Nuclear mRNA Transport
- Most of the mRNAs in eukaryotes are synthesized as long precursor RNAs in the nucleus.
- Until these are processed to functional RNAs they are not transported out of the nucleus. Only processed ones are transported out.
- First the pre-mRNAs are associated with certain proteins called hnRNPs which are involved in hnRNA processing; once processing is complete, mRNAs associate with specific proteins, such as Cap binding complex proteins, poly-A binding PAB-II, exon-joint complexes (EJC) and many others, which cover the whole length of mRNA.
- Transportation has to take place through a nuclear pore complex (NPC) found in two-unit nuclear membrane.
- The pore complex consists of veritable structural proteins as well as transport proteins.
- Transport is an active process and specific, not all RNAs are transported out, only those specified and marked are transported.
- None of the cytoplasmic RNAs that are transported into cytoplasm return to the nucleus with certain exceptions such as few snRNAs.
- RNA synthesis and many such components have to be imported via Nuclear pore complex (NPC).
- Transportation of ribosomes, tRNA, snRNA, scRNAs and other NC RNAs and mRNA is highly regulated.
- The presence of SnRNPs at splice junction sites in unspliced mRNAs prevents them from transport.
- But mRNAs with specific EJC proteins found at exon-exon splice sites are transported. Mutation in one site does not allow transportation but mutations at both sites allow transportation.
- Only capped mRNAs are transported.
- All mRNA coated with mRNPs show helically coiled forms.
- In Chironema titans, in a larva at 11 th day, many genes are expressed in large scale, which appear as chromosomal puffs and Balbiani rings, but one of the transcripts, for a glue protein for the larva, is expressed in massive amounts.
- The protein produced is responsible for the larva to glue to the surface of the substrate at the time of pupal transformation.
- This mRNA is transported in large amounts.
- The transcripts are long and covered with RNPs and coiled.
- As they transport across the pore complex, they unwind.
- In every active cell huge precursor mRNAs are found.
- The mRNAs which contain polyadenylation site and polyA are added.
- They are then transported with their cap structure in fore-front and poly-A tail at 3’end.
- Transportation is selective and determined by certain nuclear proteins.
- Adenoviral transcripts are preferably transported against host mRNAs, because the 5’ ends of these transcripts are associated with E1B and E4 proteins.
- During transportation, mRNAs are bound to ribosomes at their 5’end.
- In the case of HIV, a full length transcript is not transported until and unless it is associated with a specific REV protein at its Rev Recognizing Elements (RRE) found at 3’ end.
- The primary transcript, if it is 9 KB, with first splicing; its size is reduced to 4kb.
- This is not transported.
- But the second splicing leads to 2kb mRNA; this 2kb mRNA is transported and translated.
- The product is REV protein, which moves back into the nucleus and binds to full length mRNA at (Rev Response Element) RRE element; then only the full length HIV mRNA is transported out.
- U2 SnRNA transcribed by RNAP-II is transported but U6Sn RNA transcribed by RNAP-III is not transported for it has no cap structure.
- Sn RNAs like U1, U2, U4 and U5 are capped.
- They move out of the nucleus, where they are further methylated at cap sites (Tri methylation), associate with certain proteins such as SM (aptamers) and then they return to the nucleus.
- Many of the mRNPs transported with mRNA into cytoplasm are displaced with cytoplasmic mRNPs and nuclear mRNPs return to nucleus, a good example is PAB-II.
- Presence of poly-A or its absence does not matter for mRNA transport from nucleus to cytoplasm
- Many different kinds of Thalassemia are mainly due to their failure to transport their mRNAs out of the nucleus, because of mutations at splicing positions or because of the presence of cryptic splicing sites elsewhere.
1. Which of the following proteins is involved in the processing of hnRNA in the nucleus?
A. Cap-binding complex
B. hnRNPs
C. PAB-II
D. Exon-junction complex (EJC)
✅ Answer: B. hnRNPs
Explanation: hnRNPs are involved in the processing of heterogeneous nuclear RNA (hnRNA) before it is processed into functional mRNA.
2. What is required for the transportation of mRNA out of the nucleus?
A. The mRNA must be processed and capped
B. The mRNA must be unspliced
C. The mRNA must be bound by ribosomes
D. The mRNA must contain a poly-A tail
✅ Answer: A. The mRNA must be processed and capped
Explanation: Only processed mRNAs, which are capped and polyadenylated, are transported out of the nucleus.
3. What role does the nuclear pore complex (NPC) play in mRNA transport?
A. It is the site of RNA splicing
B. It mediates the transport of RNA molecules into the nucleus
C. It facilitates the export of RNA from the nucleus to the cytoplasm
D. It synthesizes ribosomal RNA
✅ Answer: C. It facilitates the export of RNA from the nucleus to the cytoplasm
Explanation: The nuclear pore complex allows the selective transport of mRNAs and other RNAs from the nucleus to the cytoplasm.
4. Which of the following proteins is involved in the transport of capped mRNAs?
A. Spliceosome
B. Cap-binding complex
C. PAB-II
D. Ribonuclease
✅ Answer: B. Cap-binding complex
Explanation: The cap-binding complex binds to the 5’ cap of mRNA and plays a role in mRNA transport from the nucleus.
5. Which of the following is NOT necessary for the transport of mRNA from the nucleus?
A. 5′ cap
B. Exon-junction complex (EJC)
C. Poly-A tail
D. SnRNPs
✅ Answer: D. SnRNPs
Explanation: SnRNPs are involved in splicing and not directly in the transport of mRNA from the nucleus.
6. In what form is mRNA transported out of the nucleus?
A. In a fully unprocessed form
B. Coated with specific proteins like mRNPs
C. Bound to ribosomes
D. In a helically coiled form without proteins
✅ Answer: B. Coated with specific proteins like mRNPs
Explanation: mRNAs are bound to mRNPs (mRNA-binding proteins), which help in their transport.
7. Which of the following elements is critical for the export of HIV mRNA from the nucleus?
A. Rev Response Element (RRE)
B. Poly-A tail
C. Cap-binding complex
D. Spliceosome
✅ Answer: A. Rev Response Element (RRE)
Explanation: The Rev protein binds to the Rev Response Element (RRE) to facilitate the export of unspliced HIV mRNA.
8. What is the function of the exon-junction complex (EJC) in mRNA transport?
A. It binds to the 3′ end of the mRNA
B. It marks the mRNA for degradation
C. It helps in the splicing of the mRNA
D. It is necessary for the export of mRNA from the nucleus
✅ Answer: D. It is necessary for the export of mRNA from the nucleus
Explanation: The EJC is required for proper mRNA transport, as it marks the exon-exon junctions after splicing.
9. What happens to unspliced mRNAs during mRNA transport?
A. They are transported to the cytoplasm
B. They are marked for degradation
C. They are retained in the nucleus by SnRNPs
D. They are converted into proteins in the nucleus
✅ Answer: C. They are retained in the nucleus by SnRNPs
Explanation: Unspliced mRNAs are retained in the nucleus due to the presence of SnRNPs, which prevent their transport.
10. In which of the following conditions is mRNA transport specifically regulated by nuclear proteins?
A. In cells undergoing mitosis
B. In cells expressing viral proteins like HIV Rev
C. During the first phase of transcription
D. In cells undergoing apoptosis
✅ Answer: B. In cells expressing viral proteins like HIV Rev
Explanation: Specific proteins, like the HIV Rev protein, regulate mRNA transport by binding to specific elements like RRE.
11. What is the characteristic feature of the mRNP complex involved in mRNA transport?
A. It is composed of ribosomal proteins
B. It is composed of ribosomal RNA
C. It is helically coiled
D. It is made only of tRNA
✅ Answer: C. It is helically coiled
Explanation: mRNPs, which coat mRNA during transport, form a helically coiled structure.
12. In the case of HIV mRNA, which type of mRNA is transported after splicing?
A. Full-length mRNA
B. 4 kb mRNA
C. 2 kb mRNA
D. Small viral mRNA
✅ Answer: C. 2 kb mRNA
Explanation: After the second splicing event, the 2 kb mRNA is exported and translated, producing the REV protein.
13. What happens to the mRNA from adenovirus in comparison to host mRNA?
A. It is not exported out of the nucleus
B. It is exported in preference to host mRNA
C. It is translated into a host protein
D. It remains unprocessed
✅ Answer: B. It is exported in preference to host mRNA
Explanation: Adenoviral transcripts are preferentially exported compared to host mRNAs due to their association with viral proteins such as E1B and E4.
14. Which of the following statements about polyadenylation is true for mRNA transport?
A. Only polyadenylated mRNAs can be exported to the cytoplasm
B. The absence of a poly-A tail prevents mRNA export
C. Both poly-A tail and 5’ cap are essential for mRNA export
D. Poly-A tail is added after mRNA export
✅ Answer: C. Both poly-A tail and 5’ cap are essential for mRNA export
Explanation: Both the poly-A tail and the 5’ cap are essential modifications for mRNA transport.
15. Which of the following types of RNA are typically NOT transported out of the nucleus?
A. tRNA
B. mRNA
C. snRNA transcribed by RNAP-III
D. Ribosomal RNA
✅ Answer: C. snRNA transcribed by RNAP-III
Explanation: snRNA transcribed by RNA polymerase III does not have a 5’ cap and is not transported out of the nucleus.
16. In the process of mRNA transport, which of the following modifications is important for nuclear export?
A. Splicing of the mRNA
B. 5′ cap addition
C. 3′ polyadenylation
D. All of the above
✅ Answer: D. All of the above
Explanation: All of these modifications are crucial for mRNA to be transported out of the nucleus.
17. How does the presence of mutations at both splice sites affect mRNA transport?
A. It prevents transport of mRNA
B. It allows the mRNA to be transported
C. It leads to mRNA degradation
D. It causes mRNA to remain in the nucleus
✅ Answer: B. It allows the mRNA to be transported
Explanation: Mutations at both splice sites do not hinder the transport of mRNA, while mutations at a single site would prevent export.
18. What is the role of the nuclear pore complex (NPC) in mRNA transport?
A. To export spliced mRNA to the cytoplasm
B. To assist in the transcription of mRNA
C. To degrade mRNA molecules
D. To facilitate the translation of mRNA
✅ Answer: A. To export spliced mRNA to the cytoplasm
Explanation: The NPC is responsible for the transport of processed mRNAs from the nucleus to the cytoplasm.
19. Which of the following RNA molecules is specifically required for nuclear mRNA export in eukaryotes?
A. U1 snRNA
B. U2 snRNA
C. U6 snRNA
D. EJC proteins
✅ Answer: D. EJC proteins
Explanation: Exon-junction complex (EJC) proteins are required for marking the spliced mRNA for export.
20. What is the primary function of the poly-A binding protein (PAB-II) in mRNA transport?
A. To remove introns from the mRNA
B. To facilitate the binding of mRNA to ribosomes
C. To prevent the mRNA from degradation during transport
D. To assist in splicing the mRNA
✅ Answer: C. To prevent the mRNA from degradation during transport
Explanation: PAB-II binds to the poly-A tail of mRNA, protecting it from degradation as it is transported to the cytoplasm.
21. Which of the following types of RNA is specifically transcribed by RNA polymerase II and exported from the nucleus?
A. U6 snRNA
B. U2 snRNA
C. rRNA
D. tRNA
✅ Answer: B. U2 snRNA
Explanation: U2 snRNA is transcribed by RNA polymerase II and is exported from the nucleus, unlike U6 snRNA, which is transcribed by RNA polymerase III.
22. What happens to mRNAs after they are exported to the cytoplasm?
A. They remain bound to mRNPs and are translated
B. They are degraded immediately
C. They are stored in the nucleus
D. They are packaged into ribosomal subunits
✅ Answer: A. They remain bound to mRNPs and are translated
Explanation: mRNAs in the cytoplasm are bound to cytoplasmic mRNPs, which protect them and aid in their translation.
23. Which of the following mRNA-related processes requires the action of the REV protein?
A. Splicing of HIV mRNA
B. Export of full-length HIV mRNA
C. Translation of HIV mRNA
D. Packaging of HIV RNA into new viral particles
✅ Answer: B. Export of full-length HIV mRNA
Explanation: The REV protein is required to bind to the Rev Response Element (RRE) in HIV mRNA to facilitate its export from the nucleus.
24. Which type of RNA undergoes methylation after export to the cytoplasm in eukaryotes?
A. rRNA
B. U1 snRNA
C. tRNA
D. mRNA
✅ Answer: B. U1 snRNA
Explanation: U1 snRNA undergoes further modifications, such as methylation at its cap sites, after it is exported to the cytoplasm.
25. Which of the following is true regarding the role of the nuclear pore complex (NPC) in RNA transport?
A. It mediates passive transport of all RNAs
B. It facilitates selective transport of RNAs
C. It only transports ribosomal RNAs
D. It does not allow any RNA to enter the nucleus
✅ Answer: B. It facilitates selective transport of RNAs
Explanation: The NPC is highly selective and regulates the transport of specific RNAs based on their processing and protein binding.
26. In what way does the poly-A tail influence mRNA transport?
A. It prevents the mRNA from being exported
B. It helps in the selection of mRNAs for transport
C. It removes the mRNA from the nuclear pore
D. It degrades the mRNA during transport
✅ Answer: B. It helps in the selection of mRNAs for transport
Explanation: The poly-A tail helps in marking mRNA for transport out of the nucleus, making it one of the key modifications for export.
27. What happens to mRNAs that lack a 5’ cap during nuclear transport?
A. They are not transported to the cytoplasm
B. They are degraded immediately
C. They are translated in the nucleus
D. They are exported without any further modifications
✅ Answer: A. They are not transported to the cytoplasm
Explanation: The absence of the 5’ cap prevents mRNA from being exported from the nucleus.
28. Which of the following factors can prevent the export of an mRNA from the nucleus?
A. The mRNA lacks a poly-A tail
B. The mRNA is unspliced
C. The mRNA is capped
D. The mRNA has a poly-A binding protein
✅ Answer: B. The mRNA is unspliced
Explanation: Unspliced mRNAs are retained in the nucleus, preventing their export.
29. How does the transport of mRNA differ in the case of adenoviral mRNAs?
A. Adenoviral mRNAs are never exported
B. They are exported more efficiently than host mRNAs
C. They are processed differently in the nucleus
D. They are not capped and exported
✅ Answer: B. They are exported more efficiently than host mRNAs
Explanation: Adenoviral mRNAs are preferentially exported over host mRNAs due to their association with viral proteins such as E1B and E4.
30. Which of the following proteins are involved in the transport of mRNA?
A. Poly-A binding proteins (PABPs)
B. Histone proteins
C. Ribosomal subunits
D. Spliceosomal proteins
✅ Answer: A. Poly-A binding proteins (PABPs)
Explanation: PABPs bind to the poly-A tail of mRNA and are involved in its transport from the nucleus to the cytoplasm.
31. What is the role of the Exon-junction complex (EJC) in mRNA transport?
A. It binds to the 5′ cap and facilitates translation
B. It marks the exon-exon junctions for proper splicing
C. It ensures the proper export of spliced mRNA
D. It binds to the 3′ poly-A tail for degradation
✅ Answer: C. It ensures the proper export of spliced mRNA
Explanation: The EJC binds to the exon-exon junctions after splicing, marking the mRNA for export from the nucleus.
32. Which of the following RNAs are typically NOT transported out of the nucleus?
A. rRNA
B. tRNA
C. snRNA transcribed by RNA polymerase III
D. mRNA
✅ Answer: C. snRNA transcribed by RNA polymerase III
Explanation: snRNA transcribed by RNA polymerase III does not have a cap and is typically not exported out of the nucleus.
33. Which element is essential for the transport of HIV mRNA to the cytoplasm?
A. E1B protein
B. Rev Response Element (RRE)
C. Poly-A tail
D. Cap-binding complex
✅ Answer: B. Rev Response Element (RRE)
Explanation: The RRE in the HIV mRNA is recognized by the REV protein, which facilitates its export from the nucleus.
34. Which of the following molecules is necessary for the transport of mRNA to the cytoplasm in eukaryotes?
A. Methylated 5’ cap
B. Spliceosome
C. Poly-A polymerase
D. Exon-junction complex
✅ Answer: A. Methylated 5’ cap
Explanation: A methylated 5’ cap is essential for the recognition and transport of mRNA from the nucleus to the cytoplasm.
35. What modification does U1 snRNA undergo after being exported from the nucleus?
A. Methylation at the cap site
B. Removal of the 3’ poly-A tail
C. Addition of a new 5’ cap
D. Binding to ribosomes
✅ Answer: A. Methylation at the cap site
Explanation: After being exported, U1 snRNA undergoes methylation at its cap site as part of further processing.
36. Which of the following mRNAs are typically NOT capped?
A. tRNA
B. U1 snRNA
C. U6 snRNA
D. mRNA
✅ Answer: C. U6 snRNA
Explanation: U6 snRNA does not have a 5′ cap and is not exported from the nucleus.
37. In the context of mRNA transport, what does the 5’ cap help in?
A. Stabilizing the mRNA during transport
B. Ensuring proper translation initiation
C. Marking mRNA for degradation
D. Binding mRNA to ribosomes
✅ Answer: A. Stabilizing the mRNA during transport
Explanation: The 5’ cap stabilizes the mRNA, protecting it from degradation during transport and ensuring its recognition for export.
38. Which of the following mRNAs are typically exported to the cytoplasm in a capped form?
A. Small RNAs
B. snRNAs
C. Ribosomal RNAs
D. mRNAs
✅ Answer: D. mRNAs
Explanation: Only properly processed mRNAs, which are capped and have a poly-A tail, are exported from the nucleus to the cytoplasm.
39. How is HIV mRNA exported from the nucleus?
A. By binding to a host protein
B. By binding to a REV protein at the RRE element
C. Through the action of the spliceosome
D. By using the poly-A binding protein
✅ Answer: B. By binding to a REV protein at the RRE element
Explanation: HIV mRNA is exported only when the REV protein binds to the Rev Response Element (RRE).
40. What is the role of the nuclear pore complex (NPC) in the transport of RNA?
A. It synthesizes RNA for transport
B. It is a passive channel for any RNA
C. It selectively transports only properly processed RNA
D. It degrades improperly processed RNA
✅ Answer: C. It selectively transports only properly processed RNA
Explanation: The NPC selectively transports only properly processed RNA, ensuring that only mature mRNAs are exported.
41. What prevents unspliced mRNAs from being exported to the cytoplasm?
A. The 5’ cap
B. The poly-A tail
C. The presence of SnRNPs at splice junctions
D. The presence of a ribosomal subunit
✅ Answer: C. The presence of SnRNPs at splice junctions
Explanation: The presence of SnRNPs at splice junctions in unspliced mRNAs prevents their export from the nucleus.
42. Which of the following molecules is responsible for binding to the poly-A tail of mRNA during export?
A. PABPs (Poly-A Binding Proteins)
B. E1B proteins
C. REV protein
D. Spliceosomal proteins
✅ Answer: A. PABPs (Poly-A Binding Proteins)
Explanation: Poly-A Binding Proteins (PABPs) bind to the poly-A tail of mRNA and play a role in its export from the nucleus.
43. Which of the following best describes the transport process of mRNA in eukaryotic cells?
A. It is a passive process driven by concentration gradients
B. It involves active transport through the nuclear pore complex (NPC)
C. It does not require energy
D. It only occurs in certain cell types
✅ Answer: B. It involves active transport through the nuclear pore complex (NPC)
Explanation: The transport of mRNA from the nucleus to the cytoplasm is an active, energy-dependent process that occurs through the nuclear pore complex (NPC).
44. In HIV infection, how does the virus regulate the export of its mRNA from the nucleus?
A. By attaching to the poly-A tail
B. By using the REV protein to bind to the RRE sequence
C. By utilizing the E1B protein
D. By binding to ribosomal proteins
✅ Answer: B. By using the REV protein to bind to the RRE sequence
Explanation: In HIV, the REV protein binds to the Rev Response Element (RRE) on the mRNA to facilitate its export from the nucleus.
45. What does the presence of a 5′ cap structure on mRNA ensure?
A. It prevents degradation in the cytoplasm
B. It prevents mRNA export
C. It facilitates recognition by the nuclear pore complex
D. It enhances splicing efficiency
✅ Answer: C. It facilitates recognition by the nuclear pore complex
Explanation: The 5′ cap structure helps in the recognition and binding of mRNA to the nuclear pore complex for export to the cytoplasm.
46. Which of the following mRNA modifications occurs during or after mRNA transport to the cytoplasm?
A. Splicing of introns
B. Addition of a poly-A tail
C. 5’ capping
D. Methylation of the cap site in snRNAs
✅ Answer: D. Methylation of the cap site in snRNAs
Explanation: After snRNAs are exported to the cytoplasm, they undergo methylation at the cap site as part of their modification process.
47. How do mutations at both splice sites in an mRNA affect its export from the nucleus?
A. They prevent export
B. They enhance the export of mRNA
C. They prevent translation but allow export
D. They have no effect on export
✅ Answer: B. They enhance the export of mRNA
Explanation: Mutations at both splice sites allow the mRNA to be exported, unlike mutations at a single splice site, which prevent export.
48. What is a key feature of mRNA exported from the nucleus in eukaryotic cells?
A. It contains a 5′ cap and a poly-A tail
B. It is devoid of proteins
C. It is not spliced
D. It is not associated with ribosomes
✅ Answer: A. It contains a 5′ cap and a poly-A tail
Explanation: For mRNA to be exported from the nucleus, it must have a 5′ cap and a poly-A tail, signaling it is fully processed.
49. Which of the following proteins is involved in the modification of snRNA after its export to the cytoplasm?
A. Spliceosomal proteins
B. Poly-A binding proteins
C. SM proteins
D. REV proteins
✅ Answer: C. SM proteins
Explanation: After export to the cytoplasm, snRNAs are associated with SM proteins, which assist in their further processing and return to the nucleus.
50. Which of the following statements is true about mRNA transport in eukaryotes?
A. Only capped mRNAs are exported from the nucleus
B. mRNAs with poly-A tails are not exported
C. Unspliced mRNAs are immediately exported
D. All RNAs are exported indiscriminately
✅ Answer: A. Only capped mRNAs are exported from the nucleus
Explanation: The 5′ cap is essential for the export of mRNA, and only properly processed mRNAs with a cap structure are exported from the nucleus.