Questions & explanations
1. Which level of protein structure is primarily stabilised by hydrogen bonds between carbonyl oxygen and amide hydrogen of nearby amino acids?
- Primary structure
- Quaternary structure
- Tertiary structure
- Secondary structure
Answer: Secondary structure
Secondary structure (α-helix and β-sheet) is stabilised by hydrogen bonds between the C=O of one residue and the N-H of another residue in the same polypeptide chain, typically between nearby residues. Primary structure involves peptide bonds, tertiary involves side-chain interactions, and quaternary involves subunit interactions.
2. Which test is positive for a dipeptide?
- Both Biuret and Ninhydrin tests
- Biuret test
- Xanthoproteic test
- Ninhydrin test
Answer: Both Biuret and Ninhydrin tests
A dipeptide has a free α-amino group (positive Ninhydrin) and one peptide bond. The Biuret test, though typically for two or more peptide bonds, can give a positive result with dipeptides under alkaline conditions due to the formation of a copper complex with the peptide bond. Thus both tests are positive.
3. A disaccharide on hydrolysis gives glucose and galactose. Which test will confirm its identity?
- Seliwanoff test
- Mucic acid test
- Iodine test
- Molisch test
Answer: Mucic acid test
The disaccharide is lactose, which yields glucose and galactose. The mucic acid test is specific for galactose: oxidation with HNO3 gives insoluble mucic acid crystals. Seliwanoff test distinguishes ketoses from aldoses, iodine test detects starch, and Molisch test is a general test for all carbohydrates.
4. Which of the following will give a silver mirror with Tollens reagent?
- Cellulose
- Sucrose
- Starch
- Glucose
Answer: Glucose
Glucose is a reducing sugar because its cyclic hemiacetal can open to expose a free aldehyde group. This aldehyde reduces Tollens reagent (ammoniacal silver nitrate) to metallic silver, forming a silver mirror. Sucrose, starch, and cellulose lack a free anomeric OH and are non-reducing.
5. Which of the following is NOT a difference between DNA and RNA?
- DNA contains deoxyribose sugar; RNA contains ribose sugar.
- DNA has thymine; RNA has uracil.
- DNA is double-stranded; RNA is single-stranded.
- Both contain adenine, guanine, and cytosine.
Answer: Both contain adenine, guanine, and cytosine.
Adenine, guanine, and cytosine are common to both DNA and RNA. Therefore, this is not a difference but a similarity. The other options correctly list differences: sugar (deoxyribose vs ribose), pyrimidine base (thymine vs uracil), and strandedness (double vs single).
6. Which of the following disaccharides is a reducing sugar?
- Maltose
- Sucrose
- Trehalose
- Raffinose
Answer: Maltose
Maltose has a free anomeric carbon on the second glucose unit, allowing it to open to an aldehyde form and reduce Tollens reagent. Sucrose, trehalose, and raffinose have both anomeric carbons involved in glycosidic bonds, making them non-reducing.
7. Which test gives a purple ring at the interface for all carbohydrates?
- Fehling's test
- Iodine test
- Seliwanoff's test
- Molisch's test
Answer: Molisch's test
Molisch's test uses α-naphthol and conc. H₂SO₄; a purple ring at the interface indicates any carbohydrate (mono, di, or poly). Fehling's test detects reducing sugars, Seliwanoff's test detects ketoses, and iodine test detects starch specifically.
8. Which of the following is a nucleoside?
- Adenosine monophosphate
- Adenosine
- Deoxyadenosine monophosphate
- Adenosine triphosphate
Answer: Adenosine
A nucleoside consists of a nitrogenous base attached to a pentose sugar via an N-glycosidic bond, with no phosphate group. Adenosine (adenine + ribose) fits this definition. The other options contain phosphate groups, making them nucleotides.
9. An unknown carbohydrate gives a positive Molisch test, negative Fehling test, and positive iodine test. What is it?
- Starch
- Sucrose
- Glucose
- Fructose
Answer: Starch
Molisch test is positive for all carbohydrates. Negative Fehling test indicates non-reducing nature. Positive iodine test (blue-black) is specific to starch due to amylose-iodine complex. Starch is non-reducing and gives positive iodine test.
10. Which of the following statements about carbohydrate classification is correct?
- Monosaccharides can be hydrolysed to give simpler sugars.
- Sucrose is a reducing sugar because it has a free aldehyde group.
- Polysaccharides yield many monosaccharide units on hydrolysis.
- Oligosaccharides yield 2-10 monosaccharides and are always sweet.
Answer: Polysaccharides yield many monosaccharide units on hydrolysis.
Polysaccharides, such as starch and cellulose, yield many monosaccharide units on hydrolysis. Monosaccharides cannot be hydrolysed further. Sucrose is non-reducing. Oligosaccharides may be sweet but not always (e.g., some are tasteless).
11. A 500-base-pair double-stranded DNA fragment has 20% adenine. What is the total number of hydrogen bonds in the duplex?
- 1300
- 1250
- 1350
- 1400
Answer: 1300
Chargaff's rule: %A = %T = 20%, so %G = %C = 30%. For 500 bp, total nucleotides = 1000. Number of A = T = 200, G = C = 300. A-T pairs = 200, each with 2 H-bonds → 400. G-C pairs = 300, each with 3 H-bonds → 900. Total = 400 + 900 = 1300.
12. Complete hydrolysis of sucrose (specific rotation +66.5°) gives an equimolar mixture of D-glucose (+52.5°) and D-fructose (−92°). What is the specific rotation of the hydrolysate?
- +19.75°
- −39.5°
- −19.75°
- −14.5°
Answer: −19.75°
The hydrolysate is an equimolar mixture of glucose and fructose. The specific rotation is the weighted average: (+52.5° + (−92°))/2 = −39.5°/2 = −19.75°. The sign inverts from positive (sucrose) to negative, hence the name invert sugar.