50. Whether during mitosis or meiosis, sister chromatids are held together by proteins referred to as cohesions. Such molecules must have which of the following properties?
(A) They must reattach to chromosomes during G1.
(B) They must be removed before anaphase can occur.
(C) They must be intact for nuclear envelope reformation.
(D) They must be removed before meiosis can begin.
(E) They must persist throughout the cell cycle.

46. El Niño events (A) occur when the Southern Oscillation index is high. (B) occur when barometric pressure is lower in the western Pacific than in the eastern Pacific. (C) include the appearance of warm currents on the Pacific coast of South America. (D) are always accompanied by La Niña events at the same time. (E) are accompanied by westward movement of the location of storm generation in the Pacific.

47. Mycorrhizal fungi (directly) help their plant partners acquire (A) sugars. (B) sunlight. (C) seed dispersal. (D) soil nutrients. (E) pollination.

48. dN/dt = rmaxN [(K – N)/K] represents (A) annual growth rate. (B) geometric population growth. (C) exponential growth rate. (D) logistic growth. (E) none of the choices are correct.

49. Bony marine fish (A) drink sea water. (B) secrete Na+ into the surrounding water. (C) are hyperosmotic compared to the surrounding water. (D) both drink sea water and secrete Na+ into the surrounding water. (E) drink sea water, secrete Na+ into the surrounding water, and are hyperosmotic compared to the surrounding water.

51. Analysis of the Lotka-Volterra competition model implies that two competitors can coexist only when (A) interspecific competition is stronger than intraspecific competition. (B) intraspecific competition is stronger than interspecific competition. (C) intraspecific and interspecific competition are equally strong. (D) predation or parasitism is stronger than interspecific competition. (E) actually, the model implies that two competitors can never coexist. 

52. Mouse mutations can affect an animal's appetite and eating habits. The ob gene produces a satiety factor (the hormone leptin). The db gene product is required to respond to the satiety factor (the leptin receptor). Most obese humans produce normal or increased levels of leptin without satiety. Which might provide an answer to at least some human obesity if a means to do so is found? (A) overexpression of the leptin receptor gene (B) activation of receptors for leptin (C) supplementary leptin (D) inactivation of leptin (E) mutation of the leptin receptor gene

53. The "immunotolerance" of a pregnant woman toward her unborn child is the result of (A) the complete physical separation from her cells and those of the unborn child. (B) the tenacity with which the unborn child's immune system counteracts the woman's immune system. (C) the unborn child having enough of the woman's identity so as to escape detection as foreign. (D) modern medical intervention during every pregnancy. (E) the relative quiescence of a pregnant woman's immune system compared to when she was not pregnant.

54. A mutation that inactivates the regulatory gene of a repressible operon in an E. coli cell would result in (A) complete inhibition of transcription of the structural gene controlled by that regulator. (B) continuous transcription of the structural gene controlled by that regulator. (C) irreversible binding of the repressor to the operator. (D) inactivation of RNA polymerase by alteration of its active site. (E) continuous translation of the mRNA because of alteration of its structure.

55. The alternative pathways of photosynthesis using the C4 or CAM systems are said to be compromises. What is the reason? (A) CAM plants allow more water loss, while C4 plants allow less CO2 into the plant. (B) C4 plants allow less water loss but CAM plants allow more water loss. (C) C4 and CAM plants both minimize photorespiration and optimizes the Calvin cycle. (D) C4 and CAM plants minimize both water loss and rate of photosynthesis. (E) C4 plants compromises on water loss and CAM compromises on photorespiration.

56. The role of a metabolite that controls a repressible operon is to (A) increase the production of inactive repressor proteins. (B) bind to the repressor protein and activate it. (C) bind to the repressor protein and inactivate it. (D) bind to the operator region and block the attachment of RNA polymerase to the promoter. (E) bind to the promoter region and decrease the affinity of RNA polymerase for the promoter.