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Caenorhabditis elegans is an approximately 1mm long nematode (worm) that lives in the soil and eats bacteria such as Pseudomonas. Scientists have determined that when C. elegans is hunting for food, the worms have different patterns of movement that depend on the location and quality of the food source. Scientists hypothesized that C. elegans might be attracted to certain chemicals produced by Pseudomonas. They tested this hypothesis by placing 100 C. elegans in the center of each of a series of petri dishes containing a layer of agar over which the worms could move. A drop of a test chemical diluted in ethanol was placed at one edge of each dish and an equal-sized drop of ethanol was placed at the opposite edge of the dish. The scientists determined the number of worms to reach each spot over a period of time. (a) Describe why the location and quality of food sources in the soil are sometimes likely to stimulate random movements by C. elegans, but at other times are more likely to stimulate directed movements. (b) Justify the scientists’ use of two different spots on each petri dish and the counting of worms at each of the two spots. (c) Under harsh environmental conditions, C. elegans produces a pheromone, a complex of molecules that affects other C. elegans. One effect of the pheromone is to cause developing larvae to temporarily arrest as larvae rather than mature to reproductive adults. Predict the relative amount of pheromone most likely produced by C. elegans that are only weakly attracted to one of the test chemicals. (d) Provide reasoning to justify your prediction with respect to how the predicted amount of pheromone is beneficial to the worms that are only weakly attracted to the chemical.

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At the start of mitosis, sister chromatids are held together by a complex of proteins. Separase is an enzyme that cleaves the complex, enabling the chromatids to separate during mitosis. Separase is overexpressed in many cancer cells, and scientists hypothesized that they might be able to slow or stop the growth of cancer cells by blocking the activity of separase. The scientists found a compound they named Sepin‑1 that appears to effectively cleave and thus inactivate purified separase protein in vitro (in a test tube). To test whether Sepin‑1 inhibits the growth of cancer cells, the scientists added increasing concentrations of Sepin‑1 to many different types of cancer cell lines growing in culture. A representative sample of the data they obtained is shown in Figure 1. The scientists also proposed to examine whether there is a relationship between the sensitivity of different types of cancer cells to Sepin-1, as measured by the concentration of Sepin-1 that caused 50% of the cells to die, and the relative concentration of separase in the different cell lines. A representative sample of the data is shown in Table 1. Identify the point in mitosis at which separase cleaves the protein complex that holds sister chromatid pairs together. In normal cells, separase is kept in an inactive state until it is needed. Explain how the progression of cells past sequential cell cycle checkpoints and the activity of enzymes such as separase is controlled by interactions between two major groups of regulatory proteins.

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An agricultural biologist was evaluating two newly developed varieties of wheat as potential crops. In an experiment, seedlings were germinated on moist paper towels at 20ºC for 48 hours. Oxygen consumption of the two-day-old seedlings was measured at different temperatures. The data are shown in the graph below. An agricultural biologist was evaluating two newly developed varieties of wheat as potential crops. In an experiment, seedlings were germinated on moist paper towels at 20ºC for 48 hours. Oxygen consumption of the two-day-old seedlings was measured at different temperatures. The data are shown in the graph below. In a second experiment, variety A seedlings at both temperatures were treated with a chemical that prevents NADH from being oxidized to NAD+. Predict he most likely effect of the chemical on metabolism and oxygen consumption of the treated seedlings. Explain your prediction

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