2024 AP Biology Free-Response Questions

1. Crossing over in meiosis I is required for homologous chromosomes to properly align during metaphase and segregate during the first cell division. (a) (i) Describe the function of S phase of interphase. 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 figure shows paired homologous chromosomes with centromeres labeled. In (A), before crossing over, one homolog carries the gene encoding RFP at its distal end, and the other homolog carries the gene encoding GFP at its distal end; the hotspot is located between the centromere and the fluorescent-protein genes. In (B), after crossing over within the hotspot, the chromatids have exchanged segments distal to the hotspot. (ii) Explain why some haploid cells formed after meiosis in this experiment will have only one fluorescent marker. 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 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_x̄. The bar graph shows the frequency of crossing over (%) on the y-axis (0–8) and the attached kinetochore protein treatment on the x-axis (None, CTF, IML). The None bar reaches approximately 7%, the CTF bar reaches approximately 6%, and the IML bar reaches approximately 7%. (b) (i) Identify the control group for the scientists' first experiment, shown in Figure 2. (ii) In a follow-up experiment, the scientists created a modified version of CTF in which 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. (iii) Identify the independent variable in the follow-up experiment. (c) Based on Figure 2, describe the effect on the frequency of crossing over when CTF is attached to the chromosome 8 hotspot compared with the effect when IML is attached to the hotspot. (d) (i) Predict the effect on the number of copies of chromosome 8 likely to be present in the resulting daughter cells when CTF is attached to the hotspot. (ii) Provide reasoning to justify your prediction. (iii) Explain how the presence of hotspots (Figure 1) could increase the likelihood that a population will survive in the presence of selective pressures. $\textcircled{c}$ 2024 College Board. Visit College Board on the web: collegeboard.org.