CBSE Class 12 Chemistry Biomolecules MCQs Set 14

Answer: (c) Both Tollens' reagent and Fehling's solution

Explanation:
1. Glucose has a free aldehyde group (in equilibrium with the open-chain form), so it is a reducing sugar.
2. It gives a silver mirror with Tollens' reagent and a red precipitate of Cu₂O with Fehling's solution.

Teacher's Note:
1. Any reducing sugar reduces both reagents.
2. Remember: silver mirror (Tollens) and red precipitate (Fehling).

Answer: (b) Saccharic acid

Explanation:
1. Concentrated HNO₃ is a strong oxidising agent and oxidises both the –CHO and the terminal –CH₂OH group to –COOH.
2. The dicarboxylic acid formed is saccharic acid (bromine water stops at gluconic acid).

Teacher's Note:
1. Bromine water gives gluconic acid; nitric acid gives saccharic acid.
2. n-Hexane is the product of reduction with HI, not oxidation.

Answer: (b) Mutarotation

Explanation:
1. When a freshly prepared solution of α-D-glucose or β-D-glucose stands, its specific rotation changes slowly to an equilibrium value.
2. This spontaneous change of optical rotation with time is mutarotation.

Teacher's Note:
1. Mutarotation is due to the interconversion of the α and β anomers through the open-chain form.
2. Do not confuse it with inversion, which refers to hydrolysis of sucrose.

Answer: (b) Inversion

Explanation:
1. Sucrose is dextrorotatory, +66.5°.
2. Its hydrolysis gives an equimolar mixture of D-(+)-glucose (+52.5°) and D-(–)-fructose (–92.4°), which is laevorotatory overall.
3. The mixture is called invert sugar.

Teacher's Note:
1. The enzyme invertase brings about this hydrolysis.
2. Name and sign change go together: inversion means (+) to (–).

Answer: (d) a dipolar ion carrying both a positive and a negative charge, neutral overall

Explanation:
1. In an amino acid the –COOH group transfers a proton to the –NH₂ group, giving –COO⁻ and –NH₃⁺ in the same molecule.
2. The charges balance, so the dipolar ion is electrically neutral overall.

Teacher's Note:
1. Amino acids behave like salts because they exist as zwitter ions.
2. This also explains their high melting points and water solubility.

Answer: (b) intermolecular hydrogen bonds

Explanation:
1. In the β-structure the chains are stretched out and laid side by side.
2. Neighbouring chains are joined by intermolecular hydrogen bonds between C=O and N–H groups, while in the α-helix the bonds are within one chain.

Teacher's Note:
1. Intermolecular means between different chains.
2. α-Helix: intramolecular; β-sheet: intermolecular.

Answer: (a) (a) and (b)

Explanation:
1. Purines have a double-ring structure: adenine and guanine.
2. Thymine and cytosine are single-ring pyrimidines.

Teacher's Note:
1. Remember: pure As Gold (adenine, guanine) for purines.
2. Uracil, thymine and cytosine are the pyrimidines.

Answer: (c) Both (1) and (2)

Explanation:
1. The sugar in DNA is β-D-2-deoxyribose and in RNA it is β-D-ribose.
2. DNA has the bases A, G, C and T; in RNA, uracil replaces thymine.

Teacher's Note:
1. Both differences between DNA and RNA are in the syllabus.
2. Learn them as a pair.

Answer: (a) Translation

Explanation:
1. DNA is transcribed into mRNA, and the sequence of mRNA is translated into the amino acid sequence of a protein.
2. So the RNA to protein step is translation.

Teacher's Note:
1. DNA to RNA is transcription; RNA to protein is translation.
2. Replication is DNA to DNA.

Answer: (c) Thymine

Explanation:
1. Adenine pairs with thymine through two hydrogen bonds.
2. Cytosine pairs with guanine through three hydrogen bonds.

Teacher's Note:
1. A–T has two bonds and G–C has three bonds.
2. This is why G–C rich DNA is more stable.

Answer: (d) hydrogen bonds

Explanation:
1. The α-helix is held in shape by hydrogen bonds between the C=O of one turn and the N–H of the next turn.
2. Peptide bonds form the backbone (primary structure) and do not stabilise the coiling.

Teacher's Note:
1. Secondary structure means hydrogen bonding.
2. Primary structure means peptide bonds.

Answer: (b) Thiol (–SH)

Explanation:
1. Cysteine contains an –SH side chain.
2. Two –SH groups are oxidised to give an S–S (disulphide) bridge between chains.

Teacher's Note:
1. Disulphide bridges help hold the tertiary structure.
2. Thiol is the sulphur analogue of an alcohol.

Answer: (c) Glycine

Explanation:
1. Glycine has just a hydrogen atom as its side chain (R = H).
2. This is also why it is the only optically inactive amino acid among the common ones.

Teacher's Note:
1. Glycine has no asymmetric carbon.
2. Alanine has R = CH₃.

Answer: (a) α-Keratin

Explanation:
1. Fibrous proteins like keratin have chains held side by side by hydrogen and disulphide bonds.
2. They are insoluble in water; globular proteins such as albumin dissolve.

Teacher's Note:
1. Fibrous means insoluble; globular means usually soluble.
2. Keratin occurs in hair and wool.

Answer: (b) 2nd carbon (C-2')

Explanation:
1. Ribose in RNA has –OH at C-2'.
2. 2-Deoxyribose in DNA has only H at C-2'.

Teacher's Note:
1. Deoxy means one oxygen less, at C-2'.
2. The base is attached at C-1' and phosphate at C-5'.

Answer: (d) Mutarotation

Explanation:
1. α-D-Glucose (+111°) and β-D-glucose (+19.2°) interconvert in solution through the open chain form.
2. The rotation settles at +52.5°, an effect called mutarotation.

Teacher's Note:
1. Mutarotation involves the α and β anomers.
2. Specific rotation is just a property, not a process.

Answer: (d) Quinoline

Explanation:
1. DNA contains adenine, guanine, cytosine and thymine.
2. Quinoline is not a nucleic acid base.

Teacher's Note:
1. Learn A, G, C, T for DNA and A, G, C, U for RNA.
2. Quinoline is just a heterocyclic compound.

Answer: (a) an α-amino acid

Explanation:
1. One –NH₂ group is on the carbon directly next to –COOH (the α carbon).
2. So lysine is an α-amino acid; the second amino group is in the side chain, which also makes it a basic amino acid.

Teacher's Note:
1. α means the –NH₂ is on the carbon next to –COOH.
2. All amino acids in proteins are α-amino acids.

Answer: (d) both hydrogen bonds and disulphide linkages

Explanation:
1. In fibrous proteins the chains run parallel to each other.
2. They are held together by hydrogen bonds and disulphide bonds, giving a fibre-like structure.

Teacher's Note:
1. Keratin and myosin are fibrous proteins.
2. Both bond types act together.

Answer: (d) All of these

Explanation:
1. Mono- and disaccharides are sweet and are called sugars; polysaccharides are not sweet and are non-sugars.
2. Maltose and lactose have a free hemiacetal –OH, so they reduce Fehling's solution.

Teacher's Note:
1. Check each statement separately.
2. All three statements follow from the definitions.