RNA Processing
- In eukaryotes, transcription and translation take place in different cellular compartments.
- Transcription takes place in the nucleus whereas translation takes place in the cytoplasm.
- In prokaryotes, transcription of m-RNA and translation occurs simultaneously.
- Thus, mRNA molecules undergo little to no modification after synthesis by RNA polymerase in prokaryotes.
- In contrast, tRNA and rRNA in prokaryotes undergo processing like cleavage, addition of nucleotides and chemical modification after synthesis.
- Although both prokaryotes and eukaryotes modify tRNA and rRNA,
- In eukaryotes very extensively process pre-mRNA destined to become mRNA.
- The primary transcript of an RNA polymerase is referred to as pre mRNA.
- Processing of eukaryotic pre-mRNA involves the following steps:
(1) 5′ capping
(2) 3′ cleavage and polyadenylation
(3) Splicing and RNA editing.
- The conserved eukaryotic polyadenylation signal directs cleavage at the cleavage signal and addition of a poly (A) tail to the mRNA transcript.
- Post transcriptional modification or Co-transcriptional modification is a process by which in eukaryotic cells, primary transcript RNA is converted into mature RNA.
- This process is vital for the correct translation of the genomes of eukaryotes because the primary transcript contains both exons (coding sequences) and introns which are non-coding sequences
(A) 5′ Capping
- Capping of the pre-mRNA involves the addition of 7-methylguanosine (m/G) to the 5′ end.
- To achieve this, the terminal 5′ phosphate requires removal which is done with the aid of a phosphatase enzyme
- The enzyme guanosyl-transferase then catalyses the reaction which produces the diphosphate 5′ end.
- The diphosphate 5′ end then attacks the y-phosphorus atom of a GTP molecule in order to add the guanine residue in a 5’5′ triphosphate link.
- The enzyme (guanine N7-) methyltransferase [CAP MT ase”] transfers a methyl group from S-adenosyl methionine to the guanine ring
- This type of cap. with just the m’G in position is called a cap 0 structure.
- Cap 0 is the predominant cap in unicellular organism
- In some species, a methyl group is added to the second as well as the third nucleotide of the capped mRNA.
- In these cases, the methyl groups are added to the 2′-OH groups of the ribose sugar.
- mRNA with methyl groups on the terminal 7 guanine and the 2′-OH position of the second nucleotide at the 5′ end is known as cap 1.
- This is the predominant cap in multicellular organisms.
- Similarly, if the methyl group is present at both the second and third nucleotides then it is known as Cap 2. the primary
- The cap protects the 5′ end transcript from attack by ribonucleases.
(B) 3′ Processing – Cleavage and polyadenylation
- All eukaryotic mRNAs have a series of up to 250 adenosines at their 3¹ ends called a poly-A-tail.
- These are added to the transcript by a template independent RNA polymerase called poly (A) polymerase [PAP].
- This polymerase does not act at the extreme 3′ end of the transcript but at an internal site which is cleaved to create a new 3′ end to which the poly (A) tail is added.
- The poly (A) tail appears to have several functions:
1. Export of mature mRNA from the nucleus
2. Affect the stability of some mRNAs
3. Serve as a recognition signal for the ribosome.
- A single processing complex undertakes both the cutting (cleavage) and polyadenylation.
- The signal is needed for both cleavage and polyadenylation.
- In mammals, polyadenylation is directed by a signal sequence in the mRNA it is almost invariably 5′-AAUAAA-3′.
- This sequence is located between 10 and 30 nucleotides upstream of the polyadenylation site, which is immediately after the dinucleotide 5′ CA-3′ (cleavage signal) and is followed 10-20 nucleotides later by a GU-rich region.
- Both the poly (A) signal and the GU-rich region are binding sites for multi subunit protein complexes, which are the cleavage and polyadenylation specificity factor (CPSF) and the cleavage stimulation factor (CSTF) respectively.
- Generation of the proper 3′ terminal structure requires an endonuclease (consisting of the components CFI, cleavage factor 1 and CF II. cleavage factor II) to cleave the RNA.
- Cleavage is followed by polyadenylation in a tightly coupled manner.
- The poly (A) polymerase has a non-specific catalytic activity.
- The polyadenylation reaction passes through two stages.
- First a short oligo (A) sequence (-10 residues) is added to the 3′ end
- In the second phase, the oligo (A) tail is extended to the full 200 residue length.
- This reaction requires another stimulatory factor that recognizes the oligo (A) tail.
- This additional factor includes polyadenylate binding protein (PADPII), which helps the polymerase to add the adenosines, possibly influences the length of the poly (A) tail that is synthesized and appears to play a role in maintenance of the tail after synthesis.
(C) Splicing
Intron Splicing:
- Non coding DNA are found within most eukaryotic genes Such genes have a split structure in which segments of coding sequence (called exons) are separated by non-coding sequences (intervening sequences or introns).
- The entire gene is transcribed to yield a large RNA molecule and the introns are then removed by splicing, so only exons are included in the mRNA.
- The process of excising the sequences in RNA that correspond to introns and joining of sequences corresponding to exons is called RNA splicing.
- Mechanism of RNA splicing varies depending on the types of introns.
- There are many types of pre m-RNA introns. Two types are commonly found in eukaryotic protein coding genes. These are
(i) The GU – AG and
(ii) AU-AC introns -.AU-AC intron is a rare class of introns.
Splicing of GU-AG intron
Self Splicing
- In GU-AG intron, the first two nucleotides of the intron sequence are 5′-GU-3′ (5′ splice site or donor site) and the last two 5′-AG-3′ (3′ splice site or acceptor site).
- A pyrimidine rich region, called polypyrimidine tract, near the 3′ end of the intron is found.
- In most cases, the branch point adenosine, also invariant, usually is 20-50 bases from the 3′ splice site.
- Splicing of GU-AG intron involves two transesterification reactions.
- Cleavage of the 5′ splice site occurs by a first transesterification reaction promoted by the hydroxyl group attached to the 2′ carbon of an adenosine nucleotide located within the intron sequence.
- The result of the hydroxyl attack is cleavage of the phosphodiester bond at the 5′ splice site accompanied by the formation of a new 5-2 phosphodiester bond linking the first nucleotide of the intron (the G of the 5′-GU-3′ motif) with the internal adenosine.
- This means that the intron has now been looped back on itself to create a lariat structure.
- Cleavage of the 3′ splice site and joining of the exons result from a second transesterification reaction, this one promoted by the 3′-OH group attached to the end of upstream exon.
- This group attacks the phosphodiester bond at the 3′ splice site, cleaving it and so releasing the intron as the lariat structure, which is subsequently converted back to a linear RNA and degraded.
- At the same time, the 3′ end of the upstream exon joins to the newly formed 5′ end of the downstream exon, completing the splicing process.
- A large number of ATP is consumed during the splicing reaction.
- This energy is required for assembly of the splicing apparatus.
Splicing By Spiciosome
- The central components of the splicing apparatus for GU-AG introns are the snRNAs called UI, U2, U4, U5 and U6.
- These short RNA molecules associate with proteins to form small nuclear ribonucleoproteins (snRNPs).
- The SN RNPs. together with other accessory proteins attach to the transcript and form a series of complexes,
- last one of which is the spliceosome, the structure within which the actual splicing reactions occur.
- The process of assembly of snRNP and various protein factors occurs in the following sequence.
- (i) The E complex called commitment complex initiates a splicing activity.
- This complex comprises U1-snRNP, which binds to the 5′-splice site, the U2,AF factor which binds with the polypyrimidine tract and 3′ site.
- Binding of U1 snRNP to the 5′ splice site is the first step in splicing
- (ii) The A complex called the pre-spliceosome comprises the commitment complex plus U2-snRNP attached to the branch point.
- At this stage, an association between UI-snRNP and U2-snRNP brings the 5′ splice site into close proximity with the branch point.
- (iii) The B Complex -spliceosome is formed when U4/U6-snRNP (a single snRNP containing two snRNAs) and U5-snRNP attach to the pre-spliceosome complex.
- The B1 complex is formed when U5 and U4/U6 snRNP’s binds to the A complex.
- This complex is called spliceosome and it contains all the components needed for splicing reaction.
- It is converted to B2 complex after the UI is released.
- This catalytic reaction is triggered by the release of U4 and it requires hydrolysis of ATP
- C complex -When U4 dissociates from U6 snRNP, U6 snRNP can pair with U2 snRNP to form the catalytic active site (C complex).
- This results in additional interactions that bring the 3′ splice site close to the 5′ site and the branch point All three key positions in the intron are now in proximity and the two transesterification reactions occur as a linked reaction, catalysed by U6-snRNP, completing the splicing process.
1. In eukaryotes, where does mRNA splicing occur?
A. Cytoplasm
B. Ribosome
C. Nucleus
D. Endoplasmic reticulum
✅ Answer: C. Nucleus
Explanation: In eukaryotes, transcription and RNA processing (including splicing) occur in the nucleus, whereas translation occurs in the cytoplasm.
2. The 5’ cap of eukaryotic mRNA is made of:
A. 7-methyladenosine
B. 5-methylcytosine
C. 7-methylguanosine
D. 3-methyluracil
✅ Answer: C. 7-methylguanosine
Explanation: The 5′ cap added to pre-mRNA is 7-methylguanosine, linked via a 5′ to 5′ triphosphate bond.
3. Which of the following is the correct polyadenylation signal sequence in mammals?
A. AUGAAA
B. AAUAAA
C. AATAAA
D. GUAGUA
✅ Answer: B. AAUAAA
Explanation: The canonical polyadenylation signal in most eukaryotic mRNAs is AAUAAA, located upstream of the cleavage site.
4. The enzyme responsible for adding poly-A tail to mRNA is:
A. RNA polymerase II
B. Guanyl transferase
C. Poly(A) polymerase
D. DNA ligase
✅ Answer: C. Poly(A) polymerase
Explanation: Poly(A) polymerase (PAP) adds adenosine residues to the 3′ end of pre-mRNA after cleavage.
5. Which of the following structures is formed during splicing of pre-mRNA?
A. Hairpin loop
B. Holliday junction
C. Lariat structure
D. Okazaki fragment
✅ Answer: C. Lariat structure
Explanation: During the first transesterification reaction in splicing, the intron forms a lariat structure via a 5′-2′ phosphodiester bond.
6. The first snRNP to bind the pre-mRNA during spliceosome formation is:
A. U2
B. U4
C. U5
D. U1
✅ Answer: D. U1
Explanation: U1 snRNP binds to the 5’ splice site, initiating spliceosome assembly.
7. In spliceosome-mediated splicing, which snRNA pairs with the branch point sequence?
A. U1
B. U2
C. U5
D. U6
✅ Answer: B. U2
Explanation: U2 snRNP base-pairs with the branch point sequence, placing the branch point adenosine in the correct position for the first transesterification.
8. What is the function of the 5’ cap in mRNA?
A. Enhances intron removal
B. Assists in transcription
C. Protects mRNA from degradation and assists in ribosome binding
D. Initiates polyadenylation
✅ Answer: C. Protects mRNA from degradation and assists in ribosome binding
Explanation: The 5’ cap is crucial for mRNA stability and recognition by the ribosome during translation initiation.
9. Which of the following RNA types is least likely to undergo extensive post-transcriptional modification in prokaryotes?
A. tRNA
B. rRNA
C. mRNA
D. All of the above
✅ Answer: C. mRNA
Explanation: Prokaryotic mRNA is usually translated directly after transcription and undergoes minimal processing.
10. The Cap 1 structure of eukaryotic mRNA includes methylation at:
A. Only the 7′ position of guanosine
B. Only the 2′-OH of the first nucleotide
C. Both the 7′ position of guanosine and the 2′-OH of the first nucleotide
D. Only the 3′ end
✅ Answer: C. Both the 7′ position of guanosine and the 2′-OH of the first nucleotide
Explanation: Cap 1 structure includes methylation of the guanine at the 7′ position and the 2′-OH of the first ribonucleotide.
11. The process of adding a 5’ cap to the pre-mRNA is catalyzed by which enzyme?
A. RNA polymerase
B. Guanylyl transferase
C. Poly(A) polymerase
D. Spliceosome
✅ Answer: B. Guanylyl transferase
Explanation: Guanylyl transferase catalyzes the addition of the 7-methylguanosine cap at the 5′ end of the pre-mRNA.
12. Which of the following is involved in the cleavage of pre-mRNA during polyadenylation?
A. U2 snRNP
B. Cleavage stimulation factor (CSTF)
C. RNA polymerase II
D. Guanosyl transferase
✅ Answer: B. Cleavage stimulation factor (CSTF)
Explanation: CSTF binds to the GU-rich region and assists in the cleavage of the pre-mRNA.
13. Which of the following is a common feature of both eukaryotic tRNA and rRNA processing?
A. Addition of 5’ cap
B. Polyadenylation
C. Cleavage and modification
D. Splicing
✅ Answer: C. Cleavage and modification
Explanation: Both tRNA and rRNA undergo cleavage and chemical modifications after transcription in prokaryotes and eukaryotes.
14. Which of the following statements is true regarding mRNA splicing?
A. Splicing occurs exclusively in the cytoplasm
B. Exons are spliced out and introns remain
C. Intron sequences are removed during splicing
D. Splicing is independent of snRNPs
✅ Answer: C. Intron sequences are removed during splicing
Explanation: Splicing involves removing the introns and joining the exons together.
15. The cleavage and polyadenylation signal is located at which site on the pre-mRNA?
A. 5’ end
B. Near the poly(A) tail
C. Between the 3’ splice site and the poly(A) tail
D. 3’ end
✅ Answer: C. Between the 3’ splice site and the poly(A) tail
Explanation: The cleavage and polyadenylation signal, typically AAUAAA, is found near the 3’ end of the mRNA transcript.
16. Which of the following components is NOT part of the spliceosome?
A. U1
B. U2
C. U3
D. U6
✅ Answer: C. U3
Explanation: U3 is not part of the spliceosome, while U1, U2, U4, U5, and U6 are crucial for splicing.
17. Which of the following is the function of snRNAs in the spliceosome?
A. Catalyze the addition of a 5′ cap
B. Catalyze the splicing reaction
C. Stabilize mRNA
D. Facilitate translation
✅ Answer: B. Catalyze the splicing reaction
Explanation: snRNAs, such as U1, U2, U4, U5, and U6, are involved in the catalytic process of splicing within the spliceosome.
18. The process of splicing involves the formation of a lariat structure. This occurs during the:
A. First transesterification reaction
B. Second transesterification reaction
C. Hydrolysis of ATP
D. Formation of the 5′ cap
✅ Answer: A. First transesterification reaction
Explanation: The first transesterification reaction during splicing creates a lariat structure by forming a 5′-2′ bond.
19. Which of the following best describes a ‘self-splicing’ intron?
A. An intron that is removed with the help of the spliceosome
B. An intron that can splice itself without the use of the spliceosome
C. An intron that is never removed
D. An intron that is only found in prokaryotes
✅ Answer: B. An intron that can splice itself without the use of the spliceosome
Explanation: Self-splicing introns, such as Group I and Group II introns, can catalyze their own removal without the spliceosome.
20. What is the significance of the polypyrimidine tract in splicing?
A. It signals the start of polyadenylation
B. It helps in the recognition of the 5’ splice site
C. It serves as a binding site for U2 snRNP
D. It marks the end of the 3’ exon
✅ Answer: C. It serves as a binding site for U2 snRNP
Explanation: The polypyrimidine tract is involved in the binding of U2 snRNP, which is essential for recognizing the branch point during splicing.
21. Which of the following is true about the RNA processing event of splicing?
A. It is catalyzed by RNA polymerase II
B. It involves the removal of exons
C. It removes non-coding introns from the pre-mRNA
D. It occurs exclusively in the cytoplasm
✅ Answer: C. It removes non-coding introns from the pre-mRNA
Explanation: Splicing removes introns, the non-coding regions of the gene, while exons (coding sequences) are retained.
22. Which of the following statements about the poly(A) tail is FALSE?
A. The poly(A) tail is added after mRNA cleavage
B. The poly(A) tail aids in mRNA stability
C. The poly(A) tail is encoded in the gene
D. The poly(A) tail is added by poly(A) polymerase
✅ Answer: C. The poly(A) tail is encoded in the gene
Explanation: The poly(A) tail is not encoded in the gene itself; instead, it is added post-transcriptionally.
23. Which of the following steps is involved in the addition of the 5′ cap to pre-mRNA?
A. Addition of methylguanosine to the 5′ end
B. Addition of a 7-methylguanine to the 3′ end
C. Addition of polyadenine at the 5′ end
D. Removal of introns from the 5′ end
✅ Answer: A. Addition of methylguanosine to the 5′ end
Explanation: The 5′ cap consists of a methylguanosine residue added to the 5′ end of the pre-mRNA.
24. Which enzyme is responsible for adding the poly(A) tail?
A. RNA polymerase II
B. Poly(A) polymerase
C. Guanylyl transferase
D. Spliceosome
✅ Answer: B. Poly(A) polymerase
Explanation: Poly(A) polymerase is responsible for adding the poly(A) tail to the 3’ end of the pre-mRNA after cleavage.
25. What is the function of the spliceosome?
A. To add a 5′ cap to pre-mRNA
B. To cleave the pre-mRNA at the poly(A) site
C. To remove introns and join exons
D. To bind to the ribosome for translation
✅ Answer: C. To remove introns and join exons
Explanation: The spliceosome is a complex that removes introns and joins exons to form mature mRNA.
26. Which of the following is required for both the cleavage and polyadenylation of mRNA?
A. Poly(A) polymerase
B. Cleavage stimulation factor (CSTF)
C. Spliceosome
D. RNA polymerase II
✅ Answer: B. Cleavage stimulation factor (CSTF)
Explanation: CSTF is required for both the cleavage and polyadenylation of pre-mRNA.
27. The branch point of an intron is a conserved sequence of:
A. Adenosine
B. Guanine
C. Cytosine
D. Uracil
✅ Answer: A. Adenosine
Explanation: The branch point of an intron is a conserved adenosine residue that plays a key role in the first transesterification reaction during splicing.
28. The spliceosome consists of small nuclear RNA (snRNA) and:
A. rRNA
B. snRNPs (small nuclear ribonucleoproteins)
C. mRNA
D. tRNA
✅ Answer: B. snRNPs (small nuclear ribonucleoproteins)
Explanation: The spliceosome is composed of snRNAs and snRNPs, which facilitate the splicing reaction.
29. What is the primary function of the 3′ poly-A tail?
A. It promotes mRNA degradation
B. It assists in mRNA export from the nucleus
C. It binds to ribosomal RNA
D. It inhibits mRNA translation
✅ Answer: B. It assists in mRNA export from the nucleus
Explanation: The poly-A tail plays a role in mRNA stability and facilitates the export of mRNA from the nucleus to the cytoplasm.
30. The spliceosome performs which of the following actions on pre-mRNA?
A. It modifies the 5’ cap
B. It removes introns and joins exons
C. It adds the poly-A tail
D. It synthesizes ribosomal RNA
✅ Answer: B. It removes introns and joins exons
Explanation: The spliceosome removes non-coding introns and joins coding exons together.
31. Which of the following steps is involved in the process of RNA editing?
A. Addition of a 5′ cap
B. Removal of introns
C. Modification of nucleotide sequences in RNA
D. Addition of the poly-A tail
✅ Answer: C. Modification of nucleotide sequences in RNA
Explanation: RNA editing involves the alteration of the nucleotide sequence of RNA, typically through deamination or insertion/deletion of nucleotides.
32. Which of the following is TRUE regarding the addition of the 5′ cap in eukaryotic mRNA processing?
A. It is added after the first few nucleotides of RNA synthesis
B. It is added to the 3′ end of the mRNA
C. It stabilizes the mRNA for degradation
D. It is involved in the splicing of the mRNA
✅ Answer: A. It is added after the first few nucleotides of RNA synthesis
Explanation: The 5′ cap is added early in transcription, within the first 20-30 nucleotides of RNA synthesis.
33. The 3’ poly-A tail is added to the pre-mRNA by which enzyme?
A. Poly(A) polymerase
B. RNA polymerase
C. Spliceosome
D. Guanylyl transferase
✅ Answer: A. Poly(A) polymerase
Explanation: Poly(A) polymerase adds the poly-A tail to the 3’ end of pre-mRNA after transcription.
34. Which of the following is responsible for catalyzing the splicing of pre-mRNA?
A. Ribosome
B. Spliceosome
C. RNA polymerase II
D. tRNA
✅ Answer: B. Spliceosome
Explanation: The spliceosome is a complex of RNA and protein that catalyzes the removal of introns from pre-mRNA.
35. Which of the following statements is true about the spliceosome?
A. It recognizes intron-exon boundaries by binding to specific splice sites
B. It adds the 5′ cap to mRNA
C. It synthesizes rRNA
D. It is involved in translation of mRNA
✅ Answer: A. It recognizes intron-exon boundaries by binding to specific splice sites
Explanation: The spliceosome identifies and binds to specific sequences at the intron-exon boundaries, guiding the removal of introns.
36. Which of the following modifications occur to tRNA during its processing?
A. 5’ capping
B. 3’ polyadenylation
C. Intron removal
D. 3’ CCA addition
✅ Answer: D. 3’ CCA addition
Explanation: A CCA sequence is added to the 3’ end of tRNA during its processing, necessary for its function in translation.
37. What is the function of the U1 snRNP in the spliceosome?
A. It recognizes the 5′ splice site
B. It adds the poly-A tail
C. It modifies the 5′ cap
D. It catalyzes the transesterification reaction
✅ Answer: A. It recognizes the 5′ splice site
Explanation: U1 snRNP recognizes the 5′ splice site and initiates the splicing process.
38. In which type of RNA processing is the addition of a 7-methylguanosine cap involved?
A. rRNA processing
B. tRNA processing
C. mRNA processing
D. siRNA processing
✅ Answer: C. mRNA processing
Explanation: The addition of a 7-methylguanosine cap is specific to mRNA processing in eukaryotes.
39. Which of the following steps in mRNA processing is NOT performed by RNA polymerase II?
A. Addition of the 5’ cap
B. Addition of the poly-A tail
C. Transcription of the gene
D. Splicing of exons
✅ Answer: D. Splicing of exons
Explanation: RNA polymerase II is involved in transcription, capping, and polyadenylation, but splicing is carried out by the spliceosome.
40. Which of the following is NOT a characteristic of the 5’ cap?
A. It protects mRNA from exonuclease degradation
B. It helps in the initiation of translation
C. It promotes splicing of introns
D. It is composed of a 7-methylguanosine residue
✅ Answer: C. It promotes splicing of introns
Explanation: The 5’ cap protects mRNA and helps in translation initiation, but it is not directly involved in splicing.
41. What does the branch point in intron splicing typically contain?
A. Cytosine
B. Guanine
C. Adenine
D. Uracil
✅ Answer: C. Adenine
Explanation: The branch point in splicing contains an adenosine residue, which forms a 5’–2′ phosphodiester bond during the first transesterification step of splicing.
42. Which of the following is a role of the 5′ cap in mRNA?
A. It stabilizes the mRNA for translation
B. It adds the poly-A tail to the 3′ end
C. It removes introns from the mRNA
D. It guides the mRNA to the nucleolus
✅ Answer: A. It stabilizes the mRNA for translation
Explanation: The 5′ cap stabilizes the mRNA and is involved in mRNA export and translation initiation.
43. In eukaryotes, where does mRNA splicing occur?
A. Cytoplasm
B. Nucleus
C. Endoplasmic reticulum
D. Mitochondria
✅ Answer: B. Nucleus
Explanation: mRNA splicing occurs in the nucleus before the mRNA is exported to the cytoplasm for translation.
44. What is the significance of the U6 snRNP during splicing?
A. It binds to the 5′ splice site
B. It catalyzes the second transesterification reaction
C. It adds the poly-A tail
D. It removes the 5′ cap
✅ Answer: B. It catalyzes the second transesterification reaction
Explanation: U6 snRNP plays a catalytic role in the second transesterification reaction of splicing.
45. Which sequence is typically found at the 3′ splice site in eukaryotic mRNA?
A. GU
B. AG
C. AU
D. CA
✅ Answer: B. AG
Explanation: The 3′ splice site is characterized by an AG dinucleotide, which is critical for the recognition of the spliceosome.
46. Which of the following is true regarding the process of alternative splicing?
A. It produces identical protein products from the same mRNA
B. It allows the generation of multiple protein variants from a single gene
C. It only occurs in prokaryotes
D. It is responsible for removing the 5′ cap
✅ Answer: B. It allows the generation of multiple protein variants from a single gene
Explanation: Alternative splicing allows different combinations of exons to be included in the final mRNA, leading to the production of multiple protein isoforms from one gene.
47. What happens to the introns during RNA splicing?
A. They are retained in the mRNA
B. They are transcribed into protein
C. They are removed from the mRNA
D. They are converted into rRNA
✅ Answer: C. They are removed from the mRNA
Explanation: Introns are non-coding regions that are removed from pre-mRNA during splicing.
48. What is the primary function of the exon junction complex (EJC)?
A. To remove poly-A tails
B. To mediate the splicing of introns
C. To mark the spliced mRNA for translation
D. To degrade faulty mRNA
✅ Answer: C. To mark the spliced mRNA for translation
Explanation: The exon junction complex is deposited on mRNA after splicing and is involved in the regulation of mRNA export and translation.
49. Which of the following is involved in the process of RNA capping?
A. U1 snRNP
B. RNA guanylyl transferase
C. Spliceosome
D. Poly(A) polymerase
✅ Answer: B. RNA guanylyl transferase
Explanation: RNA guanylyl transferase catalyzes the addition of the 7-methylguanosine cap to the 5′ end of the mRNA.
50. In the context of RNA processing, which of the following describes a “cis-acting element”?
A. A protein that regulates transcription
B. A regulatory sequence in the mRNA itself
C. A non-coding RNA molecule
D. An external factor that influences RNA processing
✅ Answer: B. A regulatory sequence in the mRNA itself
Explanation: A cis-acting element is a sequence in the mRNA that regulates RNA processing or stability.