43. Which of these statements correctly describes genetic engineering?
(A) Only breeding from crop plants that are resistant to pests.
(B) Using yeast to produce ethanol.
(C) Altering the DNA in crop plants so that they are resistant to herbicides.
(D) Production of insulin in the pancreas.
(E) Grafting branches from one fruit tree onto another.

44. In a cross of two individuals, both of whom have genotype AaBb, what proportion of the offspring are expected to have the genotype AABB? (A) 9/16 (B) 1/8 (C) 1/16 (D) 1/4 (E) 3/16

45. G protein-coupled receptors (GPCRs) are the most prominent and diverse family of cell-surface receptors and are therefore targets for approximately 40% of marketed drugs. The following is a typical scenario of GPCR-mediated signaling pathway: GPCR receives extracellular signals → activated GPCR acts as a guanine nucleotide exchanger (GEF) → Galpha subunit of G protein releases GDP and binds GTP → Galpha subunit binds and activates its target enzyme upon GTP hydrolysis → a secondary messenger is generated → another protein is activated → cellular responses follow. Which of the following is a common secondary messenger used in the scenario? (A) sodium ions (Na+) (B) magnesium ions (Mg2+) (C) nitric oxide (NO) (D) guanine monophosphate (GMP) (E) cyclic adenosine monophosphate (cAMP)

46. The global monitoring of influenza viruses, such as H1N1 and H5N1, is a primary focus of public health biology. Influenza viruses are classified based on two primary variables, hemagglutinin (H) and neuraminidase (N). Which of the following statements about influenza viruses is correct? (A) Influenza viruses are double-stranded DNA viruses that infect mainly humans. (B) H5N1 is currently classified as an avian flu that mainly infects birds and can cause severe disease in humans, but usually does not spread easily between people. (C) The "H" and "N" in H5N1 refer to the numbers of viral strains and enzymes, respectively. (D) The Influenza B virus is responsible for most influenza pandemics. (E) New influenza subtypes arise when small genetic mutations allow viral RNA segments to mix between different strains during infection of the same host cell.

47. The digestion and absorption of dietary fats require several coordinated processes in the human digestive system. Which of the following statements best describes how fats are digested and absorbed? (A) Bile salts emulsify fats into micelles within the intestinal lumen, allowing them to be absorbed directly into the intestinal capillaries. (B) Bile salts emulsify fats into chylomicrons within the intestinal lumen, allowing them to be absorbed directly into the hepatic portal vein. (C) Fatty acids and monoglycerides are absorbed into intestinal epithelial cells, reassembled into triglycerides, packaged into chylomicrons, and transported through lacteals before eventually entering the bloodstream. (D) Fatty acids and monoglycerides are absorbed into the intestinal capillaries and transported via the hepatic portal vein to the liver for storage. (E) Fatty acids and monoglycerides are absorbed into intestinal epithelial cells, reassembled into triglycerides, packaged into micelles, and transported into the intestinal capillaries.

48. Helper T cells play a central role in coordinating adaptive immunity. Which of the following is NOT an outcome after a naive helper T cell is activated by an antigen-presenting cell? (A) Cytotoxic T cells are activated and subsequently destroy infected cells. (B) B cells are activated and subsequently differentiate into memory B cells and antibody-secreting plasma cells. (C) Activated helper T cells secrete cytokines and undergo clonal expansion. (D) Natural killer cells are activated and subsequently destroy infected cells. (E) Clonal helper T cells secrete cytokines that activate other immune cells.

49. The ras and p53 genes encode key proteins that function in cell-signaling pathways. Mutations in ras occur in about 30% of human cancers, and mutations in p53 occur in more than 50%. Based on their physiological roles in regulating the cell cycle, the ras gene is classified as a(n) __________, and the p53 gene is a(n) __________. (A) proto-oncogene; oncogene (B) oncogene; tumor-suppressor gene (C) proto-oncogene; tumor-suppressor gene (D) tumor-suppressor gene; proto-oncogene (E) tumor-suppressor gene; oncogene

50. The postsynaptic response at a chemical synapse depends on both the neurotransmitter released from the presynaptic neuron and the receptor present on the postsynaptic membrane. Which of the following statements about neurotransmitters is NOT correct? (A) The magnitude of synaptic transmission can be influenced by the amount of neurotransmitter released. (B) Released neurotransmitter molecules can be taken back up into the presynaptic neuron. (C) A single neurotransmitter can activate both ionotropic and metabotropic receptors, producing distinct physiological effects. (D) In the adult human brain, the amino acid gamma-aminobutyric acid (GABA) is a neurotransmitter that typically depolarizes the postsynaptic neuron, making it more likely to fire an action potential. (E) Certain gaseous molecules, such as nitric oxide (NO), can act as retrograde neurotransmitters and are not stored in synaptic vesicles before release.

51. Often referred to as the "hunger hormone," __________ is a peptide hormone secreted primarily by endocrine cells of the stomach. Its concentration rises sharply before meals to stimulate appetite-regulating centers in the hypothalamus and declines following stomach distension after food intake. (A) glucagon (B) peptide YY (PYY) (C) leptin (D) ghrelin (E) insulin

52. A normal heartbeat begins with spontaneous electrical activity that coordinates atrial and ventricular contraction. Which of the following describes the correct physiological sequence of electrical conduction in the human heart? (A) signals from the sinoatrial (SA) node spreading through atria → signals delayed in the atrioventricular (AV) node → bundle branches passing signals to the heart apex → signals spreading upward throughout ventricles via Purkinje fibers. (B) signals from the atrioventricular (AV) node spreading through atria → signals delayed in the sinoatrial (SA) node → bundle branches passing signals to the heart apex → signals spreading upward throughout ventricles via Purkinje fibers. (C) signals from the sinoatrial (SA) node spreading through ventricles → signals delayed in the atrioventricular (AV) node → bundle branches passing signals to the heart apex → signals spreading upward throughout atria via Purkinje fibers. (D) signals from the sinoatrial (SA) node spreading through atria → bundle branches passing signals to the heart apex → signals delayed in the atrioventricular (AV) node → signals spreading upward throughout ventricles via Purkinje fibers. (E) signals from the atrioventricular (AV) node spreading through atria → bundle branches passing signals to the heart apex → signals delayed in the sinoatrial (SA) node → signals spreading upward throughout ventricles via Purkinje fibers.

53. Many predatory animals produce toxins to disable their prey. Some of these toxins inhibit nervous communication. Nerve impulses are transmitted along axons. Some axons are myelinated. Which type of cell produces myelin? (A) Neurone (B) Rod cell (C) Schwann cell (D) Astrocyte (E) Lymphocyte

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