19.Which of the following amino acid substitutions is most likely to disrupt the formation of a 'heptad repeat' motif in a leucine
zipper protein, assuming the substitution occurs at the 'a'or 'd' position of the (abcdefg)n sequence?
(A) Leucine to Isoleucine.
(B) Leucine to Valine.
(C) Leucine to Aspartate.
(D) Leucine to Methionine.

(E) Leucine to Alanine.

1. Transcription of the lactose operon in E. coli is stimulated by: (A) a mutation in the repressor gene that strengthens the affinity ofthe repressor for the operator. (B) a mutation in the repressor gene that weakens the affinity of the repressor for the operator. (C) a mutation in the repressor gene that weakens the affinity ofthe repressor for the inducer. (D) the presence of glucose in the growth medium. (E) binding of the repressor to the operator.

2.E. coli DNA polymerase III:(A) represents over 90% of the DNA polymerase activity in E. coli cells.(B) is the principal DNA polymerase in chromosomal DNA replication.(C) requires a free 5'-hydroxyl group as a primer.(D) can initiate replication without a primer.(E) is efficient at nick translation.

3. The function of the eukaryotic DNA replication factor PCNA (proliferating cell nuclear antigen) is similar to that oftheB-subunit of bacterial DNA polymerase III in that it:(A) forms a circular sliding clamp to increase the processivity of replication.(B) increases the speed but not the processivity of the replication complex.(C) has a 3'→ 5' proofreading activity.(D) facilitates replication of telomeres.(E) participates in DNA repair.

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

5. In homologous recombination in E. coli, the protein that moves along a double-stranded DNA, unwinding the strands ahead of it and degrading them, is: (A) RuvC protein (resolvase). (B) RecBCD enzyme. (C) RecA protein. (D) DNA ligase. (E) chi.

6. Which enzyme in the urea cycle catalyzes the reaction that provides a direct metabolic link to the TCA (Citric Acid) cycle through the production of fumarate? (A) Ornithine transcarbamoylase (OТС). (B) Argininosuccinate synthase (ASS). (C) Argininosuccinate lyase (ASL). (D) Carbamoyl phosphate synthetase I (CPS1). (E) Arginase-1 (ARG1).

7. An enzyme requires an organic non-protein component to facilitate its catalytic mechanism. Ifthis organic molecule is very tightly or even covalently bound to the enzyme's structure and remains associated with it throughout the reaction cycle, it is most specifically classified as a: (A) Apoenzyme. (B) Prosthetic group. (C) Coenzyme. (D) Holoenzyme. (E) Metalloenzyme.

8. In the context of cellular physiology, which of the following transport mechanisms is specifically characterized by the use ofa pre-existing electrochemical gradient (such as a sodium gradient) to drive the movement of a solute against its own concentration gradient? (A) Primary active transport. (B) Simple diffusion. (C) Osmosis. (D) Facilitated diffusion. (E) Secondary active transport.

9. Phospholipids are the primary structural components of the cell membrane bilayer. Which of the following descriptions regarding the function of phospholipids in the membrane bilayer is the most accurate? (A) Phospholipids, through their amphipathic nature, spontaneously form a barrier that prevents the free movement of water-soluble substances in and out of the cell. (B) The primary function of phospholipids is to act as active transport pumps, directly consuming ATP to move ions across the membrane. (C) Phospholipids remain in a fixed, static state within the membrane to provide mechanical rigidity and replace the need for a cytoskeleton. (D) The main function of phospholipids is to act as enzymes that catalyze the translation and modification of membrane (E) proteins. Phospholipids use their hydrophilic tails to create channels in the membrane core to facilitate the simple diffusion of large polar molecules.

10. Pyridoxal phosphate (PLP) is one of the most versatile cofactors in human metabolism. Which of the following catalytic processes is PLP-dependent and involves the formation of a Schiff base intermediate with a lysine residue in the enzyme's active site? (A) The carboxylation of pyruvate to form oxaloacetate. (B) The oxidative deamination of glutamate to release free ammonia. (C) The transamination of amino acids to generate a-keto acids. (D) The activation of fatty acids for ẞ-oxidation. (E) The conversion of methyl malonyl-CoA to succinyl-CoA.

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