Question 1
Topic – 4.3 Signal Transduction Pathways
(a) – Topic – 2.5 Membrane Transport
(b) – Topic – 4.3 Signal Transduction Pathways
(c) – Topic – 4.3 Signal Transduction Pathways
(d) – Topic – 4.3 Signal Transduction Pathways
The binding of an extracellular ligand to a G protein-coupled receptor in the plasma membrane of a cell triggers intracellular signaling (Figure 1, A). After ligand binding, GTP replaces the GDP that is bound to Gsa, a subunit of the G protein (Figure 1, B). This causes Gsa to activate other cellular proteins, including adenylyl cyclase that converts ATP to cyclic AMP (cAMP). The cAMP activates protein kinases (Figure 1, C). In cells that line the small intestine, a cAMP-activated protein kinase causes further signaling that ultimately results in the secretion of chloride ions (\( C1^{-}\)) from the cells. Under normal conditions, Gsa hydrolyzes GTP to GDP, thus inactivating adenylyl cyclase and stopping the signal (Figure 1, A).
Individuals infected with the bacterium Vibrio cholerae experience severe loss of water from the body (dehydration). This is due to the effects of the bacterial cholera toxin that enters intestinal cells. Scientists studied the effects of cholera toxin on four samples of isolated intestinal cell membranes containing the G protein-related signal transduction components shown in Figure 1. GTP was added to samples II and IV only; cholera toxin was added to samples III and IV only. The scientists then measured the amount of cAMP produced by the adenylyl cyclase in each sample (Table 1).
TABLE 1. AMOUNT OF cAMP PRODUCED FROM INTESTINAL CELL MEMBRANES IN THE ABSENCE
OR PRESENCE OF CHOLERA TOXIN
(a) Describe one characteristic of a membrane that requires a channel be present for chloride ions to passively cross the membrane. Explain why the movement of chloride ions out of intestinal cells leads to water loss.
(b) Identify an independent variable in the experiment. Identify a control in the experiment. Justify why the scientists included Sample III as a control treatment in the experiment.
(c) Based on the data, describe the effect of cholera toxin on the synthesis of cAMP. Calculate the percent change in the rate of cAMP production due to the presence of cholera toxin in sample IV compared with sample II.
(d) A drug is designed to bind to cholera toxin before it crosses the intestinal cell membrane. Scientists mix the drug with cholera toxin and then add this mixture and GTP to a sample of intestinal cell membranes. Predict the rate of cAMP production in pmol per mg adenylyl cyclase per min if the drug binds to all of the toxin. In a separate experiment, scientists engineer a mutant adenylyl cyclase that cannot be activated by
Gsa. The scientists claim that cholera toxin will not cause excessive water loss from whole intestinal cells that contain the mutant adenylyl cyclase. Justify this claim.
▶️Answer/Explanation
1(a) Describe one characteristic of a membrane that requires a channel be present for chloride ions to passively cross the membrane.
Accept one of the following:
• The interior of the membrane/phospholipid tail is nonpolar.
• The interior of the membrane/phospholipid tail is not charged.
• The interior of the membrane/phospholipid tail is hydrophobic.
Explain why the movement of chloride ions out of intestinal cells leads to water loss.
Accept one of the following:
• The space outside of the cells becomes hypertonic/hyperosmotic compared with the cells, so water moves out of the cells.
• The space outside of the cells would have a lower water potential compared with the cells, so water will move out of the cells.
1(b) Identify an independent variable in the experiment.
Accept one of the following:
• The presence or absence of cholera toxin
• The presence or absence of GTP
Identify a negative control in the experiment.
Accept one of the following:
• The sample lacking both cholera toxin and GTP /sample I
• The samples that lack cholera toxin /samples I and II
• The sample that lacks cholera toxin but contains GTP /sample II
• The samples that lack GTP /samples I and III
Justify why the scientists included Sample III as a control treatment in the experiment.
Accept one of the following:
• (Sample III serves as a control) to compare cAMP production with that of the sample having cholera toxin and GTP /sample IV .
• Comparing sample III and sample IV enables the scientists to evaluate whether the activity of cholera toxin requires GTP/acts via the G protein pathway .
1(c) Based on the data, describe the effect of cholera toxin on the synthesis of cAMP .
Accept one of the following:
• Cholera toxin increases the production of cAMP in the presence of GTP ( IV vs II ).
• Cholera toxin has no effect on the production of cAMP in the absence of GTP ( III vsI ).
Calculate the percent change in the rate of cAMP production due to the presence of cholera toxin in sample IV compared with sample II .
• 1,170% [(127−10)/10 = 11.7 × 100]
1(d) A drug is designed to bind to cholera toxin and prevent the toxin from crossing the intestinal cell membrane. Scientists mix the drug with cholera toxin and then add this mixture and GTP to a sample of intestinal cell membranes. Predict the rate of cAMP production in pmol per mg adenylyl cyclase per min if the drug binds to all of the toxin.
• The rate will be 10 ( pmol per mg adenyl cyclase per min ). In a separate experiment, scientists engineer a mutant adenylyl cyclase that cannot be activated by Gsα . The scientists claim that cholera toxin will not cause excessive water loss from whole intestinal cells that contain the mutant adenylyl cyclase. Justify this claim.
• (Even in the presence of the toxin) cAMP will not be produced (by this pathway), the protein kinases will not be activated, and Cl− ions will not be secreted (and less water will leave the intestinal cells).
Question 2
Topic – 5.1 Meiosis
(a) – Topic – 1.3 Introduction to Macromolecules
(b) – Topic – 5.1 Meiosis
(c) – Topic – 5.1 Meiosis
(d) – Topic – 5.1 Meiosis
During meiosis, double-strand breaks occur in chromatids. The breaks are either repaired by the exchange of genetic material between homologous nonsister chromatids, which is the process known as crossing over (Figure 1A), or they are simply repaired without any crossing over (Figure 1B). Plant breeders developing new varieties of corn are interested in determining whether, in corn, a correlation exists between the number of meiotic double-strand chromatid breaks and the number of crossovers.
Using specialized staining and microscopy techniques, scientists counted the number of double-strand chromatid breaks and the number of crossovers in the same number of meiotic gamete-forming cells of six inbred strains of corn (Table 1).
TABLE 1. NUMBER OF CHROMATID DOUBLE-STRAND BREAKS AND AVERAGE NUMBER OF
CROSSOVERS IN INBRED STRAINS OF CORN
(a) The double-strand breaks occur along the DNA backbone. Describe the process by which the breaks occur.
(b) Using the template in the space provided for your response, construct an appropriately labeled graph that represents the data in Table 1 and allows examination of a possible correlation between double-strand
breaks and crossovers. Based on the data, determine whether corn strains I, II, and III differ in their average number of crossovers.
(c) Based on the data, describe the relationship between the average number of double-strand breaks and the average number of crossovers in the strains of corn analyzed in the experiment.
(d) Crossing over (Figure 1A) creates physical connections that are required for proper separation of homologous chromosomes during meiosis. A diploid cell with four pairs of homologous chromosomes undergoes meiosis to produce four haploid cells. Crossing over occurs between only three of the pairs. Predict the number of chromosomes most likely present in each of the four haploid cells. Provide reasoning to justify your prediction. Explain how plant breeders can use the information in Table 1 to help develop new varieties of corn.
▶️Answer/Explanation
2(a) The double-strand breaks occur along the DNA backbone. Describe the process by which the breaks occur.
Accept one of the following:
• (Enzymatic) hydrolysis occurs between the sugars and phosphates/nucleotides.
• The covalent bonds between the sugars and phosphates/nucleotides are broken.
2(b) Using the template in the space provided for your response, construct an appropriately labeled graph that represents the data in Table 1 and allows examination of a possible correlation between double-strand breaks and crossovers.
• Appropriate axis scaling
Using the template in the space provided for your response, construct an appropriately labeled graph that represents the data in Table 1 and allows examination of a possible correlation between double-strand breaks and crossovers.
• Accurately plotted X,Y graph with separate points for the average number of crossovers for each strain Using the template in the space provided for your response, construct an appropriately labeled graph that represents the data in Table 1 and allows examination of a possible correlation between double-strand breaks and crossovers.
• Accurate error barz Based on the data, determine whether corn strains I , II , and III differ in their average number of crossovers.
• There is no (statistical) difference (in the average number of crossovers) between strains II and III. Strain I is higher/different (in the average number of crossovers) compared with strains II and III.
2(c) Based on the data, describe the relationship between the average number of doublestrand breaks and the average number of crossovers in the strains of corn analyzed in the experiment.
• (In general) there is a direct correlation/positive relationship (between the number of double-strand breaks and the number of chromatid crossovers).
2(d) Crossing over (Figure 1A) creates physical connections that are required for proper separation of homologous chromosomes during meiosis. A diploid cell with four pairs of homologous chromosomes undergoes meiosis to produce four haploid cells. Crossing over occurs between only three of the pairs. Predict the number of chromosomes most likely present in each of the four haploid cells.
• Two cells will have three/n−1 chromosomes; two cells will have five/n+1 chromosomes.Provide reasoning to justify your prediction.
• During meiosis I , (three homologous pairs separate normally, and) one pair does not separate/experiences nondisjunction. In meiosis II , the sister chromatids separate normally.
Explain how plant breeders can use the information in Table 1 to help develop new varieties of corn.
Accept one of the following:
• Because crossing over increases genetic diversity, the plant breeders can breed strains with high crossover numbers/double-strand breaks.
• They can increase the number of double-stranded breaks, which may lead to more crossovers that increase genetic variation.
Question 3
Topic – 3.4 Photosynthesis
(a) – Topic – 3.1 Enzymes
(b) – Topic – 3.4 Photosynthesis
(c) – Topic – 3.4 Photosynthesis
(d) – Topic – 3.4 Photosynthesis
Fireflies emit light when the enzyme luciferase catalyzes a reaction in which its substrate, D-luciferin, reacts to form oxyluciferin and other products (Figure 1). In order to determine the optimal temperature for this enzyme, scientists added ATP to a solution containing D-luciferin, luciferase, and other substances needed for the reaction. They then measured the amount of light emitted during the first three seconds of the reaction when it was carried out at different temperatures.
(a) Describe a characteristic of the luciferase enzyme that allows it to catalyze the reaction.
(b) Identify the dependent variable in the experiment.
(c) State the null hypothesis for the experiment.
(d) A student claims that, as temperature increases, there will be an increase in the amount of light given off by the reaction in the first three seconds. Support the student’s claim.
▶️Answer/Explanation
3(a) Describe a characteristic of the luciferase enzyme that allows it to catalyze the reaction.
Accept one of the following:
• It has an active site/a shape that can bind with the substrate(s)/brings reactants together.
• It has a charge that is compatible with the substrate(s).
3(b) Identify the dependent variable in the experiment.
• The amount of light emitted
3(c) State the null hypothesis for the experiment.
• Temperature has no effect on the amount of light emitted.
3(d) A student claims that, as temperature increases, there will be an increase in the amount of light given off by the reaction in the first three seconds. Support the student’s claim.
Accept one of the following:
• Higher temperature increases the frequency of collisions/interactions between molecules, resulting in an increase in reaction rate.
• The higher temperature results in a change to the active site that enhances substrate binding.
Question 4
Topic – 7.4 Population Genetics
(a) – Topic – 7.4 Population Genetics
(b) – Topic – 7.2 Natural Selection
(c) – Topic – 8.1 Responses to the Environment
(d) – Topic – 7.4 Population Genetics
Existing isolated brook trout populations in Newfoundland, Canada, were once part of a larger population that was fragmented at the end of the most recent glaciation period about 10,000 to 12,000 years ago. Researchers investigated 14 naturally separated stream populations of brook trout. They found that the populations are all genetically distinct and show differences in morphology.
(a) Describe the prezygotic barrier that results in these genetically distinct populations.
(b) Brook trout with longer fins are able to swim faster than brook trout with shorter fins. In one of the Newfoundland streams, the main prey of the brook trout evolved to move faster. For brook trout living in this stream, explain the difference in fitness between longer-finned individuals and shorter-finned individuals.
(c) If two morphologically and behaviorally distinct populations of brook trout remain isolated for many generations, predict the likely impact on both populations.
(d) Researchers claim that there are more genetic differences between any two current brook trout populations than there are between any single current population and the ancestral brook trout population from which all the trout are descended. Provide reasoning to justify their claim.
▶️Answer/Explanation
4(a) Describe the prezygotic barrier that results in these genetically distinct populations.
• Geographic isolation prevents gene flow between the populations.
4(b) Brook trout with longer fins are able to swim faster than brook trout with shorter fins. In one of the Newfoundland streams, the main prey of the brook trout evolved to move faster. For brook trout living in this stream, explain why there is a difference in fitness between longer-finned individuals and shorter-finned individuals.
• Individuals with longer fins are more likely to capture prey and reproduce.
4(c) If two morphologically and behaviorally distinct populations of brook trout remain isolated for many generations, predict the likely impact on both populations.
Accept one of the following:
• The two populations will become separate species.
• The two populations will continue diverging (behaviorally/morphologically/genetically).
4(d) Researchers claim that there are more genetic differences between any two current brook trout populations than there are between any single current population and the ancestral brook trout population from which all the trout are descended. Provide reasoning to justify their claim.
Accept one of the following:
• Each single population has accumulated mutations/experienced genetic drift (distinguishing it from the ancestral population). The mutations each population accumulated are likely to differ (as a result of different selective pressures).
• Allele production (as a result of random mutation) and genetic drift/selection by local environmental conditions has resulted in a collection of alleles unique to each population.