49. Which of the following terms best characterizes catabolite repression associated with the lactose operon in E. coli?
(A) inducible system.
(B) repressible system.
(C) negative control.
(D) positive control.
(E) constitutive.

45. During meiosis ll, (A) cytokinesis results in the formation of a total of two cells. (B) sister chrorriatids of each chromosome are separated from each other. (C) homologous chromosomes pair up. (D) homologous chromosomes separate. (E) sister chromatids exchange parts.

46. Which of these is NOT found in the epidermis? (A) stratified epithelium.(B) blood vessels.(C) tight cell junctions. (D) keratin.  (E) melanin.

47. The first vertebrates are most closely related phylogenetically to which of the following phyla? (A) Echinodermata. (B) Annelida. (C) Mollusca. (D) Arthropoda. (E) None of these.

48. The classic Hershey and Chase (1952) experiment which offered evidence in support of DNA being the genetic material in bacteriophage made use of which of the following labeled components? (A) phosphorus and sulfur. (B) phosphorus and nitrogen. (C) phosphorus and oxygen. (D) nitrogen and oxygen. (E) nitrogen and sulfur.

50. Developmental similarities observed in most vertebrates are probably due to (A) chance. (B) the similar environments in which they live. (C) homeoboxes. (D) their phylogenetic histories. (E) homeoboxes and their phylogenetic histories.

51. Which of the following statements concerning biogeochemical cycles is NOT correct? (A) Plants and algae can use two inorganic forms of nitrogen: ammonium and nitrate. (B) Photosynthesis by plants and phytoplankton removes substantial amounts of atmospheric CO2 each year. (C) The largest accumulations of phosphorus are in sedimentary rocks of marine origin. There are also large quantities of phosphorus in soils, in the oceans and in organisms. (D) Under anaerobic conditions, nitrifying bacteria use NO3- in their metabolism instead of O2, releasing N2 in a process known as nitrification. (E) The major pathway for nitrogen to enter an ecosystem is via nitrogen fixation, the conversion of N2 by bacteria to forms that can be used to synthesize nitrogenous organic compounds.

52. The breathing in human is controlled by all of the following mechanisms EXCEPT (A) The sensors in the walls of the aorta and carotid arteries in the neck detect changes in blood pH and send nerve impulses to the medulla. (B) The sensors in the aorta and carotid arteries also detect changes in O2 levels in the blood and signal the medulla to increase the breathing rate. (C) Nerves from a breathing control center in the medulla oblongata of the brain send impulses to the diaphragm and rib muscles, stimulating them to contract and causing inhalation. (D) Sensors in the medulla detect changes in the pH of the blood and interistial fluid bathing the surface of the brain. (E) The control center in the medulla sets the basic rhythm, and a control center in the pons moderates it, smoothing out the transitions between inhalations and exhalations.

53. Which of the following statements concerning appetite-regulating hormones is NOT correct? (A) Leptin suppresses appetite as its level increase. (B) The hormone PYY, secreted by the small intestine after meals, acts as an appetite suppressant. (C) Insulin can suppress appetite by acting on the brain. (D) Ghrelin is one of the signals that triggers feelings of hunger as mealtimes approach. (E) Amino acids or fatty acid in the duodenum trigger the release of Cholecystokinin (CCK), an appetite suppressant.

54. Which of the following statements about both C4 and crassulacean acid metabolism (CAM) plant is INCORRECT? (A) Both adaptations are characterized by preliminary incorporation of CO2 into organic acids, followed by transferring of CO2 to the Calvin cycle. (B) In C4 and CAM plants, carbon fixation and the Calvin cycle occur in the same cells at different time. (C) The C4 and CAM pathways are two evolutionary solutions to the problem of maintaining photosynthesis with stomata partially or completely closed on hot, dry days. (D) The C4 plants are so named because they preface the Calvin cycle with an alternate mode of carbon fixation that forms a four-carbon compound as its first product. (E) The mesophyll cells of CAM plants store the organic acids they make during the night in their vacuoles until morning, when the stomata close.

55. Which of following statements concerning the mechanism of long-term potentiation (LTP) in the vertebrate brain is INCORRECT? (A) LTP involves an increase in the strength of synaptic transmission that occurs when presynaptic neurons produce a brief, high-frequency series of action potentials. (B) The presynaptic neuron release acetylcholine (ACh). ACh binds to its receptor, opening the receptor channel  and depolarizing the postsynaptic membrane. (C) Ca2 + diffuses into the postsynaptic neuron. Ca2 + initiates the phosphorylation of the receptor, making them more responsive. (D) stimulates the postsynaptic neuron to produce NO. (E) NO diffuses into the presynaptic neuron, causing it to release more neurotransmitters.