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