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.

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.

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.

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.

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.

17. 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.

18. Which of the following inhibitors can have its inhibitory effect overcome by increasing substrate concentration? (A) Competitive inhibitor. (B) Non-competitive inhibitor. (C) Uncompetitive inhibitor. (D) Irreversible inhibitor. (E) None of the above.

19. Which of the following statements about CRISPR-Cas9 is FALSE? (A) CRISPR-Cas9 can be used to introduce specific mutations in a precise manner. (B) Off-target effects, where unintended DNA sequences are edited, are a potential concern with CRISPR-Cas9. (C) CRISPR-Cas9 is primarily limited to editing the genomes of simpler organisms such as bacteria and yeast. (D) gRNA directs the Cas9 protein to a complementary target DNA sequence for cleavage. (E) CRISPR-Cas9 has the potential to treat genetic diseases by repairing or correcting disease-causing mutations.