10. Which of the following substances is the starting material for de novo purine nucleotide synthesis?
(A) Aspartate.
(B) Glycine.
(C) Glutamine.
(D) All of the above.
(E) None of the above.

6. Which of the following enzymes is directly responsible for the elongation of fatty acid chains during de novo fatty acid synthesis? (A) Acetyl-CoA carboxylase. (B) Acyl-CoA dehydrogenase. (C) Fatty acid synthase. (D) B-ketoacyl-ACP synthase. (E) Carnitine acyltransferase.

7. Which enzyme catalyzes the rate-limiting step in the citric acid cycle and is inhibited by ATP? (A) Citrate synthase. (B) a-ketoglutarate dehydrogenase complex. (C) Isocitrate dehydrogenase. (D) Malate dehydrogenase. (E) Succinate dehydrogenase.

8. Regarding eukaryotic translation and the structure and function of tRNA, which of the following statements is correct? (A) During translation, the small ribosomal subunit is responsible for peptide bond formation, while the large subunit scans and pairs the ribosome with the start codon of the mRNA. (B) During the recognition and attachment of the correct amino acid to tRNA, aminoacyl-tRNA synthetase verifies the side-chain structure of the amino acid and the anticodon of the tRNA to ensure specificity. (C) The E-site of the ribosome is the entry site for aminoacyl-tRNA, while the P-site is the location where decoding occurs. (D) Eukaryotic translation initiation requires eIF2 (eukaryotic initiation factor 2) to assist in loading the initiator tRNA into the large ribosomal subunit and directly position the start codon on the mRNA via the 5' cap structure. (E) The TYC arm of tRNA interacts with the ribosome to help tRNA enter the ribosome, while the DHU arm is responsible for the specific recognition by aminoacyl-tRNA synthetase.

9. Which of the following substances is NOT a precursor or intermediate in cholesterol synthesis? (A) Acetyl-CoA. (B) Acetoacetyl-CoA. (C) Mevalonic acid. (D) Squalene. (E) Phosphatidic acid.

11. Which of the following mechanisms is primarily involved in repairing double-stranded breaks in DNA? (A) Non-homologous end joining (NHEJ). (B) Homologous recombination. (C) Base excision repair (BER). (D) Nucleotide excision repair (NER). (E) Both A and B.

12. Which of the following substances can activate phosphofructokinase-1 (PFK-1) and stimulate glycolysis? (A) ATP. (B) Citrate. (C) AMP. (D) Fatty acid. (E) Acetyl-CoA.

13. Which of the following statements about cyclin-dependent kinases is INCORRECT? (A) Different types of cells regulate the cell cycle through distinct CDK complexes with specific cyclins. (B) CDK activity fluctuates depending on the stage of the cell cycle. (C) CDK activity is controlled by mechanisms such as gene expression, phosphorylation, and protein degradation. (D) Cyclins are essential regulators of CDK activity. (E) CDKs regulate proteins that drive cell cycle progression.

14. Which of the following reactions is common to both gluconeogenesis and glycolysis? (A) Phosphoenolpyruvate to pyruvate. (B) Glucose-6-phosphate to fructose-6-phosphate. (C) Fructose 1,6-bisphosphate to glyceraldehyde 3-phosphate and dihydroxyacetone phosphate. (D) Dihydroxyacetone phosphate to glyceraldehyde 3-phosphate. (E) Pyruvate to oxaloacetate.

15. How is the electron transport chain related to oxidative phosphorylation? (A) The electron transport chain serves as the electron donor for oxidative phosphorylation. (B) Oxidative phosphorylation provides protons as the electron acceptor for the electron transport chain. (C) These two processes are entirely separate. (D) Both processes occur simultaneously in the cytoplasm. (E) None of the above.

16. The pentose phosphate pathway plays a dual role in cellular metabolism. It generates _____ for anabolic reactions and _____ as a precursor for nucleotide biosynthesis. Which of the following correctly fills in the blanks? (A) ATP; fructose-6-phosphate. (B) NADPH; fructose-1,6-bisphosphate. (C) NADPH; ribose-5-phosphate. (D) NADH; glyceraldehyde-3-phosphate. (E) NADH; ribulose-5-phosphate.