Proteins and nucleic acids are the two biomolecules NEET tests most heavily within the biomolecules chapter — and the common trap is terminology confusion between structural levels, bond types, and nucleotide/nucleoside naming.
Proteins — what matters for NEET:
Proteins are polymers of amino acids joined by peptide bonds (formed by dehydration between –COOH of one amino acid and –NH₂ of the next). There are 20 standard amino acids. NCERT (Class 11 Biology Chapter 9, page 112) emphasises that protein structure operates at four levels:
Nucleic acids — what matters for NEET:
DNA and RNA are polymers of nucleotides. Each nucleotide = nitrogenous base + pentose sugar + phosphate group. A nucleoside lacks the phosphate (base + sugar only). DNA uses deoxyribose; RNA uses ribose. Bases: purines (Adenine, Guanine — double ring) and pyrimidines (Cytosine, Thymine in DNA / Uracil in RNA — single ring).
Watch-out: NEET distractors frequently swap nucleotide/nucleoside definitions, confuse secondary with tertiary structure stabilising bonds, or misattribute purine vs pyrimidine ring structures. Precision in terminology is the difference between +4 and –1.
Select an option to see the explanation. Wrong answers show why your choice was tempting — and name the exact trap it exploits.
A peptide bond is formed between which two functional groups of adjacent amino acids?
Answer: D. A peptide bond forms by a condensation reaction between the carboxyl group (–COOH) of one amino acid and the amino group (–NH₂) of the next, releasing water (NCERT Class 11 Biology Chapter 9, page 106).
Why A is wrong: A is wrong because –NH₂ to –NH₂ would not form a peptide linkage; no natural enzyme catalyses amine-amine condensation in protein synthesis.
Why B is wrong: B is wrong because R-group interactions contribute to tertiary structure (disulphide bonds, hydrophobic interactions), not to primary-sequence peptide linkage.
Why C is wrong: C is wrong because –COOH to –COOH condensation forms an acid anhydride, not a peptide bond.
Which level of protein structure is stabilised primarily by hydrogen bonds between backbone C=O and N–H groups (not R-group interactions)?
Answer: A. Secondary structure (α-helix and β-sheet) is stabilised by hydrogen bonds between peptide backbone carbonyl (C=O) and amide (N–H) groups (NCERT Class 11 Biology Chapter 9).
Why B is wrong: B is wrong because primary structure is determined solely by the sequence of amino acids held by covalent peptide bonds, not H-bonds.
Why C is wrong: C is wrong because tertiary structure involves R-group interactions — disulphide bonds, hydrophobic interactions, ionic bonds, and H-bonds between side chains, not backbone-to-backbone H-bonds.
Why D is wrong: D is wrong because quaternary structure refers to the spatial arrangement of multiple polypeptide subunits, stabilised by the same types of non-covalent interactions as tertiary but at the inter-subunit level.
A protein is assembled from 4 identical polypeptide subunits, and each subunit is stabilised internally by 3 disulphide bonds. What is the total number of disulphide bonds in the fully assembled protein, and what is the highest level of structural organisation it exhibits?
Answer: B. Each of the 4 subunits contains 3 disulphide bonds, so the assembled protein has 4 × 3 = 12 disulphide bonds in total. Because the protein is built from more than one polypeptide subunit, it exhibits quaternary structure — the highest level of protein organisation (NCERT Class 11 Biology, Chapter 9, page 112).
Why A is wrong: A is wrong because it adds the subunit count and the per-subunit bond count (4 + 3 = 7) instead of multiplying them (4 x 3 = 12), and it also mislabels a multi-subunit assembly as tertiary structure.
Why C is wrong: C is wrong because it gets the disulphide-bond total right (12) but mislabels the structure as tertiary — any protein assembled from more than one polypeptide subunit is, by definition, quaternary, not tertiary.
Why D is wrong: D is wrong because it reports only the disulphide bonds within a single subunit (3) and fails to multiply by the number of subunits (4) to get the protein-wide total.
A nucleoside differs from a nucleotide in that a nucleoside lacks:
Answer: C. A nucleoside = base + sugar. A nucleotide = base + sugar + phosphate group. Therefore the nucleoside lacks the phosphate (NCERT Class 11 Biology Chapter 9).
Why A is wrong: A is wrong because both nucleoside and nucleotide contain a nitrogenous base; removing the base would leave neither structure.
Why B is wrong: B is wrong because both nucleoside and nucleotide contain a pentose sugar; the sugar is the component that links base to phosphate.
Why D is wrong: D is wrong because hydrogen bonds are intermolecular interactions (e.g., between DNA strands), not a structural component of a single nucleotide or nucleoside.
Which of the following is a purine base found in both DNA and RNA?
Answer: B. Adenine is a purine (double-ring structure) present in both DNA and RNA. Guanine is the other purine. Cytosine, Thymine, and Uracil are all pyrimidines (NCERT Class 11 Biology Chapter 9).
Why A is wrong: A is wrong because cytosine is a pyrimidine (single-ring), not a purine — though it is present in both DNA and RNA.
Why C is wrong: C is wrong because thymine is a pyrimidine found only in DNA (replaced by uracil in RNA).
Why D is wrong: D is wrong because uracil is a pyrimidine found only in RNA (replaced by thymine in DNA).
The sugar in DNA nucleotides is deoxyribose. What distinguishes deoxyribose from ribose?
Answer: B. Deoxyribose lacks one oxygen at the 2' carbon (hence "deoxy" = without oxygen) compared to ribose which has a hydroxyl (–OH) at C-2' (NCERT Class 11 Biology Chapter 9).
Why A is wrong: A is wrong because it states the opposite — deoxyribose has FEWER oxygen atoms than ribose, not more.
Why C is wrong: C is wrong because both deoxyribose and ribose are pentose (5-carbon) sugars, not hexose (6-carbon).
Why D is wrong: D is wrong because the phosphate group's attachment point is a feature of the nucleotide linkage, not a defining structural difference between the two sugars themselves.
Disulphide bonds that stabilise the tertiary structure of a protein form between R-groups of which amino acid?
Answer: A. Cysteine contains a thiol (–SH) group in its R-chain. Two cysteine residues can form a covalent disulphide bond (–S–S–) that stabilises tertiary structure (NCERT Class 11 Biology Chapter 9).
Why B is wrong: B is wrong because glycine has the simplest R-group (a single hydrogen atom) and cannot form disulphide bonds.
Why C is wrong: C is wrong because alanine's R-group is a methyl group (–CH₃), which is hydrophobic but cannot form disulphide linkages.
Why D is wrong: D is wrong because valine has a branched hydrophobic R-group (isopropyl) — it contributes to hydrophobic interactions but has no sulphur atom for disulphide bond formation.
A student lists the following as correct base-pairing rules in DNA:
(i) A pairs with T via 2 hydrogen bonds
(ii) G pairs with C via 3 hydrogen bonds
(iii) A pairs with U via 2 hydrogen bonds
Which statement(s) apply specifically to DNA?
Answer: C. In DNA, A pairs with T (2 H-bonds) and G pairs with C (3 H-bonds). Statement (iii) describes A–U pairing, which occurs in RNA, not DNA. Therefore only (i) and (ii) apply to DNA (NCERT Class 11 Biology Chapter 9).
Why A is wrong: A is wrong because it includes (iii) — an RNA-specific pairing — and excludes (i), which is a fundamental DNA rule (A–T pairing).
Why B is wrong: B is wrong because statement (iii) describes A–U pairing which is specific to RNA; uracil replaces thymine only in RNA, not DNA.
Why D is wrong: D is wrong because it excludes (ii); G–C pairing with 3 hydrogen bonds is a defining feature of DNA base-pairing and must be included.
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Given
A polypeptide chain of 150 amino acids is folded into a functional protein. The protein has two polypeptide chains (identical) assembled together.
Required
(a) How many peptide bonds in one chain?
(b) What is the minimum structural level this protein exhibits?
Concept
A polypeptide of n amino acids has (n – 1) peptide bonds. If multiple polypeptide subunits assemble, the protein exhibits quaternary structure.
Formula
Number of peptide bonds = n – 1, where n = number of amino acids.
Substitution
Peptide bonds = 150 – 1 = 149.
Calculation
149 peptide bonds per chain. Two identical chains assembled = quaternary structure by definition (any multi-subunit arrangement is quaternary).
Final answer
(a) 149 peptide bonds per chain.
(b) Quaternary structure (two subunits).
Note: 150 and 2 are exact counting numbers and do not affect significant-figure considerations.
Common trap
Students often answer "150 peptide bonds" by forgetting to subtract 1. The bond forms BETWEEN amino acids — so n amino acids produce only (n – 1) bonds. Also, some students call a two-chain assembly "tertiary" — but any multi-subunit protein, even a dimer, is quaternary.
Similar NEET-style question
"Insulin has two polypeptide chains (A-chain: 21 amino acids, B-chain: 30 amino acids) linked by disulphide bonds. What is the total number of peptide bonds in one molecule of insulin, and what level of structural organisation does it represent?"
Answer: (21–1) + (30–1) = 20 + 29 = 49 peptide bonds; quaternary structure.
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Primary structure (sequence), secondary (α-helix, β-sheet), tertiary (3D fold, e.g. myoglobin), quaternary (multi-subunit, e.g. haemoglobin). 20 standard amino acids; essential vs non-essential.
-- NCERT Class 11 Biology, Ch. 9, p. 112More in Cell: Structure and Function: 14 exam traps and mistakes · 1 formula · 1 question pattern from its other lessons.
5 questions from NEET 2020, 2022, 2023, 2025, 2026. Answers verified against NTA official keys.
Identify the substances having glycosidic bond and peptide bond, respectively in their structure :
All 69 past-paper questions from Cell: Structure and Function →
Page numbers are the ones printed in the current NCERT textbook (2023 rationalised edition), unless marked pre-2023. The books are free at ncert.nic.in.
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