The Concentration of a Particular Protein, X, in a Normal

question 23

Multiple Choice

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

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