This set of Molecular Biology Multiple Choice Questions & Answers (MCQs) focuses on “Introns are Removed from RNA by Splicing – 1”.
1. The splice site is found in __________
a) 3’ end of exon
b) 5’ end of intron
c) Within the exon
d) Within the intron
View Answer
Explanation: The splice site is found in the borders between intron and exon. They are marked by specific nucleotide sequences and are found on the borders of the intron that is the 3’ and 5’ ends. They are known as 3’ splice site and 5’ splice site respectively.
2. During splicing which of the following sites is not recognized by the splicosome?
a) Donor
b) Branch point
c) Acceptor
d) G-rich site
View Answer
Explanation: The splicosome splice site recognizes three specific sites. They are 5’ splice site or the donor, the branch point site and the 3’ splice site known as the acceptor.
3. The 3’ end of an intron is marked by __________
a) Donor site
b) Acceptor site
c) AT rich site
d) Branch point site
View Answer
Explanation: The 3’ end of an intron is marked by the acceptor site also known as the 3’ splice site. It is a track of pyrimidines that is recognized by the splicosome complex.
4. The highly conserved base at the branch point site is __________
a) A
b) U
c) G
d) C
View Answer
Explanation: The highly conserved base at the branch point site is A whereas the 3’ splice site is AG rich and the 5’ splice site is GU rich. These highly conserved sequences are found within the intron itself as the bases in the exons are constrained by the need of encoding specific amino acid and protein products.
5. How many consensus sequences for splicing are found in an exon?
a) 1
b) 2
c) 3
d) 0
View Answer
Explanation: None of the consensus sequences for splicing are found in an exon. The highly conserved base at the branch point site is A whereas the 3’ splice site is AG rich and the 5’ splice site is GU rich. These highly conserved sequences are found within the intron itself.
6. To achieve splicing of an intron how many phosphodiester bonds are formed __________
a) 1
b) 2
c) 3
d) 0
View Answer
Explanation: The splicing is achieved by the two successive transesterification reactions. For the formation of these two new phosphodiester bonds, two existing phosphodiester bonds are broken thus no net change in the number of bond is observed.
7. The first transesterification reaction between the branch point site and the 5’ splice site is a __________ reaction.
a) SN1
b) SN2
c) E1
d) E2
View Answer
Explanation: The first transesterification reaction is triggered by the 2’-OH of the conserved A at the branch site. This group acts as a nucleophile to attack the phosphoryl group of the conserved G in the 5’ splice site. This is an SN2 reaction that proceeds through pentavalent phosphorus intermediate.
8. The second transesterification reaction occurs between __________
a) The 3’ splicing site of intron and 5’ splice site of intron
b) The 3’ splicing site of intron and the branch point site
c) The 5’ splicing site of intron and 3’ end of exon
d) The 5’ end of exon and the 3’ splice site
View Answer
Explanation: The second transesterification reaction occurs between the 5’ end of exon and the 3’ splice site. In this case the newly liberated 3’-OH of the 5’ exon acts as a nucleophile. It attacks the phosphoryl group at the 3’ splice site thus joining the two exons by a phosphodiester bond.
9. The transesterification reaction is ATP independent.
a) True
b) False
View Answer
Explanation: In the two reaction steps, there is no net gain in the number of chemical bonds – two phosphodiester bonds are broken, and two new ones made. As it is just a question of shuffling of bonds, no energy input is demanded by the chemistry of this process.
10. In trans-splicing the excised intron forms a lariat structure.
a) True
b) False
View Answer
Explanation: The lariat structure of intron is formed in case of the general splicing mechanism. In case of trans-splicing the excised intron forms a Y – shaped branch structure.
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