19. In the nitrogen excretion and the urea cycle, which of the following amino acids are essential for humans?
(A) alanine
(B) aspartic acid
(C) asparagine
(D) serine
(E) threonine

20. In the reoxidation of QH2 by purified ubiquinone-cytochrome c reductase (Complex III) from heart muscle, the overall stoichiometry of the reaction requires 2 mol of cytochrome c per mole of QH2 because: (A) cytochrome c is a one-electron acceptor, whereas QH2 is a two-electron donor. (B) cytochrome c is a two-electron acceptor, whereas QH2 is a one-electron donor. (C) cytochrome c is water soluble and operates between the inner and outer mitochondrial membranes. (D) heart muscle has a high rate of oxidative metabolism, and therefore requires twice as much cytochrome c as QH2 for electron transfer to proceed normally. (E) molecules of cytochrome c must first combine physically before they are catalytically active.

21. During oxidative phosphorylation, the proton motive force that is generated by electron transport is used to: (A) create a pore in the inner mitochondrial membrane. (B) generate the substrates (ADP and Pi) for the ATP synthase. (C) induce a conformational change in the ATP synthase. (D)oxidize NADH to NAD+. (E) reduce O2 to H2O.

22. Photosynthetic phosphorylation and oxidative phosphorylation appear to be generally similar processes, both consisting of ATP synthesis coupled to the transfer of electrons along an electron carrier chain. Which of the following is not true of both processes? (A) Both contain cytochromes and flavins in their electron carrier chains. (B) Both processes are associated with membranous elements of the cell. (C) Both use oxygen as a terminal electron acceptor. (D) Each represents the major route of ATP synthesis in those cells in which it is found. (E) Protons are pumped from the inside to the outside of both mitochondria and chloroplast membranes.

23. Which of these can be synthesized by plants but not by humans? (A) Stearate (18:0) (B) Palmitate (16:0) (C) Phosphatidylcholine (D) Pyruvate (E) Linoleate [18:2(9,12)]

24. One amino acid directly involved in the purine biosynthetic pathway is: (A) alanine. (B) aspartate. (C) glutamate. (D) leucine. (E) tryptophan.

25. The synthesis of purine and pyrimidine nucleotides differ in that: (A) ATP is required in the synthesis of purines but not in the synthesis of pyrimidines. (B) purine biosynthesis starts with the formation of PRPP, whereas pyrimidines incorporate the PRPP near the end of the pathway. (C) purine formation requires a THF derivative, whereas pyrimidine formation does not. (D) pyrimidine biosynthesis is tightly regulated in the cell, whereas purine biosynthesis is not. (E) pyrimidines go through many steps, adding a single carbon or nitrogen each time, whereas the basic skeleton for purines is formed by two main precursors.

26. In base-excision repair, the first enzyme to act is: (A) AP endonuclease. (B) Dam methylase. (C) DNA ligase. (D) DNA glycosylase. (E) DNA polymerase.

27. In homologous genetic recombination, RecA protein is involved in: (A) formation of Holliday intermediates and branch migration. (B) introduction of negative supercoils into the recombination products. (C) nicking the two duplex DNA molecules to initiate the reaction. (D) pairing a DNA strand from one duplex DNA molecule with sequences in another duplex, regardless of complementarity. (E) resolution of the Holliday intermediate.

28. The excision (splicing) of many group I introns requires, in addition to the primary transcript RNA: (A) a cytosine nucleoside or nucleotide and a protein enzyme. (B) a guanine nucleoside or nucleotide (only). (C) a protein enzyme only. (D) a small nuclear RNA and a protein enzyme. (E) ATP, NAD, and a protein enzyme.

29. The signal sequences that direct proteins to the nucleus are: (A) always at the amino terminus of the targeted protein. (B) cleaved after the protein arrives in the nucleus. (C) not located at the ends of the peptide, but in its interior. (D) glycosyl moieties containing mannose 6-phosphate residues. (E) the same as those that direct certain proteins to lysosomes.

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