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.

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.

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.

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

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.

57In a series of mapping experiments, the recombination frequencies for four different linked genes of Drosophila were determined as shown in the figure. What is the order of these genes on a chromosome map? (A) cn-rb-b-vg (B) b-rb-cn-vg (C) rb-cn-vg-b (D) vg-b-rb-cn (E) vg-cn-b-rb

58. The steps below refer to various stages in transmission at a chemical synapse: 1. The synaptic vesicles release neurotransmitter into the synaptic cleft. 2. The ligand-gated ion channels open. 3. An action potential depolarizes the membrane of the axon terminal. 4. Calcium ions rush into neuron's cytoplasm. 5. Neurotransmitter binds with receptors associated with the postsynaptic membrane. Which sequence of events is CORRECT? (A) 1 → 2 → 3 → 4 → 5 (B) 3 → 4 → 1 → 5 → 2 (C) 3 → 1 → 5 → 4 → 2 (D) 4 → 3 → 1 → 2 → 5 (E) 5 → 1 → 2 → 4 → 3

59. Eukaryotic telomeres replicate differently than the rest of the chromosome. Which of the following is the cause? (A) DNA polymerase that cannot replicate the leading strand template to its 5' end. (B) The evolution of telomerase enzyme. (C) Gaps left at the 3' end of the lagging strand because of the need for a 5' onto which nucleotides can attach. (D) Gaps left at the 3' end of the lagging strand because of the need for a primer. (E) Gaps left at the 5' end of the lagging strand because of the need for a 3' onto which nucleotides can attach.