This set of Cell Biology Multiple Choice Questions & Answers (MCQs) focuses on “Gene Expression Control – Post-translational Control”.
1. Degradation of cellular unwanted proteins is carried out in _________
Explanation: Cellular proteins are degraded in the proteasomes.They are made up of 2 different protein complexes(two α subunit and two β subunit). Lysosomes however degrade extracellular proteins and other macromolecules.
2. How many β-subunits are there in proteasomes?
Explanation: In proteasomes, there are four polypeptide rings stacked one on top of the other and a cap attached at either side of the stack. The two central rings are β-subunits are the main site for proteolysis.
3. Proteins that terminate in arginine are short-lived.
Explanation: Although exact factors that determine the longevity of a protein are not yet clear. One of the determinants is amino acid sequence at the N-terminus of the polypeptide. Polypeptides that terminate in arginine and lysine are short-lived.
4. “Degron” is a __________
c) amino acid sequence
d) nucleic acid sequence
Explanation: For post-translational control, it is important that regulatory proteins are degraded in time. Certain proteins contain a specific internal amino acid sequence in their polypeptide chain, that ensure timely degradation.
5. Ubiquitin is a ___________________
c) nuclear body
d) aggragated complex
Explanation: Ubiquitin is an important protein that performs functions in diverse cellular processes. It is a small and highly conserved protein. A single attached ubiquitin can serve as a sorting signal leading the associated polypeptide to a specific pathway.
6. A number of ubiquitin molecules must be attached to ensure enzymatic degradation of a polypeptide.
Explanation: A single attached ubiquitin functions as a sorting signal while for ensuring enzymatic degradation of polypeptides, a number of ubiquitin molecules must be enzymatically transferred to the polypeptide.
7. For enzymatic degradation, Ubiquitin is transferred from the carrier protein to _______________ residue.
Explanation: Ubiquitination is the process of adding ubiquitin molecules to damaged or old proteins. The last Amino acid of ubiquitin is attached to a lysine residue on the substrate protein.
8. Which part of the proteasome recognizes a polyubiquitinated protein?
d) end caps
Explanation: Once the protein is polyubiquitinated, it is ready for enzymatic degradation by the proteasome. The cap of proteasome recognizes the protein and removes the ubiquitin chain and unfolds the target protein.
9. In the proteasome, the unfolded protein is threaded through narrow opening in the ring of ___________________
b) sigma factor
Explanation: In the proteasome after removal of ubiquitin chain the unfolded linear peptide moves through the narrow opening in the rings of alpha-subunits and passed to the central chamber where degradation takes place.
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