NUMS — Medical & Dental Entry Test
Take the NUMS Biology — Biological Molecules and Enzymes (Set 7): 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. An enzyme follows Michaelis–Menten kinetics with Km = 4 mM and Vmax = 120 µmol/min. At what substrate concentration is the reaction velocity 90 µmol/min?
Answer: B. 12 mM
Explanation: v = Vmax[S]/(Km+[S]); 90 = 120[S]/(4+[S]) gives 90×4 = 30[S], so [S] = 12 mM.
Q2. Chemical analysis of a nucleic acid from a virus gives these base percentages: A 23%, G 32%, C 21%, T 24%. Which description fits the sample best?
Answer: D. Single-stranded DNA, as its A and T values and its G and C values are not equal
Explanation: Thymine marks it as DNA. In a double helix A = T and G = C (Chargaff's rules), but here 23 ≠ 24 and 32 ≠ 21, so the bases are not paired: the DNA is single-stranded.
Q3. In amylopectin an α-1,6 branch point occurs on average once in every 25 glucose residues, and in glycogen once in every 10. About how many branch points would a chain of 1200 glucose residues have in each polymer?
Answer: C. 48 in amylopectin and 120 in glycogen
Explanation: Branch points = residues ÷ spacing: 1200 ÷ 25 = 48 for amylopectin and 1200 ÷ 10 = 120 for glycogen. Glycogen is the more highly branched.
Q4. A protein is made of four polypeptide chains that together contain 400 amino acid residues. How many peptide bonds does the whole molecule contain? (Assume no other covalent cross-links are counted.)
Answer: C. 396
Explanation: Each chain of n residues has n−1 peptide bonds, so 400 residues in 4 chains give 400 − 4 = 396.
Q5. Boiling an egg white turns it from clear to opaque white. Which change explains this?
Answer: B. Hydrogen bonds and weak interactions break, so the protein unfolds and clumps
Explanation: Heat disrupts hydrogen bonds, ionic and hydrophobic interactions. The primary structure with its peptide bonds is not hydrolysed, but the unfolded chains clump together.
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Q6. A double-stranded DNA fragment contains 2000 nucleotides in total, of which 30% are guanine. How many hydrogen bonds hold the two strands together? (A–T pairs have 2, G–C pairs have 3.)
Q7. One strand of a DNA duplex has the composition A 20%, G 30%, T 25%, C 25%. What is the percentage of guanine in the whole duplex?
Q8. Analysis of a viral genome gives A 24%, T 36%, G 20%, C 20%. What is the best conclusion?
Q9. Four double-stranded DNA samples have the following GC contents. Which melts (strands separate) at the highest temperature?
Q10. A student adds iodine solution to a colourless solution and sees a red-brown colour. After boiling with dilute acid and neutralising, the product gives a brick-red precipitate with Benedict's reagent. The solution most likely contains:
Q11. A sample gives no colour change with Benedict's reagent. After boiling with dilute HCl and neutralising with sodium hydrogencarbonate, it gives a brick-red precipitate. Which carbohydrate is most likely present?
Q12. Which solution gives a violet colour when treated with dilute copper sulphate in alkaline conditions (Biuret test)?
Q13. A linear glucan chain contains 1200 glucose residues joined by glycosidic bonds. How many water molecules are released when it is built from free glucose, and how many are needed to hydrolyse it back completely?
Q14. A bacterial culture grows well on D-glucose but not on L-glucose, although both have the same formula. The most likely reason is that:
Q15. Triglycerides made entirely of palmitic acid (saturated) and entirely of linoleic acid (two cis double bonds) are compared at 25 °C. Which statement is most likely correct?
Q16. Collagen contains glycine at about every third position. What is the main reason for this regularity?
Q17. Adult haemoglobin contains four polypeptide chains. Which statement about its structure is correct?
Q18. A dialysed enzyme preparation has lost its activity. Adding the boiled dialysate (the small molecules that passed through the membrane) restores the activity, but adding boiled enzyme does not. What can be concluded?
Q19. A dietary deficiency of niacin would most directly reduce the synthesis of which coenzyme?
Q20. During glycolysis an enzyme moves a phosphate group from carbon 3 to carbon 2 of the same molecule (3-phosphoglycerate to 2-phosphoglycerate), with no redox change and no other change. Which class does this mutase belong to?
Q21. A product accumulates in an enzyme assay as follows: 12 µmol at 1 min, 22 µmol at 2 min, 30 µmol at 3 min, 36 µmol at 4 min, 40 µmol at 5 min (0 µmol at 0 min). Which statement about the reaction is correct?
Q22. The rate of an enzyme-catalysed reaction below its optimum is 5 µmol/min at 15 °C. Assuming a temperature coefficient (Q₁₀) of 2 and no denaturation up to 35 °C, what is the rate at 35 °C?
Q23. The activity of an enzyme is 20% of maximum at pH 4, 100% at pH 7 and 10% at pH 10. The fall in activity at pH 10 is best explained by:
Q24. In a reaction with excess substrate, the enzyme concentration is doubled. How do Vmax and Km change?
Q25. In a metabolic pathway, excess isoleucine (the end product) switches off the first enzyme, threonine deaminase, by binding to a site away from the active site. This is an example of:
Q26. An enzyme is inhibited by a compound that closely resembles its substrate. The inhibition is reduced when the substrate concentration is greatly increased. The inhibitor is acting as:
Q27. Pepsin is secreted as pepsinogen, which becomes active only in the stomach lumen when acid removes part of the chain. What is the main advantage of this arrangement?
Q28. An RNA nucleotide is identified by the presence of which pair of features?
Q29. Insects such as pond skaters walk on the surface of a pond without sinking. Which property of water is mainly responsible?
Q30. Two enzymes act on the same substrate. Enzyme P reaches half of Vmax at 0.5 mM substrate and enzyme Q at 5 mM. Which statement is correct?