G proteins are a family of receptor proteins that are involved in transmitting signals from outside a cell to inside a cell. When a signaling molecule binds to a G protein, the G protein is activated. The G protein then activates an enzyme that produces a second messenger called cAMP.

Which of the following describes a critical role of cAMP during the transduction stage of a G protein signal transduction pathway?

Which of the following steps in a signaling pathway typically occurs first once a chemical messenger reaches a target cell?

During a fight-or-flight response, epinephrine is released into the body’s circulatory system and transported throughout the body. Some cells exhibit a response to the epinephrine while other cells do not.

Which of the following justifies the claim that differences in components of cell signaling pathways explain the different responses to epinephrine?

Signal transduction may result in changes in gene expression and cell function, which may alter phenotype in an embryo. An example is the expression of the SRY gene, which triggers the male sexual development pathway in mammals. This gene is found on the Y chromosome.

Researchers have discovered details about apoptosis (programmed cell death) by studying embryologic development of a nematode worm, Caenorhabditis elegans. Apoptosis is a normal developmental process in C. elegans. They found several genes involved in apoptosis, including ced−9 and ced−3. The ced−3 gene was found to promote cell death, and ced−9 to inhibit it. The ced−9 gene serves as a regulator that prevents apoptosis in the absence of a signal promoting apoptosis.

Which of the following statements best justifies the claim that changes in the expression of ced−9 in C. elegans can affect regulation of apoptosis in the cell?

The insulin receptor is a transmembrane protein that plays a role in the regulation of glucose homeostasis. The receptor’s extracellular domain binds specifically to the peptide hormone insulin. The receptor’s intracellular domain interacts with cellular factors. The binding of insulin to the receptor stimulates a signal transduction pathway that results in the subcellular translocation of GLUT4, a glucose transport protein that is stored in vesicles inside the cell. A simplified model of the insulin receptor–signaling pathway is shown in Figure 1.

Which of the following statements best predicts the effect of a loss of function of the insulin receptor’s intracellular domain?

The beta-2 adrenergic receptor is a membrane-bound protein that regulates several cellular processes, including the synthesis and breakdown of glycogen. The receptor binds specifically to the hormone epinephrine. The binding of epinephrine to the beta-2 adrenergic receptor triggers a signal transduction cascade that controls glycogen synthesis and breakdown in the cell. A simplified model of the signal transduction cascade is represented in Figure 1.

Which of the following outcomes will most likely result from the inactivation of the beta-2 adrenergic receptor?

Fibroblast growth factor receptors (FGFRs) are transmembrane proteins that regulate cellular processes such as cell proliferation and differentiation. The extracellular domains of FGFR proteins bind specifically to signaling molecules called fibroblast growth factors. The intracellular domains of FGFR proteins function as protein kinases, enzymes that transfer phosphate groups from ATP to protein substrates.

FGFR activation occurs when binding by fibroblast growth factors causes FGFR proteins in the plasma membrane to become closely associated with each other. The association of two FGFR proteins stimulates protein kinase activity, which triggers the activation of intracellular signaling pathways.

A simplified model of FGFR activation is represented in Figure 1.

Which of the following changes in the FGFR signaling pathway is most likely to result in uncontrolled cell proliferation?

Phosphofructokinase (PFK) is a key enzyme in glycolysis. ATPis one of the two substrates for the reaction catalyzed by PFK. ATP is also an allosteric regulator of PFK. Figure 1 shows the enzyme-substrate interactions of PFK.

A researcher found a mutation that resulted in the PFK enzyme being unable to bind ATP to the allosteric site. Which of the following best predicts the effect of the mutation?

A person’s blood glucose level fluctuates during the day, as represented in Figure 1. Two hormones, insulin and glucagon, are directly involved in regulating the blood glucose level to maintain a healthy level. Insulin acts to lower the blood glucose level, and glucagon acts to increase the blood glucose level.

Which of the following best predicts what will happen to the blood glucose level if the person has another meal at 5 p.m.?

Blood clots are formed by a positive feedback loop. Two pathways exist, the extrinsic and intrinsic pathways, which converge during clot formation. There are many clotting factors involved, most of which are proteins. Vitamin K

is required for the formation of the active form of several of the clotting factors, including Factor X. A simplified model of the blood clotting process is shown in Figure 1.

Warfarin is a drug used to treat certain blood clots. Warfarin blocks the formation of the active form of vitamin K-dependent clotting factors. Based on the model, which of the following best predicts the effects of warfarin on a patient?

A model of the typical life cycle of a cell is shown in Figure 1. Scientists have estimated that it takes yeast cells approximately 20 hours to complete the entire cycle. Table 1 shows the amount of time in each phase of the life cycle for yeast cells.

Table 1. Amount of time spent in each stage of the cell cycle by yeast cells

Stage M G1 S G2

Time (hours) 2 10 5 3

Based on Table 1, what percent of the life cycle of yeast cells is spent in DNA replication?

Researchers grew seedlings of corn, Zea mays, in loose and compact sand. The researchers measured the amount of time required for the cells in the growing root tips of the seedlings to double in number. The mean cell doubling times for the two groups of seedlings are shown in Figure 1.

Figure 1. Mean cell doubling times for the growing root tips of Zea mays seedlings planted in loose or compact sand

Based on the sample means, which of the following conclusions about the cells in the growing root tips of Zea mays seedlings is best supported by the results of the experiment?

The relative amounts of DNA present in the nucleus of a cell at four different stages of the life cycle are shown in Figure 1.

Figure 1. Relative amounts of DNA present in the nucleus of a cell

Based on Figure 1, which of the following statements correctly links a stage of the cell cycle with the event occurring at that stage?

Researchers tracked the amount of DNA (measured in picograms) over time beginning with a single cell and continuing through several rounds of cell division. The researchers observed threadlike chromosomes prior to cell division. The threadlike chromosomes disappeared from view shortly after each division. The amount of DNA

in picograms per cell over several rounds of cell division is shown in Figure 1.

Figure 1. Amount of DNA in picograms per cell over several rounds of cell division

Which of the following statements is consistent with the data in Figure 1?

A cell culture commonly used in research was selected to study the effect of a specific virus on the timing of cell cycle phases. Two separate cultures were started, one untreated and one inoculated with the virus. Both cultures were incubated under identical conditions. After a period of time, 200 cells from each culture were observed and classified as shown in Table 1.

Table 1. Number of normal and infected cells found in three phases of the cell cycle

Which of the following most accurately describes an observation and an effect of the viral infection indicated by the data in Table 1?

A student used microscopy to investigate the relative lengths of the different stages of mitosis. The student prepared slides of cells isolated from a growing onion root tip and viewed the slides under a dissecting microscope. The student then made diagrams of cells that were in different stages of mitosis and counted the number of cells that were in each of those stages. The student’s data are presented in Table 1.

Based on the data, the percent of the mitotic cells that were in metaphase is closest to which of the following?

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?

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?

Figure 1 represents the relative time and sequence of the phases of the cell cycle.

Which statement best predicts why a cell’s progression through the cell cycle might be halted at the G1/S checkpoint?