Erythropoietin (EPO) is a glycoprotein hormone that stimulates the production of erythrocytes in red bone marrow. Although EPO is primarily produced and released by the kidneys in response to low tissue levels of oxygen, several other tissues, including the liver and neurons of the central nervous system (CNS), can produce EPO. EPO binds the EPO receptor (EPOR) in erythrocyte precursor cells, causing them to differentiate into mature erythrocytes that are released into circulation. As the oxygen-carrying capacity of the blood increases, secretion of EPO by the kidneys decreases. Neurons in the CNS also express EPOR, and EPO has been shown to decrease apoptosis of both erythrocyte precursor cells and CNS neurons. EPO also promotes angiogenesis, the production of new blood vessels. The human EPO gene has been cloned and expressed in vitro. The recombinant gene product (rHuEPO) is frequently administered to patients who have anemia resulting from either kidney failure or chemotherapy. More recently, it has been shown that a significant number of tumors express EPOR, even though the healthy tissue from which the tumor was derived does not. Experiments in mice indicate that EPO prevents apoptosis and increases angiogenesis in at least some types of EPOR-positive tumors. Expression of the rHuEPO gene in E. coli bacteria produced an EPO protein that did not increase erythrocyte production when injected into humans. The most likely reason for this observation is that: O A. prokaryotic ribosomes interpret the genetic code in a completely different manner than do eukaryotic ribosomes. O B. E. coli cannot glycosylate EPO in the same way that it is glycosylated by eukaryotic cells. O C. bacteria are unable to secrete eukaryotic proteins. O D. only viruses contain the necessary cellular machinery to properly express recombinant proteins. Assume that a certain dominant mutation in the EPO gene exists such that a person who carries this mutant EPO allele has a higher- than-normal number of circulating erythrocytes. Which of the following best describes a mechanism by which this mutation could have its effect? O A. The promoter of the mutant EPO allele is defective, and the allele is not transcribed. OB. The mutant EPO allele produces a protein that has an increased affinity for EPOR. O C. The mRNA produced by the mutant EPO allele is degraded before translation can occur. O D. The mutant EPO allele produces a protein that is unable to bind EPOR. In which of the following cellular locations does EPO most likely initially bind EPOR in erythrocyte precursor cells? O A. Cytosol O B. Endoplasmic reticulum O C. Nucleus OD. Plasma membrane Certain types of kidney tumors continuously produce and release EPO. Such tumors most likely have which of the following effects, if any, on erythrocyte production? O A. The liver will take over the process of regulating erythrocyte production. OB. Erythrocyte production within the bone marrow will cease. O C. Constant stimulation of erythrocyte production will occur within the bone marrow. O D. There is no effect; erythrocyte production will continue to be regulated by the kidneys based on oxygen levels within the body. An increase in which of the following physiological variables is most likely to cause an increase in the amount of EPO released by the kidneys in a healthy human adult? O A. Amount of aerobic exercise the person performs OB. Total amount of circulating hemoglobin O C. Rate of erythrocyte maturation O D. Cardiac output (volume of blood pumped by the heart per minute)