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
(c) The decrease in the level of protein X in the normal cell is most probably due to protein degradation, because levels of mRNA remain constant. The inability of the mutant cell to divide could be due to a mutation that inhibits protein degradation. This would be achieved by the removal of sites for attachment of ubiquitin, which targets proteins for destruction. Choices (a), (b), and (d) would probably not produce the results described, because without the production of a functional protein X, the mutant cells could not grow in size.
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