AP Sample Student Responses and Scoring Commentary

2024 AP Biology ® Sample Student Responses and Scoring Commentary Inside: Free-Response Question 1 ☑ Scoring Guidelines ☑ Student Samples ☑ Scoring Commentary © 2024 College Board. College Board, Advanced Placement, AP, AP Central, and the acorn logo are registered trademarks of College Board. Visit College Board on the web: collegeboard.org. AP Central is the official online home for the AP Program: apcentral.collegeboard.org. AP® Biology 2024 Scoring Guidelines Question 1: Interpreting and Evaluating Experimental Results with Experimental Design 9 points Crossing over in meiosis I is required for homologous chromosomes to properly align during metaphase and segregate during the first cell division. Some regions of a chromosome called hotspots display a higher frequency of crossing over than other regions do. Crossing over is suppressed in chromosomal regions near the centromeres. The centromere region of a duplicated chromosome includes a collection of proteins that form a structure called the kinetochore. Scientists hypothesized that one or more of these kinetochore proteins are responsible for suppressing crossing over around the centromere. To investigate their hypothesis, scientists modified chromosome 8 in yeast such that, in each cell, one chromosome from the pair of homologous chromosome 8s contained the gene encoding red fluorescent protein (RFP), while the other chromosome from the pair contained the gene encoding green fluorescent protein (GFP). Cells expressing RFP emit (give off) red light, and cells expressing GFP emit green light. Models of the modified chromosome 8 both before and after crossing over are shown in Figure 1. Figure 1. Models of modified chromosome 8 used in the experiment (A) before and (B) after crossing over occurs at the hotspot The scientists then investigated whether attaching individual kinetochore proteins to a specific DNA sequence present in a known crossing-over hotspot on chromosome 8 affected the frequency of crossing © 2024 College Board AP® Biology 2024 Scoring Guidelines over at this location. In their first experiment, they examined three groups of yeast cells containing the modified chromosome 8. Group 1 contained no kinetochore proteins attached to the hotspot, group 2 contained the kinetochore protein CTF attached to the hotspot, and group 3 contained the kinetochore protein IML attached to the hotspot. For each group, the scientists determined the frequency of crossing over between the RFP and GFP genes. To determine the frequency, the scientists added the number of cells emitting both red and green light to the number of cells that emitted no light and divided by the total number of cells (Figure 2). Figure 2. The frequency of crossing over in a hotspot on yeast chromosome 8 for cell groups treated with different kinetochore proteins. Error bars represent ±2SE X . (a) Describe the function of S phase of interphase. 1 point Accept one of the following: • (The function of S phase is to) replicate/duplicate/synthesize the DNA/chromosomes. • (The function of S phase is to) double the amount of DNA. Explain why some haploid cells formed after meiosis in this experiment will have only 1 point one fluorescent marker. Accept one of the following: • Some cells will receive a chromosome/(sister) chromatid that did not undergo crossing over/recombination (in the hotspot). • Two (or a multiple of two) crossing-over events occurred (in the hotspot). • Each daughter cell will receive one of the four sister chromatids. Two of the sister chromatids shown in Figure 1 have only one fluorescent protein gene. • The two chromosomes that were not involved in crossing over will have only the RFP gene or the GFP gene. After meiosis II , a gamete with one of these chromosomes will have only the RFP or the GFP gene. Total for part (a) 2 points (b) Identify the control group for the scientists’ first experiment, shown in Figure 2. Accept one of the following: • Group 1 • (The group called) None • Cells with no (kinetochore) protein (attached to the chromosome) 1 point © 2024 College Board AP® Biology 2024 Scoring Guidelines In a follow-up experiment, the scientists created a modified version of CTF in which 1 point the DNA-binding portion had been removed. They compared the frequency of crossing over in yeast cells in the presence and absence of unmodified CTF with that in yeast cells in the presence and absence of the modified CTF protein (data not shown). In the follow-up experiment, justify why the scientists used a modified CTF protein that is unable to bind to DNA as a control. Accept one of the following: • (Using a modified CTF enabled the scientists) to determine whether DNA binding of the CTF/kinetochore protein affects/inhibits crossing over/recombination. • (Using a modified CTF enabled the scientists) to determine whether just the presence of the CTF/kinetochore protein is enough to affect/inhibit the crossing over/recombination frequency. 1 point Identify the independent variable in the follow-up experiment. Accept one of the following: • The type of CTF used • The presence or absence of (modified/unmodified) CTF Total for part (b) 3 points (c) (d) Based on Figure 2, describe the effect on the frequency of crossing over when CTF is 1 point attached to the chromosome 8 hotspot compared with the effect when IML is attached to the hotspot. Accept one of the following: • CTF attachment results in a decreased/lower frequency of crossing over/recombination, (whereas IML had no effect). • IML attachment results in an increased/a higher frequency of crossing over/recombination (compared with CTF attachment). Predict the effect on the number of copies of chromosome 8 likely to be present in 1 point the resulting daughter cells when CTF is attached to the hotspot. Accept one of the following: • There will be zero/two (copies). • There will be one less/one extra (copy). Provide reasoning to justify your prediction. 1 point • Cells (with attached CTF molecules) undergo crossing over/recombination at a lower frequency, so it is more likely that nondisjunction would occur/chromosomes would not separate properly. Explain how the presence of hotspots (Figure 1) could increase the likelihood that a 1 point population will survive in the presence of selective pressures. • Hotspots would increase genetic diversity; therefore, it would be more likely that some individuals would survive and reproduce. Total for part (d) 3 points Total for question 1 9 points © 2024 College Board Q1 Sample A 1 of 3 Q1 Sample A 2 of 3 Q1 Sample A 3 of 3 Q1 Sample B 1 of 2 Q1 Sample B 2 of 2 Q1 Sample C 1 of 2 Q1 Sample C 2 of 2 AP® Biology 2024 Scoring Commentary Question 1 Note: Student samples are quoted verbatim and may contain spelling and grammatical errors. Overview The stimulus of Question 1 explained that crossing over is necessary for proper alignment and segregation of chromosomes during meiosis I , and that crossing over frequency is higher in chromosomal regions called hotspots. Figure 1 showed a pair of homologous chromosomes labeled with fluorescent proteins. The primary experiment measured crossing-over frequency in the presence of different kinetochore proteins. A follow-up experiment then tested modification of one of these proteins to determine what function/domain of the protein might be affecting the crossing-over frequency. In part (a)(i) responses were expected to describe the function of the S phase of interphase (Science Practices Skill 1.A; Learning Objective [LO] IST-1.B from the AP Biology Course and Exam Description [CED]). In part (a)(ii) responses were expected to explain why some of the haploid cells resulting from meiosis in this experiment would have only one fluorescent marker (Skill 1.C; LO IST-1.F). In part (b) responses were expected to demonstrate an understanding of experimental design (Skill 3.C). In part (b)(i), responses were expected to identify a control group. In part (b)(ii), responses were expected to justify the purpose for a control treatment in the context of the follow-up experiment, and in part (b)(iii), responses were expected to identify the independent variable in this follow-up experiment. In part (c) responses were expected to describe data from a graph (Figure 2) that presented results of the first experiment (Skill 4.B). In part (d)(i), responses were expected to predict that, when a kinetochore protein that reduces crossing over is attached to the chromosomal hotspot, resulting daughter cells would be more likely to have a missing or extra chromosome (Skill 3.B; LO SYI-3.C). In part (d)(ii) responses were expected to justify the prediction by reasoning that nondisjunction would be more likely (Skill 6.C; LO SYI-3.C). In part (d)(iii) responses were expected to explain the relationship between the experimental results and a broader biological theory by explaining that crossing over could increase the likelihood of a population surviving “in the presence of selective pressures” (Skill 6.D; LOs EVO1.E, SYI-3.D, and IST-1.H). Sample: 1A Score: 8 The response earned 1 point in part (a)(i) for describing the function of S phase as “DNA is replicated.” The response earned 1 point in part (a)(ii) for explaining that “not all chromatids experience crossing over” and some gamete cells “will have chromosomes with...just one fluorescent protein marker.” The response earned 1 point in part (b)(i) for identifying Group 1. The response earned 1 point in part (b)(ii) for justifying that “it can show them that it is the CTF protein binding to the DNA that is causing the decrease in frequency of crossing over.” The response earned 1 point in part (b)(iii) for identifying “the presence or absence of either modified or unmodified CTF protein.” The response earned 1 point in part (c) for describing that “There is a decrease in the frequency of crossing over when CTF is attached to the...hotspot.” The response did not earn a point in part (d)(i) because it does not predict a specific difference in the number of copies of chromosome 8. The response earned 1 point in part (d)(ii) for justifying… that “Since CTF protein decreases the frequency of crossing over, there is then a higher chance... [of] nondisjunction.” The response earned 1 point in part (d)(iii) for explaining that hotspots increase “genetic variation” and, as a result, “one individual might be fitter than another.” © 2024 College Board. Visit College Board on the web: collegeboard.org. AP® Biology 2024 Scoring Commentary Question 1 (continued) Sample: 1B Score: 6 The response earned 1 point in part (a)(i) for correctly describing the function of S phase. The response did not earn a point in part (a)(ii) because even though it explains that “not all genes are transferred,” this will not necessarily produce a sister chromatid with only one copy of the fluorescent marker. There is no mention that the chromatid that did not cross over was the one passed on to the cell. The response earned 1 point in part (b)(i) for identifying group 1 as the control group. The response did not earn a point in part (b)(ii) because it does not justify that the control determines whether the binding of CTF to the DNA inhibits crossing over frequency. The response earned 1 point in part (b)(iii) for correctly identifying the independent variable. The response earned 1 point in part (c) for describing the effect on the frequency as “lower.” The response earned 1 point in part (d)(i) for predicting that there will be “2 daughter cells with 2 copies of chromosome 8” (and at least one cell with zero copies of chromosome 8). The response earned 1 point in part (d)(ii) justifying the prediction because of the increased probability of nondisjunction due to lower crossing over frequency. Sample: 1C Score: 3 The response earned 1 point in part (a)(i) for describing that the function of the S phase “is for DNA replication.” The response did not earn a point in part (a)(ii) because it does not explain why some cells will have only one fluorescent marker. The response earned 1 point in part (b)(i) for identifying the control group as “the group...that contained no kinetochore proteins.” The response did not earn a point in part (b)(ii) because it does not correctly justify why the modified CTF was used as a control. The response did not earn a point in part (b)(iii) because it incorrectly identifies the independent variable. The response earned 1 point in part (c) for describing the effect as a lower frequency of crossing over when CTF is attached. The response did not earn a point in part (d)(i) because it incorrectly predicts “more chromosome 8 copies.” The response did not earn a point in part (d)(ii) because it does not correctly justify the prediction. The response did not earn a point in part (d)(iii) because it does not explain that an increase in genetic variation increases the likelihood of individuals surviving and reproducing, © 2024 College Board. Visit College Board on the web: collegeboard.org.