21. Which structure prevents blood flowing back into the atria during ventricular contractions?
(A)atrioventricular valves
(B)semilunar valves
(C)sinoatrial node
(D) carotid blood shunts
(E) foramen ovale

17. Which of the following statements best describes DNA polymerase? (A)It is an enzyme required to produce a primer needed for DNA replication. (B) It is an enzyme that catalyzes the addition of nucleotides to the 5′ end of a growing DNA strand. (C) It is an enzyme that catalyzes the addition of nucleotides to the 3′ end of a growing DNA strand. (D) It is an enzyme required to glue pieces of DNA fragments together. (E) It is an enzyme that polymerizes the DNA and keeps the replication fork open.

18. A DNA specimen that contains 30% guanine has ________ thymine. (A)10% (B)20% (C)30% (D) 40% (E) The amount of thymine cannot be determined.

19. Bile is crucial for ______ digestion because of its function in _____. (A)protein, protecting the stomach lining (B)fat, solubilizing glycerol:fatty acid bonds (C)nucleic acids, uncoiling DNA strands (D) fat, emulsification (E) protein, lyophilization

20. Which animal would you predict would have the highest mass-specific metabolic rate? (A)a lizard (B)a squid (C)an elephant (D) a goose (E) a hummingbird 

22. Which of the following statements about RNA polymerase in bacteria is INCORRECT? (A)It binds the promoter region of a gene. (B)It is bound to a sigma factor during initiation of transcription. (C)It synthesizes RNA. (D) It catalyzes the addition of nucleotides to the 5′end of a growing RNA strand. (E) It dissociates from the DNA at the terminator.

23. Surgical removal of the pancreas would (A)increase the acidity of the small intestine. (B)prevent the production of chyme in the stomach. (C)eliminate the ability to store bile in the gallbladder. (D) block the secretion of bile. (E) All of these changes would be a result of pancreas removal.

24. What protein is involved in synthesizing messenger RNA in eukaryotes? (A)RNA polymerase I (B)RNA polymerase II (C)RNA polymerase III (D) sigma factor (E) both RNA polymerase I and II

25. The respiratory centers that control ventilation of the lungs in mammals are located in the (A)brainstem. (B)hypothalamus. (C)cerebral cortex. (D) bronchi. (E) neck.

26. The ascending portion of the loop of Henle is (A)permeable to water and impermeable to sodium. (B)permeable to water and permeable to sodium. (C)impermeable to water and permeable to sodium. (D) impermeable to water and impermeable to sodium. (E) the site of active transport of water.

27. MicroRNAs (miRNAs) (A)are long RNA molecules. (B)promotes post-translational modification of proteins. (C)are found only in animals. (D) are cut by an enzyme called transcriptase. (E) silence the expression of specific mRNAs.