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 mRNA in the mutant cells are normal. Which of the following mutations in the gene for X could explain these results?
(a) the introduction of a stop codon that truncates protein X at the fourth amino acid
(b) a change of the first ATG codon to CCA
(c) the deletion of a sequence that encodes sites at which ubiquitin can be attached to the protein
(d) a change at a splice site that prevents splicing of the RNA
Answer: C