COMPETITIVE EXAM MCQs SERIES of LIFE SCIENCES for UGC-CSIR NET/JRF, SLET, GATE, and other entrance tests – MOLECULES AND THEIR INTERACTION RELEVANT TO BIOLOGY – Biomolecules Structure and Key Functions.
Syllabus Outline
- Carbohydrates – classification, monosaccharides, disaccharides, polysaccharides
- Lipids – fatty acids, triacylglycerols, phospholipids, sterols
- Proteins – structure, amino acids, levels of protein organisation
- Nucleic acids – structure of DNA and RNA, nucleotide components
- Vitamins – water- and fat-soluble, biological roles, deficiencies
- Biological functions of major biomolecules
- Structure-function relationship in biomolecules
This quiz contains concept-based, most frequently asked 25 MCQs of “MOLECULES AND THEIR INTERACTION RELEVANT TO BIOLOGY – Biomolecules Structure and Key Functions”. Each question has a single correct/most appropriate answer.
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1. The Ramachandran plot shows the allowed φ (phi) and ψ (psi) angles in proteins. Why does Proline occupy a restricted region on this plot?
A) Its side chain bonds to the backbone nitrogen, restricting rotation and favouring β-sheet regions.
B) Its cyclic side chain limits φ to about –60°, linking the α-carbon to the amide nitrogen.
C) Its small side chain allows both αR and αL conformations equally.
D) Its flexible ring permits access to disallowed regions.
2. A research team models a 15-residue peptide containing a non-standard residue, X. Analysis of the Ramachandran plot shows that X is the only residue whose dihedral angles (phi, psi) fall into a region that is small, discrete, and equally represented in both the left and right quadrants of the plot, excluding the Glycine region. What is the most likely structural identity of residue X?
A) A derivative of L-Glutamate with an unusually bulky side chain.
B) A residue that introduces significant beta-turn structure.
C) A D-amino acid stereoisomer.
D) A Cysteine residue involved in an intra-peptide disulfide bond.
3. Glycine residues are often critical for protein flexibility because their presence allows the polypeptide backbone to access regions on the Ramachandran plot that are disallowed for other L-amino acids. This is because:
A) Glycine can form more stable hydrogen bonds, lowering the energy barrier for certain dihedral angles.
B) Glycine lacks a C-beta atom, thereby minimising steric hindrance across the phi and psi angles.
C) Glycine preferentially stabilises the cis conformation of the peptide bond.
D) Glycine’s small size preferentially stabilises the A-form alpha-helix.
4. Why are antiparallel beta-sheets usually more stable than parallel beta-sheets?
A) Antiparallel sheets bury more hydrophobic residues away from water
B) Antiparallel sheets form linear, stronger hydrogen bonds between strands
C) Antiparallel sheets contain more proline, which stabilises the structure
D) Parallel sheets need short loops that reduce flexibility and stability
5. A protein whose activity depends solely on its tertiary structure is treated with a high concentration of urea. Which of the following stabilisation elements would remain intact after the urea treatment, but would be susceptible to subsequent reduction by beta-mercaptoethanol?
A) Ionic bonds between exposed surface residues.
B) Covalent disulfide bonds between Cysteine residues.
C) Intramolecular hydrogen bonds form alpha-helices.
D) Hydrophobic interactions driving the collapse of nonpolar residues to the core.
6. The denaturation of a globular protein is an endothermic process because energy is required to break the stabilising non-covalent interactions. For the unfolding process to be thermodynamically favourable at physiological temperatures, which thermodynamic factor must dominate?
A) A large negative change in entropy, reflecting the high order of the unfolded state.
B) A large negative change in enthalpy, indicating bond formation.
C) A positive change in entropy, where the increase in solvent and chain disorder overcomes the positive Delta H.
D) A very high concentration of denaturant that artificially increases Keq for folding.
7. A protein is least stable when the pH equals the pKa of residues that form salt bridges in its core. Why?
A) Hydrophobic groups become more exposed to water at this pH
B) The protein backbone dissolves better in water at this pH
C) The charged residues lose their charges, so they can’t form stabilising ionic bonds anymore
D) The protein’s enzymatic activity is highest at this pH, breaking it down faster
8. Two enzymes, E1 and E2, catalyse the same reaction with the following kinetic parameters:
E1: Km = 100 µM, Vmax = 500 µmol/s.
E2: Km = 20 µM, Vmax = 100 µmol/s.
If both enzymes are present at the same concentration in a cell where the substrate concentration is 1 µM, which enzyme will be more efficient?
A) E1, because its maximal velocity is higher.
B) E2, because its substrate affinity (1/Km) is much higher than E1 and [S]≪Km.
C) Both will have equal efficiency since their Vmax/Km ratios are similar.
D) Efficiency cannot be determined without knowing the turnover number (kcat).
9. The catalytic efficiency of an enzyme is best represented by the ratio kcat/Km. This ratio is particularly significant when comparing different substrates for the same enzyme or when the substrate concentration is much lower than the Km. A high kcat/Km value indicates:
A) High specificity but low reaction velocity at saturating conditions.
B) Low substrate affinity but fast product release.
C) High reaction velocity and high affinity for the substrate.
D) A mechanism involving irreversible covalent inhibition.
10. In an experiment, the activity of three proteins (P1, P2, P3) acting as proteases was measured at varying substrate concentrations, yielding Michaelis-Menten kinetics. The concentration used was: P1 (1 mg/ml), P2 (4 mg/ml), P3 (2 mg/ml). If all three proteins exhibit the same intrinsic catalytic efficiency (kcat/Km) but Protein P2 displays the highest observed Vmax, which conclusion regarding the proteins is most appropriate?
A) Protein P1 has the highest turnover rate (kcat).
B) Protein P3 has the lowest Km.
C) Protein P2 has the highest molecular weight.
D) Protein P2 has the highest concentration, leading directly to the highest Vmax.
11. In DNA melting experiments, the increase in absorbance at 260 nm (hyperchromic effect) occurs mainly because:
A) Phosphodiester bonds break, releasing nucleotides as strands separate.
B) Base stacking between purine and pyrimidine rings decreases as strands separate.
C) G–C rich regions absorb more than A–T regions.
D) The decreased hydration of the phosphate backbone in the random coil state.
12. Two equal-length DNA duplexes are compared: Sequence 1 has a Tm of 65°C in 100 mM NaCl, and Sequence 2 has 15% more G–C content. When NaCl concentration is reduced to 10 mM, what happens?
A) Tm of Sequence 2 increases, while Sequence 1 remains unchanged.
B) Both Tm values decrease, but Sequence 2 still has a higher value than Sequence 1.
C) Both Tm values increase due to reduced charge screening, but Sequence 2 still has a higher Tm than Sequence 1.
D) Tm of Sequence 1 becomes higher than Sequence 2 due to A–T stacking.
13. Which of the following non-canonical DNA structures often requires a high concentration of Mg2+ or polyamines for stabilisation, and is characterised by a left-handed helix with a zig-zag backbone?
A) A-DNA structure.
B) B-DNA structure.
C) Z-DNA structure.
D) Cruciform DNA structure.
14. The stability of a short DNA duplex is best predicted by the Nearest-Neighbour (NN) model rather than by simply counting G–C pairs because the NN model considers:
A) The kinetic rate of hybridisation.
B) Sequence-dependent stacking interactions between adjacent base pairs.
C) The energy required to disrupt the structure, which depends on nearest-neighbour base pairs.
D) The equilibrium constant of base tautomerization, which depends on nearest-neighbour base pairs.
15. What happens to the optical density of DNA at 260 nm when it undergoes denaturation?
A) It increases.
B) It decreases.
C) It first decreases and then increases.
D) It shows no change.
16. The process of DNA duplex formation from two single strands is generally characterised by:
A) ΔH > 0 and ΔS > 0
B) ΔH > 0 and ΔS < 0
C) ΔH < 0 and ΔS < 0
D) ΔH < 0 and ΔS > 0
17. The stability of DNA secondary structures mainly depends on base stacking interactions governed by van der Waals forces and base geometry. These stacking interactions are strongest in:
A) B-DNA, where the bases are tilted and partially displaced from the helix axis, creating moderate stacking.
B) A-DNA, where the 2′-OH group produces tighter turns and stronger base overlap.
C) Single-stranded regions where the bases are freely exposed to the solvent.
D) Regions containing a high ratio of purine-pyrimidine steps, regardless of sugar pucker.
18. The formation of a stable nucleic acid duplex at a specific temperature (T) requires that the free energy change ΔG be negative. According to the Gibbs equation (ΔG = ΔH – T x ΔS), if duplex formation is exothermic (ΔH < 0) and results in a net decrease in molecular disorder (ΔS < 0):
A) The reaction is entropy-driven, favouring low temperatures.
B) The reaction is enthalpy-driven, and high temperatures will favour the product.
C) The reaction is enthalpy-driven, favouring low temperatures
D) The reaction is entropy-driven, irrespective of temperature.
19. Cellulose and Amylose are both homopolymers of D-glucose. The profound difference in their physical properties—cellulose serving as a rigid structural fibre and Amylose as a readily digestible energy store—is primarily determined by:
A) The number of branch points per 100 residues (alpha-1,6 linkages).
B) The use of fructose monomers in cellulose synthesis.
C) The total length of the polymer chain and cross-linking pattern.
D) The anomeric configuration of the glycosidic linkage at C1.
20. The plasma membrane shows distinct lipid asymmetry, essential for translating extracellular signals into intracellular responses. Which lipid pair is mainly confined to the cytosolic face of the membrane and participates in key signalling pathways?
A) Phosphatidylcholine and Sphingomyelin.
B) Cardiolipin and Phosphatidylcholine
C) Cholesterol and Glycolipids.
D) Phosphatidylserine and Phosphatidylinositol 4-phosphate (PIP).
21. Thiamine Pyrophosphate (TPP), the active coenzyme form of Vitamin B1 (Thiamine), plays a critical role in metabolic pathways. TPP is required as a cofactor for which general type of reaction?
A) Decarboxylation of alpha-keto acids
B) Carboxylation reactions, transferring CO2
C) Redox reactions involving hydride transfer.
D) One-carbon transfer reactions involving formyl groups.
22. Vitamin K is essential for the post-translational modification of specific coagulation factors. Its active form serves as a cofactor for the enzyme that catalyses:
A) Hydroxylation of Proline and Lysine residues.
B) Reduction of disulfide bonds in nascent proteins.
C) Gamma-Carboxylation of Glutamate residues.
D) Deamination of Asparagine residues.
23. NAD+ and FAD are essential coenzymes derived from the water-soluble vitamins Niacin (B3) and Riboflavin (B2), respectively. Although both function in cellular respiration, the key biochemical distinction in their electron-carrying capacity is that:
A) NAD+ accepts a hydride ion (H–, two electrons), while FAD can accept one electron at a time in radical mechanisms.
B) NAD+ transfers electrons directly to oxygen, while FAD transfers electrons to complexes in the electron transport chain.
C) FAD always remains covalently bound to its apoenzyme, while NAD+ is freely diffusible.
D) NAD+ operates only in anabolic pathways, and FAD operates only in catabolic pathways.
24. Tetrahydrofolate (THF), the reduced coenzyme form of Folic Acid (Vitamin B9), is crucial for various one-carbon transfer reactions. A major metabolic role of THF is its direct involvement in the biosynthesis of:
A) Fatty acids and membrane lipids.
B) Purine and Pyrimidine nucleotides
C) Purine nucleotides and Thymidylate.
D) Steroid hormones.
25. Both Folic Acid (B9) and Cobalamin (B12) deficiencies can lead to macrocytic anaemia. This shared pathology arises because the B12-dependent enzyme Methionine Synthase is essential for:
A) Covalently modifying the ribosomal subunits necessary for haemoglobin synthesis.
B) Regenerating active THF from N5-methyl THF
C) Reducing iron absorbed from the diet.
D) Directly participating in the G-C base pairing mechanism.
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References
- Nelson, David L. & Cox, Michael M. (2021). Lehninger Principles of Biochemistry, W. H. Freeman, 8th Edition
- Voet, Donald, Voet, Judith G., & Pratt, Charlotte W. (2018). Voet’s Principles of Biochemistry, Wiley, 5th Edition
- Berg, Jeremy M., Tymoczko, John L., & Stryer, Lubert (2023.) Biochemistry, W. H. Freeman, 10th Edition
- Palmer, Trevor & Bonner, Philip L. (2007). Enzymes: Biochemistry, Biotechnology, Clinical Chemistry, Horwood Publishing, 2nd Edition
- Upadhyay, Avinash, Upadhyay, K., & Nath, Nirmalendu (2023). Biophysical Chemistry: Principles and Techniques, Himalaya Publishing House, 4th Edition
