Question
The epidermal growth factor receptor EGFR is a cell surface receptor. When a growth factor binds to EGFR, the receptor is activated. The activated EGFR triggers a signal transduction pathway, which leads to increased frequency of cell division.
Which of the following best predicts the effect of a mutation that causes EGFR to be active in the absence of a growth factor?
A. Increased apoptosis will lead to abnormal growth of the tissue.
B. Increased cell division will lead to the formation of a tumor.
C. Cells will exit the cell cycle, entering a non-dividing \(G_0\) phase.
D. Fewer cells will be in any of the stages of mitosis.
▶️Answer/Explanation
Ans: B
Growth factors are important signaling molecules that trigger cells to divide at the correct rate and correct time. An EGFR that is active when a growth factor is absent would lead to increased cell division. Uncontrolled cell growth will likely result in a tumor.
Question
The tumor suppressor protein p53 binds to DNA and activates target genes, which results in the synthesis of p21, CD95, and other proteins. The p21 protein promotes cell-cycle arrest, whereas the CD95 protein promotes apoptosis.
Which of the following will most likely result from a loss of p53 function?
A. Rapid cell growth without cell division
B. Immediate activation of apoptosis pathways
C. Uncontrolled cell proliferation
D. Increased expression of p53 target genes
▶️Answer/Explanation
Ans: C
A loss of p53 function will result in unregulated cell proliferation because the p21 protein will not be synthesized. In the absence of the p21 protein, a cell will continue to progress through the cell cycle without stopping.
Question
Figure 1 represents the relative time and sequence of the phases of the cell cycle.
Figure 1. Representation of the cell cycle and identification of the \(G_1/S\)
checkpoint
Which statement best predicts why a cell’s progression through the cell cycle might be halted at the \(G_1/S\) checkpoint?
A. Spindle fibers have not correctly attached to chromosomes.
B. There are not enough nucleotides available to construct new DNA.
C. Damage occurred to DNA when it was being copied in \(G_1\).
D. Proteins necessary for M phase of the cell cycle have not been produced.
▶️Answer/Explanation
Ans: B
DNA replication, which occurs during S phase, requires free nucleotides. If nucleotides are unavailable, the G1/S checkpoint halts the cell cycle to conserve energy.
Question
Glycogen synthetase kinase 3 beta is a protein kinase that has been implicated in many types of cancer. Depending on the cell type, the gene for glycogen synthetase kinase 3 beta (GSK3β) can act either as an oncogene or as a tumor suppressor.
Which of the following best predicts how GSK3β mutations can lead to the development of cancer?
A. Cells with inactive GSK3β fail to trigger apoptosis.
B. Cells with inactive GSK3β fail to proceed past the \(G_2\)/M checkpoint.
C. Cells with overactive GSK3β are more likely to repair DNA damage.
D. Cells with overactive GSK3β have longer cell cycles.
▶️Answer/Explanation
Ans: A
If apoptosis cannot be initiated in cells with GSK3β mutations, those cells are more likely to proliferate, which could lead to cancer.
Question
Cancer cells behave differently than normal body cells. For example, they ignore signals that tell them to stop dividing.
Which of the following conditions will most likely cause a normal body cell to become a cancer cell?
A. The environment already contains cancer cells.
B. The environment has an abundance of nutrients.
C. The environment lacks signals that would otherwise tell the cell to stop dividing.
D. The environment contains mutagens that induce mutations that affect cell-cycle regulator proteins.
▶️Answer/Explanation
Ans: D
Cancerous cells often produce abnormal forms of cell-cycle regulators, allowing them to divide even when it is not appropriate to do so.