NUMS — Medical & Dental Entry Test
Take the NUMS Biology — Biological Molecules and Enzymes (Set 5): 30 multiple-choice questions (40 minutes) with instant scoring and a full answer review.…
What this test covers
The first 5 questions of this test, with the correct answer and explanation.
Q1. A single polypeptide chain is built from 120 amino acids joined end to end with no branching. How many water molecules are released while its primary structure is formed?
Answer: B. 119
Explanation: Each peptide bond releases one water. A linear chain of n residues has n − 1 bonds, so 120 − 1 = 119.
Q2. A DNA fragment of 1,000 base pairs contains 280 adenine nucleotides. How many hydrogen bonds hold its two strands together (2 per A–T pair, 3 per G–C pair)?
Answer: A. 2,720
Explanation: A = T = 280 pairs; G = C = 720 pairs. Hydrogen bonds = 280×2 + 720×3 = 560 + 2160 = 2,720.
Q3. An enzyme has Km = 3 mM and Vmax = 50 μmol/min. At what substrate concentration will its initial rate be 40 μmol/min?
Answer: A. 12 mM
Explanation: v = Vmax[S]/(Km+[S]); v/Vmax = 40/50 = 0.8 = [S]/(3+[S]), so 0.8×3 = 0.2[S] and [S] = 12 mM (four times Km).
Q4. An enzyme is assayed with and without an inhibitor. At 1 mM substrate the rate falls from 50 to 25 units with the inhibitor, but at 200 mM substrate both assays give almost identical rates near the maximum. Which inhibitor behaviour does this show?
Answer: B. Competitive inhibition, overcome by excess substrate
Explanation: If a very high substrate concentration restores the normal maximum rate, Vmax is unchanged and only apparent Km rises. This is competitive inhibition, in which substrate and inhibitor compete for the same active site.
Q5. In a pathway A → B → C → D, the final product D normally slows enzyme 1 by binding to a site away from its active site. A mutant enzyme 1 can no longer bind D at that site but still catalyses A → B. What is the expected result?
Answer: A. D keeps being made, and may accumulate beyond need
Explanation: Allosteric feedback inhibition is lost, so enzyme 1 is never switched off by D and the pathway runs on regardless of the level of D.
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Q6. The rate of an enzyme-catalysed reaction at 25 °C is 4 units. Assuming a temperature coefficient (Q10) of 2 and no denaturation, what rate is expected at 45 °C?
Q7. A starch solution gives a blue-black colour with iodine and no change with Benedict's reagent. After incubation with saliva, it gives no blue-black colour but a brick-red precipitate on heating with Benedict's reagent. What explains this?
Q8. A sugar solution gives no colour change with Benedict's reagent. After boiling with dilute HCl, neutralising, and re-testing, it gives a brick-red precipitate. The sugar was most probably
Q9. A child with bleeding gums and slow wound healing has collagen that cannot be properly cross-linked because proline residues are not hydroxylated. Which dietary deficiency is the most likely cause?
Q10. How many different tripeptides could be assembled if each of the three positions can be occupied by any one of the 20 common amino acids, repeats allowed?
Q11. A cell makes 5 triglyceride molecules from glycerol and fatty acids. How many water molecules are released in total?
Q12. Olive oil is liquid at room temperature while butter fat is solid. Which structural difference best accounts for this?
Q13. The waxy layer on a leaf surface is made of long-chain fatty acids joined to long-chain alcohols. Which bond type joins these two components?
Q14. A person running on a hot day loses heat as sweat evaporates from the skin. Which property of water makes this cooling so effective?
Q15. A rabbit survives on grass rich in cellulose although its own cells make no cellulase. Cellulose can be digested because
Q16. Glycogen can be mobilised faster than a long unbranched glucose polymer of the same size. The main reason is that glycogen has
Q17. The exoskeleton of an insect and the cell wall of a mushroom both contain the same structural polysaccharide. It is made of
Q18. A nucleic acid sample analysed by chemists has the base composition A 23%, U 27%, G 31%, C 19%. What is the best description?
Q19. Sample X of DNA contains 30% G + C and sample Y contains 60% G + C. When both are heated slowly in solution, which statement is correct?
Q20. Hexokinase transfers a phosphate group from ATP to glucose. To which class of enzymes does it belong?
Q21. A person on a diet poor in niacin shows impaired activity of many dehydrogenases. Which coenzyme is directly affected?
Q22. The pancreas releases trypsinogen, not active trypsin. What is the biological advantage of this arrangement?
Q23. A mutation changes an amino acid far from an enzyme's active site, yet the enzyme loses activity. The most likely explanation is that the mutation
Q24. An enzyme is heated at 70 °C for 10 minutes, then cooled to 37 °C. Its activity does not return. The best explanation is that
Q25. Reaction rate is measured at a fixed, limited substrate concentration while the enzyme concentration is raised step by step. The rate first rises in proportion and then stops rising. What is the best interpretation?
Q26. Glycine has an isoelectric point of about 6. If it is placed in an electric field at pH 2, which statement is correct?
Q27. Silk fibroin is strong but hardly stretches, while hair keratin stretches under tension. Which structural feature explains the silk property?
Q28. A haemoglobin molecule has four polypeptide chains, each with one haem group binding one O₂. How many O₂ molecules can 250 fully saturated haemoglobin molecules carry?
Q29. A drug that blocks cholesterol synthesis in the body would be expected to reduce production of which pair of hormones?
Q30. Malonate is added to isolated mitochondria that are oxidising succinate. Malonate closely resembles succinate and binds the active site of succinate dehydrogenase. Which result is expected?