57. Photosynthesis ceases when leaves wilt, mainly because___________.
(A) the chlorophyll of wilting leaves breaks down
(B) flaccid mesophylI cells are incapable of photosynthesis
(C) stomata close, preventing CO₂ from entering the leaf
(D) photolysis. the water-splitting step of photosynthesis, cannot occur when there is a water deficiency
(E) accumulation of CO₂in the leaf inhibits enzymes 

53. Which of the following vitamins is correctly associated with its use? (A) Vitamin C - curing rickets (B) Vitamin A - incorporated into the visual pigment of the eye (C) Vitamin D - calcium removal from bone (D) Vitamin E - protection of skin from cancer (E) Vitamin K - production of white blood cells

54. The MHC (Major Histocompatibility Complex) is important in a T cell's ability to___________. (A) distinguish self from nonself (B) recognize specific parasitic pathogens (C) identify specific bacterial pathogens (D) identify specific viruses (E) recognize differences among types of cancer

55. Which combination of hormones helps a mother to produce milk and nurse her baby? (A) Prolactin and calcitonin (B) Oxytocin and prolactin (C) Follicle-stimulating hormone and luteinizing hormone (D) Luteinizing hormone and oxytocin (E) Oxytocin, prolactin, and luteinizing hormone

56. Dog breeders maintain the purity of breeds by keeping dogs of different breeds apart when they are fertile. This kind of isolation is most similar to which of the following reproductive isolating mechanisms? (A) Reduced hybrid fertility (B) Hybrid breakdown (C) Mechanical isolation (D) Habitat isolation (E) Gametic isolation

58. Which of the following pathways is most likely taken by newly synthesized histones? (A) Rough endoplasmic reticulum →Golgi complex →secretory vesicle (B) Rough endoplasmic reticulum →Golgi complex →nucleus (C) Rough endoplasmic reticulum → smooth endoplasmic reticulum →nucleus (D) Cytoplasm → nucleus (E) Cytoplasm →rough endoplasmic reticulum →Golgi complex nucleus

59. In the communication link between a motor neuron and a skeletal muscle, which of the following descriptions is right? (A) The motor neuron is considered the presynaptic cell and the skeletal muscle is the postsynaptic cell. (B) The motor neuron is considered the postsynaptic cell and the skeletal muscle is the presynaptic cell. (C) Action potentials are possible on the motor neuron but not the skeletal muscle. (D) Action potentials are possible on the skeletal muscle but not the motor neuron. (E) The motor neuron fires action potentials but the skeletal muscle is not electrochemically excitable,

60. In nerves, vesicles can move the length of an axon at a rate that far exceeds that which would be predicted for simple diffusion. Which of the following models best explains vesicular movement in these cells? (A) Depolymerization of actin microfilaments attached to vesicles pulls the vesicles toward the site of depolymerization. (B) Vesicles are propelled by fluid movement generated by changes in osmotic potential within the cells. (C) Vesicles are moved by alternate contraction and relaxation of actin-myosin "muscle" complexes. (D) Vesicles, by virtue of their net negative charge, are attracted to positively charged regions of the cell. (E) Vesicles are attached to the protein kinesin, which slides along microtubules by an ATP dependent process.

61. Which kinds of the following nucleic acid can form viral genome? (1) double-stranded DNA(2) single-stranded DNA(3) single-stranded RNA (A)(1) only (B) (1) and (2) (C)(1) and(3) (D) None of the above is correct (E) All of the above are correct

62. Retroviral vectors are more popular for somatic gene therapy than other viral vectors because: (A) They replicate faster than most other viruses. (B) They contain several copies of their DNA genome in the virus particle. (C) They can integrate themselves into the host cell DNA. (D) Their replication is more accurate than that of most other viruses. (E) Their DNA has extensive sequence homology with normal cellular DNA.

63. cAMP regulates the transcription of many genes. What is the major mechanism for this action? (A) It induces the phosphorylation of transcription factors. (B) It binds directly to cAMP response elements in promoters and enhancers. (C) It mediates this effect by increasing the calcium concentration in the cytoplasm and the nucleus. (D) It binds directly to nuclear transcriptionfactors. (E) It induces the phosphorylation of STAT proteins, thus enabling them to translocate into the nucleus.