. Which three snRNPs together make the tri-snRNP particle?
a) U1, U2, U4
b) U2, U4, U5
c) U1, U5, U6
d) U4, U5, U6
Explanation: After the formation of the A complex, it rearranges itself in such a fashion that all the 3 splice sites come close together. This is achieved when U4, U5 and U6 snRNPs join the A complex. Together these three snRNPs are called the tri-snRNP particle.
3. Which of the following pairs is held together by the complementary base pairing in the B complex of splicosome?
a) U1, U2
b) U2, U4
c) U4, U6
d) U2, U5
Explanation: U4 and U6 snRNPs are held together by the complementary base pairing between their RNA components in the tri-snRNP particle. The U5 snRNP is more loosely associated through protein:protein interactions. The entry of this tri-snRNP into the A complex converts it into the B complex.
4. Which of the following complexes produces the active site of the splicosome?
a) A complex
b) A complex rearrangement
c) B complex
d) C complex
Explanation: U4 is released from the complex, allowing U6 to interact with U2. This arrangement, called the C complex, produces the active site.
5. Formation of the active site only in the C complex is a necessity for avoiding wrong splicing.
Explanation: Formation of the active site only in the C complex is a necessity for avoiding the chance of any aberrant splicing. Linking the formation of the active site to the successful completion of earlier steps in the splicosome assembly makes it highly likely that the active site is available only at legitimate splice sites.
6. Rearrange the order of the steps in the splicing mechanism.
i) Transesterification of 3’ splice site
ii) Formation of lariat complex
iii) Formation of active site
iv) Release of snRNPs
a) iii, i, ii, iv
b) iii, ii, i, iv
c) ii, iii, i, iv
d) ii, iv, iii, i
Explanation: Formation of the active site juxtaposes the 5’ splice site and the branch point site facilitating the first transesterification reaction. The second reaction, between the 5’ and 3’ splice sites, is aided by U5 snRNP, which helps bring together the two exons. This thus facilitates the complete formation of the lariat complex. The final step involves the release of the lariat complex and the snRNPs from the complex.
7. Group II introns are spliced by the splicosome complex.
Explanation: Group II introns are spliced by the RNA enzyme encoded by the intron. Nuclear pre-mRNA is the only mRNA that utilizes the splicosome complex for its splicing mechanism.
8. The branch point site conserved sequence for group II intron is ___________
Explanation: The branch point site conserved sequence for group II intron is G. they are very rare and are found in the nuclear rRNA of some eukaryotes along with some organelle gene and a few prokaryotic gene.
9. Which of the following groups can perform self splicing?
a) mRNA, rRNA
b) rRNA, mtRNA
c) tRNA, mRNA
d) rRNA, tRNA
Explanation: Self splicing is observed in two groups of very rare intron that are, group I and group II introns. The attain splicing by ribozyme encoded by the intron and are found in some eukaryotic genes along with some organelle gene and a few prokaryotic gene.
10. Self-splicing is achieved by the formation of specific structures within the introns.
Explanation: A typical self-splicing intron is between 400 to 1000 nucleotides long, and much of the sequence is critical for the splicing reaction. This sequence requirement holds because the intron must fold into a precise structure to perform the reaction chemistry.
Sanfoundry Global Education & Learning Series – Molecular Biology.
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