The concentration of a particular protein X in a normal human cell rises gradually from a low point, immediately after cell division, to a high point, just before cell division, and then drops sharply. The level of its mRNA in the cell remains fairly constant throughout this time. Protein X is required for cell growth and survival, but the drop in its level just before cell division is essential for division to proceed. You have isolated a line of human cells that grow in size in culture but cannot divide, and on analyzing these mutants, you find that levels of X protein do not decrease. Which of the following mutations could explain these results?
a. A mutation that results in the loss of an enzyme that adds a ubiquitin tag to the protein.
b. a mutation in gene X that results in the loss of polyA addition to its mRNA
c. a mutation in gene X that changes the sequence that encodes sites at which ubiquitin can be attached to the protein
d. Both A and C

Ubiquitination is a post-translational modification that leads to the attachment of ubiquitin (Ub) monomers to a protein target, a cellular process required for protein degradation in the proteasomes. The Ub monomers and/or poly-ubiquitin chains are added to specific amino acid residues (e.g., Lysine) in the protein substrate. A proteasome is a protein complex that recognizes and degrades ubiquitinated (unneeded or damaged) proteins via proteolytic cleavage reactions, i.e., via chemical reactions that break peptide bonds between amino acids. In consequence, mutations that affect the ubiquitination process may also alter the degradation of the target protein (in this case, protein X).