This set of Cell Biology Questions and Answers for Aptitude test focuses on “Nucleus – Transport of Molecules Through Nucleopores”.
1. Processing of mRNA includes __________
a) 5′ capping, 3′ polyadenylation and RNA splicing
b) 3′ capping, 5′ polyadenylation and RNA splicing
c) 5′ capping and 3′ polyadenylation
d) 5′ capping and RNA splicing
Explanation: After transcription, the pre-mRNA is modified to produce matured mRNA. The modification of mRNA includes the following- 5′ methul guanosine capping that protects the mRNA from RNases, 3′ polyadenylation and splicing of the introns of pre-mRNA.
2. The ability of importins and exportins to transport molecules in and out of the nucleoporins is regulated by ______________
d) Both ATPases and GTPases
Explanation: Impotins and exportins are responsible for the transport of molecules inside the nucleus and outside to the cytoplasm respectively. This transport is regulated by GTPases, enzymes that hydrolyze guanosine triphosphate (GTP) to release energy. The major GTPase used in nuclear transport is ‘Ran’, which can bind either GTP or GDP.
3. Which of the following human cells are multinucleated?
d) None of the mentioned
Explanation: Multinucleated cells have multiple nucleus. In humans, skeletal muscle cells which are also known as myocytes, become multinucleated during development. The results in the arrangement of nuclei near the periphery of the skeletal muscle cells and thus, allowing maximum intracellular space for myofibrils.
4. Why are red blood cells anucleated?
a) Presence of nucleus leads to heavier cells that cannot flow in blood stream
b) Absence of nucleus leads to increased space for haemoglobin that carries molecular oxygen
c) Presence of nucleus leads to increased proliferation of RBCs which harms the body
d) All of the mentioned
Explanation: Red blood cells (RBCs) present in human blood stream do not contain nucleus. Absence of nucleus leads to more space for stuffing of haemoglobin that carries molecular oxygen. This thus increases the oxygen-carrying capacity in blood. If the nucleus was present, haemoglobin content will be limited around the periphery. Furthermore, due to absence of nucleus, RBCs contain a disk-like depression that increases the surface are for haemoglobin content.
5. Which of the following first binds to the NLS protein sequence?
c) CAS protein
d) NLS detecting protein
Explanation: The molecules that contain Nucleus Localization Signal (NLS) sequence, are immediately ushered into the nucleus through nucleopore. The first step of this transport involves binding of importin-alpha to the NLS sequence, which then acts as a bridge for the attachment of importin-beta.
6. Which of the following displaces Importin-alpha from the cargo transported inside the nucleus?
Explanation: Once the complex composed of importin-alpha—importin-beta—molecule enters the nucleus, the importin-alpha and importin-beta has to be removed. RanGTP binds to Importin-beta and displaces it from the complex. Then the Cellular Apoptosis Susceptibility (CAS) protein which is an exportin and is bound to RanGTP, displaces Importin-alpha from the molecule with NLS sequence.
7. The import cycle of cargo or molecule inside the nucleus requires the hydrolysis of ___
a) 1 GTP
b) 1 ATP
c) 2 GTPs
d) 3 GTPs
Explanation: The import of cargo or molecule inside the nucleus is an active transport. This is because the whole of import cycle needs the energy from the hydrolysis of two Guanosine triphosphate (GTP) molecules.
8. Export of molecules from inside the nucleus to the cytoplasm needs hydrolysis ________
a) 1 GTP
b) 1 ATP
c) 2 GTPs
d) 3 GTPs
Explanation: The export of protein molecules from nucleus to cytoplasm is done due to the presence of Nuclear Export Sequence (NES). NES of the protein molecule can bind with an exportin and RanGTP to form a heterotrimeric complex. This complex is then transported outside nucleus by active transport as it requires the hydrolysis of one GTP molecule.
9. Export of proteins with exportin CRM1 can be inhibited by ___________
a) Leptomycin A
b) Leptomycin B
c) Leptomycin D
d) Leptomycin H
Explanation: CRM1 is an exportin which binds to the Nuclear Export Sequence (NES) of protein molecules destines to be transported out of the nucleus. The export process with exportin CRM1 can be stopped by Leptomycin B.
10. Which of the following cellular RNA is not dependent on RanGTP during transport outside nucleus?
Explanation: All and cellular RNAs such as tRNAs, rRNAs, snRNAs and miRNAs except mRNA are dependent on RanGTP for their transport outside the nucleus. Conserved mRNA export factors such as Mex67/Tap (large subunit) and Mtr2/p15 are required for mRNA transport. In higher eukaryotes, mRNA recruits the protein TREX for spliced mRNA sequences.
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