NUMS — Medical & Dental Entry Test

NUMS Biology — Biological Molecules and Enzymes (Set 3)

Take the NUMS Biology — Biological Molecules and Enzymes (Set 3): 30 multiple-choice questions (40 minutes) with instant scoring and a full answer review.…

What this test covers

30 NUMS Biology

Sample questions with answers

The first 5 questions of this test, with the correct answer and explanation.

Q1. A hexapeptide is formed from six amino acids. How many water molecules are released during its formation?
- A. 4
- B. 5 ✓
- C. 7
- D. 6
Answer: B. 5
Explanation: Each peptide bond forms by condensation releasing one water molecule; n amino acids give n − 1 = 5 bonds.
Q2. A polypeptide with 100 amino acids is cleaved completely into free amino acids by hydrolysis. How many water molecules are consumed?
- A. 98
- B. 100
- C. 101
- D. 99 ✓
Answer: D. 99
Explanation: Complete hydrolysis breaks all 99 peptide bonds, using one water per bond.
Q3. A mutation replaces glutamic acid (polar, charged) with valine (non-polar) at position 6 of the β-chain of haemoglobin. What is the most direct consequence?
- A. Haemoglobin S polymerises at low oxygen, deforming red cells ✓
- B. The peptide bond at position 6 can no longer form between neighbouring residues
- C. The α-chains can no longer be synthesised in the red cell
- D. Haemoglobin loses its haem group completely during synthesis
Answer: A. Haemoglobin S polymerises at low oxygen, deforming red cells
Explanation: The hydrophobic valine creates a sticky patch that lets deoxy-HbS fibres form, causing sickling.
Q4. In a double-stranded DNA molecule, 28% of the bases are adenine. What percentage of the bases is guanine?
- A. 28%
- B. 72%
- C. 44%
- D. 22% ✓
Answer: D. 22%
Explanation: A = T = 28%, so A + T = 56%; G + C = 44%, so G = 22%.
Q5. A DNA fragment of 2000 base pairs contains 600 cytosine bases. How many adenine bases does it contain?
- A. 1000
- B. 400
- C. 1400 ✓
- D. 800
Answer: C. 1400
Explanation: C = G = 600, so G + C = 1200 of 4000 total bases; A + T = 2800, so A = 1400.

The other 25 questions

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Q6. Which feature distinguishes a nucleoside from a nucleotide?
- A. A nucleoside lacks the nitrogenous base
- B. A nucleoside lacks the pentose sugar
- C. A nucleoside lacks the phosphate group
- D. A nucleoside has two phosphate groups
Q7. Sucrose is NOT a reducing sugar because:
- A. it contains no hydroxyl groups in either ring
- B. the anomeric carbons of glucose and fructose are tied up in the glycosidic bond
- C. it contains a single ketone group only
- D. it is a polysaccharide made of many units
Q8. A sample gives a brick-red precipitate with Benedict's reagent only after boiling with dilute HCl and neutralising. The sample most likely contains:
- A. sucrose
- B. fructose
- C. glucose
- D. lactose
Q9. Which polysaccharide has the most highly branched structure, with the greatest number of branch points per unit length?
- A. Glycogen
- B. Amylopectin
- C. Cellulose
- D. Amylose
Q10. Why can humans digest starch but not cellulose?
- A. Cellulose contains α(1→4) bonds
- B. Cellulose is made of galactose
- C. They lack an enzyme that hydrolyses β(1→4) glycosidic bonds
- D. Starch has no glycosidic bonds
Q11. Which pair is correctly matched?
- A. Maltose – glucose + fructose
- B. Lactose – two glucose units
- C. Sucrose – glucose + galactose
- D. Lactose – glucose + galactose
Q12. Compared with a saturated fatty acid of the same length, an unsaturated fatty acid with a cis double bond has:
- A. a lower melting point because of the kink in its chain
- B. a smaller number of carbon atoms
- C. a higher melting point because of tighter packing
- D. no hydrogen atoms
Q13. Which statement about phospholipids in water is correct?
- A. They orient with tails towards water
- B. They orient with hydrophilic heads towards water and tails away from it
- C. They dissolve as single molecules at all concentrations
- D. They form covalent bonds with water
Q14. Which lipid is a precursor of steroid hormones and bile salts?
- A. Cholesterol
- B. Beeswax
- C. Lecithin
- D. Triacylglycerol
Q15. Hydrolysis of one molecule of a triglyceride by lipase completely into its components yields:
- A. one glycerol and two fatty acids
- B. one glycerol and three fatty acids
- C. three glycerol and three fatty acids
- D. three glycerol and one fatty acid
Q16. Which is a feature of competitive inhibition?
- A. Vmax is unchanged but apparent Km increases
- B. Both Vmax and Km decrease
- C. Inhibition cannot be reversed by adding more substrate
- D. Vmax decreases and Km is unchanged
Q17. The substrate concentration at which an enzyme works at half its maximum velocity is called:
- A. the turnover number
- B. the isoelectric point
- C. the Michaelis constant (Km)
- D. the activation energy
Q18. An enzyme reaction rate stops increasing when substrate is raised further. The best explanation is:
- A. the product has become a coenzyme
- B. the enzyme is denatured by substrate
- C. all active sites are saturated with substrate
- D. activation energy has risen
Q19. Methotrexate resembles folic acid and blocks dihydrofolate reductase. This is an example of:
- A. feedback activation
- B. irreversible denaturation
- C. competitive inhibition
- D. non-competitive inhibition
Q20. Heavy metal ions such as Hg²⁺ inhibit enzymes by binding –SH groups away from the active site and altering shape. Increasing substrate does not restore activity. This is:
- A. product inhibition by a substrate analogue
- B. substrate-level activation
- C. non-competitive inhibition
- D. competitive inhibition
Q21. Pepsin works best at pH about 2, trypsin at about 8. If pepsin is moved into the small intestine, it loses activity mainly because:
- A. it is converted back into inactive pepsinogen at that pH
- B. its peptide bonds are spontaneously hydrolysed by water
- C. the temperature drops sharply in the small intestine
- D. the alkaline pH changes the ionisation of its side chains and disrupts its shape
Q22. In an experiment, enzyme activity doubles for every 10 °C rise from 20 °C to 40 °C, then falls sharply at 60 °C. The fall at 60 °C is due to:
- A. decreased kinetic energy of molecules
- B. thermal denaturation of the enzyme
- C. exhaustion of substrate at low temperature
- D. loss of coenzyme by oxidation of water
Q23. Many digestive proteases are secreted as inactive precursors (zymogens). The main advantage is that:
- A. the enzymes need no substrate
- B. the enzymes work faster at low temperature
- C. the precursors are cofactors
- D. the secreting cells and ducts are not digested
Q24. In end-product (feedback) inhibition of a metabolic pathway, the final product typically inhibits:
- A. the gene for the first substrate permanently
- B. the first enzyme of the pathway, often allosterically
- C. all enzymes of the pathway by denaturing them
- D. the last enzyme by competing with water molecules
Q25. Niacin deficiency impairs many dehydrogenases because niacin is a component of:
- A. haem
- B. FAD only
- C. coenzyme A
- D. NAD⁺
Q26. A prosthetic group differs from a coenzyme in that it is:
- A. a type of substrate
- B. tightly and permanently bound to the enzyme
- C. always an inorganic ion
- D. loosely bound and easily dialysed
Q27. The complete enzyme, active and containing its cofactor, is called the:
- A. isozyme
- B. apoenzyme
- C. holoenzyme
- D. zymogen
Q28. The reaction A + B → AB is catalysed by an enzyme that lowers the activation energy. Which quantity is unchanged by the enzyme?
- A. The rate of the reaction
- B. The overall free-energy change of the reaction
- C. The energy of the transition state
- D. The time taken to reach equilibrium
Q29. The biuret test for protein gives a violet colour because:
- A. copper(II) ions complex with peptide bonds in alkaline solution
- B. lipids dissolve the dye
- C. reducing sugars reduce Cu²⁺ to Cu₂O
- D. iodine binds helical amylose
Q30. Water's high specific heat capacity is important to organisms because it:
- A. buffers cells and bodies against rapid temperature change
- B. makes water a non-polar solvent
- C. prevents hydrogen-bond formation
- D. lowers its surface tension