The following question is a case-based question. Read the case carefully and answer the questions that follow.

Question

Q. The following question is a case-based question. Read the case carefully and answer the questions that follow. Ribose and 2-deoxyribose have an important role in biology. Among the most important derivatives are those with phosphate groups attached at the 5 position. Mono-, di- and tri-phosphate forms are important, as well as 3-5 cyclic monophosphates. Purines and pyrimidines form an important class of compounds with ribose and deoxyribose. When these purine and pyrimidine derivatives are coupled to a ribose sugar, they are called nucleosides. Answer the following questions :

(a) What products would be formed when DNA is hydrolysed ? How is DNA different from RNA with reference to a structure ?

(b) Differentiate between nucleotide and nucleoside.

(c)

(i) Mention two important functions of nucleic acid.

(OR)

(c)

(ii) Name the linkage which joins two nucleotides. Name the base that is found in nucleotide of RNA but not in DNA.

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Concept understanding — Nucleic Acid Components

Nucleic Acid Components: The Alphabet of Life

Imagine you want to write a book that contains all the instructions for building and running a living organism. You'd need an alphabet — a set of letters that can be combined in endless ways to form words, sentences, and chapters. In living cells, that alphabet is made of nucleic acids (DNA and RNA), and each "letter" is called a nucleotide.

The Big Picture: What Are Nucleic Acids?

Nucleic acids are long, chain-like molecules that store and transmit genetic information. DNA holds the master blueprint; RNA helps execute it. But both are built from the same basic building block: the nucleotide.

Note

Think of a nucleotide as a single Lego brick. DNA and RNA are long chains of these bricks, each brick slightly different from the next.

The Three Parts of a Nucleotide

Every nucleotide has three components, like a three-part key:

A phosphate group — a small, negatively charged group ( $\text{PO}_4^{3-}$ ). It acts like the "handle" that links nucleotides together.
A sugar — either ribose (in RNA) or deoxyribose (in DNA). This is the "body" of the brick.
A nitrogenous base — a ring-shaped molecule containing nitrogen. This is the "colored part" that carries the actual information.

The sugar and base together form a nucleoside. When you add the phosphate, you get a nucleotide.

Nucleoside = Sugar + Base Nucleotide = Sugar + Base + Phosphate

The Two Families of Bases

The bases come in two structural types:

Purines (double-ring structures): Adenine (A) and Guanine (G)
Pyrimidines (single-ring structures): Cytosine (C), Thymine (T) (only in DNA), and Uracil (U) (only in RNA)

Tip

A mnemonic: Purines are Pure as All Gold (A and G). Pyrimidines are CUT (C, U, T).

DNA vs. RNA: The Key Differences

Feature	DNA	RNA
Sugar	Deoxyribose (missing one oxygen)	Ribose (has that oxygen)
Bases	A, G, C, T	A, G, C, U
Structure	Double-stranded helix	Usually single-stranded
Function	Stores genetic information	Carries and executes instructions

Watch out

A common mistake: thinking "nucleoside" and "nucleotide" are the same. Remember: nucleotide has the phosphate; nucleoside does not. ATP (adenosine triphosphate) is a nucleotide — it's the energy currency of the cell.

Why This Matters

The sequence of bases along a DNA strand spells out the genetic code. A change in even one base (a mutation) can alter a protein, sometimes with dramatic consequences — like sickle cell anemia, where a single base change in the hemoglobin gene causes red blood cells to deform. …

Why this formula?

Nucleic Acid Components: Understanding the "Why" Behind the Key Relationships

Let’s start with the big picture: Nucleic acids (DNA and RNA) are polymers made of nucleotides. Each nucleotide has three parts: a nitrogenous base, a pentose sugar, and a phosphate group. The key formulas and relationships in this topic arise from how these parts are linked and how they behave chemically.

1. The Basic Composition Formula

What it says:

A nucleotide = Base + Sugar + Phosphate

Why this holds:

Chemical necessity: The sugar (ribose in RNA, deoxyribose in DNA) has a 5-carbon ring. The base attaches to the 1' carbon (via a glycosidic bond), and the phosphate attaches to the 5' carbon (via an ester bond).
Directionality: This creates a 5' → 3' linkage in the polymer. The phosphate of one nucleotide bonds to the 3' carbon of the next sugar.
Reasoning: Without the phosphate, you have a nucleoside (base + sugar). Adding the phosphate makes it a nucleotide — the monomer that can polymerize.

Key takeaway: The formula isn’t arbitrary — it reflects the specific carbon positions on the sugar that allow for stable, directional chain formation.

2. Chargaff’s Rules (for DNA)

What it says:

In double-stranded DNA:

$[A] = [T]$
$[G] = [C]$
$[A] + [G] = [T] + [C]$

Why this holds:

Base pairing: Adenine (A) forms two hydrogen bonds with Thymine (T). Guanine (G) forms three hydrogen bonds with Cytosine (C).
Structural constraint: The DNA double helix has a constant width (20 Å). A purine (A or G) always pairs with a pyrimidine (T or C) — otherwise the helix would bulge or narrow.
Derivation: If every A on one strand must pair with a T on the opposite strand, then the number of A equals number of T in the whole molecule. Same for G and C.
Consequence: The sum of purines equals sum of pyrimidines ( $A+G = T+C$ ).

Why it’s not just a rule: It’s a geometric and energetic necessity — hydrogen bonding and helix stability force this equality.

3. The Phosphodiester Bond Energy Formula

What it says:

Formation of a phosphodiester bond requires ~30 kJ/mol of energy (from ATP).

Why this holds:

Mechanism: The 3' hydroxyl of one nucleotide attacks the α-phosphate of a nucleotide triphosphate (e.g., ATP). This releases pyrophosphate (PPi).
Energy source: The hydrolysis of PPi to two inorganic phosphates ( $PPi \rightarrow 2P_i$ ) is highly exergonic (ΔG ≈ -30 kJ/mol). This drives the reaction forward.
Reasoning: The bond itself is a covalent ester linkage — strong but not spontaneously formed. The energy comes from breaking a high-energy phosphate bond in the triphosphate.

Key insight: The formula isn’t about the bond’s strength — it’s about the thermodynamic cost of making it in a cell.

4. The Melting Temperature ( $T_m$ ) Formula

What it says:

$T_m$ (in °C) ≈ $4(G+C) + 2(A+T)$ for short oligonucleotides.

Why this holds:

Hydrogen bonds: G-C pairs have 3 H-bonds, A-T pairs have 2 H-bonds. More H-bonds = more energy needed to separate strands.
Stacking interactions: G-C base pairs also have stronger π-stacking (aromatic ring overlap) than A-T.
Derivation: The formula is empirical — it comes from measuring $T_m$ for many sequences. The coefficients (4 and 2) reflect the relative stability contributed by each base pair.
Limitation: For long DNA, this simple formula fails because nearest-neighbor interactions matter more.

Why it works: It’s a linear approximation of the free energy needed to break all base pairs, weighted by H-bond count.

5. The Central Dogma (Information Flow)

What it says:

DNA → RNA → Protein

Why this holds: …

(a) DNA hydrolysis → 2‑deoxyribose + phosphoric acid + bases A, G, C, T; DNA differs from RNA in having deoxyribose (not ribose) and thymine (not uracil), and is double‑stranded. (b) Nucleoside = base + sugar, nucleotide = base + sugar + phosphate. (c)(i) Nucleic acids store genetic information and direct protein synthesis. OR (c)(ii) The joining linkage is a 3′–5′ phosphodiester bond; the RNA‑only base is uracil.

Every nucleic acid is a polymer of nucleotides, each built from a nitrogenous base, a pentose sugar and a phosphate group. Hydrolysis simply reverses that assembly.

Products of DNA hydrolysis. Water breaks two kinds of bonds — the phosphodiester bonds between nucleotides and the glycosidic bonds between sugar and base. Complete hydrolysis therefore liberates:

2‑deoxyribose (a 5‑carbon sugar lacking the –OH at C‑2′),
phosphoric acid,
the four bases adenine, guanine, cytosine and thymine.

Watch out

List thymine, not uracil, for DNA. Uracil belongs to RNA.

How DNA differs from RNA structurally.

Feature	DNA	RNA
Sugar	2‑deoxyribose (no 2′‑OH)	ribose (2′‑OH present)
Bases	A, G, C, T	A, G, C, U
Strands	double helix (antiparallel)	usually single

The 2′‑OH of ribose makes RNA more reactive and less stable, which is why DNA is the long‑term store of information.

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Concept understanding — Biochemical Bonds

Biochemical Bonds: The Glue That Holds Life Together

Imagine you're building with LEGO bricks. Some bricks click together tightly and never come apart unless you really yank them. Others snap together lightly and can be pulled apart with a gentle tug. Some bricks don't even click — they just stick because of static cling or magnetism.

Biochemical bonds are exactly like that. They are the forces that hold atoms together inside the molecules of your body — your DNA, proteins, fats, and carbohydrates. Without these bonds, you'd literally fall apart into a pile of individual atoms.

The Core Idea

Atoms bond because being bonded is more stable (lower energy) than being alone. Think of it like this: a single atom is like a person standing alone in a cold room. Bonding is like huddling together for warmth — you lose some freedom of movement, but you gain stability.

In biochemistry, we care about four main types of bonds. They differ in strength, how they form, and what they do in living systems.

1. Covalent Bonds — The Strong, Permanent LEGO Clicks

This is the strongest bond in biology. Two atoms share electrons — like two people holding the same umbrella. Each atom contributes one or more electrons, and they both "own" the pair.

Key properties:

Very strong (100–400 kJ/mol)
Forms the backbone of all biomolecules
Takes a lot of energy (or enzymes) to break

Where you find it:

The carbon-carbon bonds in your DNA's sugar-phosphate backbone
The peptide bonds linking amino acids into proteins
The bonds within a glucose molecule

Note

A single covalent bond shares 2 electrons. A double bond shares 4. Triple bonds are rare in biology but exist (e.g., in cyanide).

2. Ionic Bonds — The Static Cling of Opposites

Some atoms steal electrons from others. When that happens, one atom becomes positively charged (lost an electron) and the other becomes negatively charged (gained one). Opposite charges attract — that's an ionic bond.

Key properties:

Moderate strength (5–100 kJ/mol in dry conditions)
Very weak in water (because water molecules get in between)
Easily broken by changes in pH or salt concentration

Where you find it:

In salt bridges that help proteins fold into their correct shape
Between the phosphate groups of DNA and positively charged proteins (histones)

Watch out

Ionic bonds are often called "bonds" but in water they behave more like attractions. Don't confuse them with covalent bonds — they're much weaker in biological fluids.

3. Hydrogen Bonds — The Gentle, Reversible Magnets

This is the most important weak bond in biology. A hydrogen atom that's already covalently bonded to an electronegative atom (like oxygen or nitrogen) gets a slight positive charge. It then gets attracted to another electronegative atom nearby.

Think of it like a weak magnet — it holds things together but can be easily undone.

Key properties:

Weak individually (5–30 kJ/mol)
But many together can be very strong
Easily broken by heat or changes in pH
Directional — they only work when atoms are properly aligned

Where you find it:

Between the two strands of DNA (this is what holds the double helix together)
In protein folding (between amino acids in the backbone)
Between water molecules (giving water its unique properties)

Important

Hydrogen bonds are the reason DNA can unzip for replication. If DNA used covalent bonds between strands, it would be impossible to separate without destroying the molecule.

4. Van der Waals Interactions — The Fleeting, Accidental Touches

Even neutral atoms have temporary, uneven distributions of electrons. These create tiny, momentary charges that attract nearby atoms. It's like two people accidentally brushing shoulders in a crowd — brief, weak, but real.

Key properties:

Extremely weak (0.5–5 kJ/mol per interaction)
Only work when atoms are very close (within 0.3–0.4 nm)
Add up significantly when many atoms are packed together

Where you find it:

In the hydrophobic core of proteins (where oily amino acids pack tightly)
Between lipid tails in cell membranes
In enzyme-substrate binding (helps "grip" the substrate)

Putting It All Together: A Biological Example

Consider a protein in your body. It's a long chain of amino acids held together by covalent peptide bonds. That chain then folds into a specific shape. The folding is guided by:

Hydrogen bonds between backbone atoms (forming alpha helices and beta sheets)
Ionic bonds between charged side chains …

Why this formula?

Biochemical Bonds: Why the Key Formulas Hold

Biochemical bonds are the forces that hold atoms together in biomolecules. The key formulas come from electrostatics and quantum mechanics — not from biology itself. Let's break down the why behind the most important ones.

1. Ionic Bond Energy: Coulomb's Law

Formula:

E = \frac{k \cdot q_1 \cdot q_2}{r}

Why it holds:

Opposite charges attract — this is a fundamental law of physics (Coulomb's law).
In a biochemical context, consider a sodium ion ( $\text{Na}^+$ ) and a chloride ion ( $\text{Cl}^-$ ). The energy released when they come together is directly proportional to the product of their charges ( $q_1 q_2$ ) and inversely proportional to the distance ( $r$ ) between them.
The constant $k$ accounts for the medium (water vs. vacuum). In water, the effective force is weaker because water molecules partially shield the charges — this is why ionic bonds in biology are often weaker in aqueous environments.

Key insight: The formula is not arbitrary — it's derived from the inverse-square law of electrostatics, integrated over the distance the charges move toward each other.

2. Covalent Bond Energy: The Morse Potential (Approximation)

Formula (simplified):

E = D_e \left(1 - e^{-a(r - r_0)}\right)^2

Why it holds:

Covalent bonds arise from shared electrons between atoms. The energy is not a simple inverse-square law because electrons are delocalized.
The Morse potential is an empirical formula that captures two key observations:
1. At equilibrium distance ( $r_0$ ): Energy is minimum ( $E = 0$ in this form).
2. If atoms are pulled apart ( $r \to \infty$ ): Energy approaches $D_e$ (the bond dissociation energy).
3. If atoms are pushed too close ( $r \to 0$ ): Energy skyrockets due to Pauli repulsion (electrons can't occupy the same space).
The exponential term $e^{-a(r - r_0)}$ models the rapid drop in attractive force as distance increases — this comes from quantum mechanical overlap of electron clouds.

Key insight: The formula is a curve fit to quantum mechanical calculations, not a first-principles derivation. But it works because it respects the physics: attraction at long range, repulsion at short range, and a stable minimum.

3. Hydrogen Bond Energy: Dipole-Dipole Interaction

Formula (approximate):

E \approx -\frac{2 \mu_1 \mu_2}{4 \pi \epsilon_0 r^3} \cdot \cos\theta

Why it holds:

A hydrogen bond (e.g., between water molecules) is not a true bond — it's a strong dipole-dipole interaction.
The dipole moment ( $\mu$ ) arises because oxygen is more electronegative than hydrogen, creating partial charges ( $\delta^+$ and $\delta^-$ ).
The energy depends on:
- Strength of dipoles ( $\mu_1 \mu_2$ )
- Distance ( $r$ ) — falls off as $1/r^3$ , much faster than ionic bonds ( $1/r$ )
- Orientation ( $\cos\theta$ ) — strongest when dipoles are aligned head-to-tail

Key insight: The $1/r^3$ dependence comes from the derivative of the dipole field. Unlike point charges, dipoles have a field that decays faster — this is why hydrogen bonds are directional and weaker than covalent bonds.

4. Van der Waals Interaction: Lennard-Jones Potential

Formula:

E = 4\epsilon \left[ \left(\frac{\sigma}{r}\right)^{12} - \left(\frac{\sigma}{r}\right)^6 \right]

Why it holds:

Van der Waals forces arise from temporary fluctuations in electron distribution — even nonpolar molecules have instantaneous dipoles. …

Every nucleic acid is a polymer of nucleotides, each built from a nitrogenous base, a pentose sugar and a phosphate group. Hydrolysis simply reverses that assembly.

(b) Nucleoside vs nucleotide

Nucleoside = nitrogenous base + pentose sugar only (e.g. adenosine, deoxyadenosine). The base is attached to C‑1′ of the sugar by a β‑glycosidic bond.
Nucleotide = nucleoside + one or more phosphate groups esterified at C‑5′ (e.g. AMP, dATP).

Tip

"‑oside" = sugar + base only; "‑otide" = oside + phosphate.

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Same / Similar Concept — real previous-year questions on the same or a closely similar concept, not this exact question.

Showing the 12 most recent of 56 on this concept.

CBSE 2026Set 56/2/11 markMCQ
$Q. The base which is present in DNA but not in RNA is : (A) Guanine (B) Cytosine (C) Thymine (D) Adenine$
›Reveal solutionSolution
The key difference between DNA and RNA is that DNA contains thymine (T) while RNA contains uracil (U) in its place. The base present in DNA but absent in RNA is thymine, which corresponds to option (C).

Why This Question Tests a Fundamental Distinction

DNA and RNA are both nucleic acids, but they serve different roles in the cell — DNA stores genetic information, while RNA helps express it. One of the clearest chemical differences between them lies in their nitrogenous bases. Both use adenine (A), guanine (G), and cytosine (C), but they differ in the fourth base: DNA uses thymine (T), and RNA uses uracil (U). This is not a trivial detail — it affects base pairing, stability, and even how enzymes distinguish the two molecules.

The question asks you to spot which base is exclusive to DNA. Let’s walk through the reasoning.

List the bases common to both DNA and RNA.

Adenine (A), guanine (G), and cytosine (C) appear in both nucleic acids. So options (A), (B), and (D) are present in RNA as well — they cannot be the answer.

Identify the base unique to DNA.

Thymine (T) is found in DNA but is replaced by uracil (U) in RNA. This substitution is a defining chemical difference. Therefore, thymine is the base present in DNA but not in RNA.

Confirm by elimination. …
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CBSE 2026Set 56/2/11 markMCQ
$\alpha$
›Reveal solutionSolution
Proteins are linear chains of $\alpha$ -amino acids linked by peptide bonds — a specific type of covalent bond formed between the carboxyl group of one amino acid and the amino group of the next, with the elimination of water. The correct answer is (B) Peptide Bond.

Why This Question Tests a Core Biochemical Idea

The question is about the primary structure of proteins — the simplest level of protein organization. A protein is a polymer, and like any polymer, it has repeating units (monomers) held together by a specific chemical linkage. The monomers here are $\alpha$ -amino acids, and the linkage that joins them is not just any covalent bond — it has a special name because of how it forms and its unique properties.

Many students get confused because all the options are types of bonds found in biomolecules. The trick is to match the bond to the specific monomers and the reaction that creates it.

Step-by-Step Reasoning

Identify the monomers. Proteins are built from $\alpha$ -amino acids. Each amino acid has a central carbon ( $\alpha$ -carbon) bonded to an amino group ( $-\mathrm{NH}_2$ ), a carboxyl group ( $-\mathrm{COOH}$ ), a hydrogen atom, and a variable side chain ( $R$ ). The key functional groups for linking are the $-\mathrm{NH}_2$ and $-\mathrm{COOH}$ groups.

Understand how two amino acids join. When two amino acids link, the carboxyl group of the first amino acid reacts with the amino group of the second. This is a condensation reaction (also called dehydration synthesis) — a molecule of water ( $\mathrm{H}_2\mathrm{O}$ ) is removed. The oxygen from the carboxyl group and two hydrogens from the amino group form the water.

Name the resulting bond. The chemical bond that forms between the carbon of the first amino acid's carboxyl group and the nitrogen of the second amino acid's amino group is called an amide bond. In biochemistry, this specific amide bond between amino acids is universally known as a peptide bond. The structure is $-\mathrm{CO}-\mathrm{NH}-$ .

Eliminate the other options.

(A) Covalent Bond — This is too broad. A peptide bond is a covalent bond, but the question asks for the specific name of the bond joining amino acids in proteins. "Covalent bond" is the general category, not the precise answer.

(C) Glycosidic Bond — This bond joins monosaccharides to form carbohydrates (e.g., starch, cellulose). It involves a sugar's anomeric carbon and an alcohol or another sugar. It has nothing to do with amino acids.

(D) Coordinate Bond — This is a special type of covalent bond where both shared electrons come from the same atom. It is found in coordination complexes (e.g., metal ions with ligands) and is not the standard linkage in protein backbones. …
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CBSE 2026Set ANNUAL1 markMCQ
$Q. Which of the following is not a pyrimidine base?$
(a) Adenine
(b) Uracil
(c) Thymine
(d) Cytosine
›Reveal solutionSolution
Adenine is a purine, not a pyrimidine; the other three listed bases (cytosine, uracil, thymine) are pyrimidines.

Nitrogenous bases found in nucleic acids are classified into two structural families:

Purines: fused bicyclic (six-membered + five-membered ring) heterocycles — these are Adenine (A) and Guanine (G).

Pyrimidines: single six-membered heterocyclic ring — these are Cytosine (C), Thymine (T, found in DNA), and Uracil (U, found in RNA in place of thymine). …
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CBSE 2026Set ANNUAL1 mark
$Q. Write any one difference between DNA and RNA.$
›Reveal solutionSolution

DNA and RNA differ in their sugar component, strand structure, and one nitrogenous base.

One clear structural difference: the sugar in DNA is deoxyribose (lacking an -OH at C2'), whereas the sugar in RNA is ribose (has an -OH at C2'). (Other valid differences: DNA is usually double-stranded/helical while RNA is usually single-stra …

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CBSE 2026Set ANNUAL1 mark
$Q. Read the following paragraph and answer the question given below: Complete hydrolysis of DNA/RNA gives a pentane sugar, phosphoric acid and nitrogen containing heterolysis base. In DNA, sugar is β-D-2-deoxyribose and in RNA, sugar is β-D-ribose. DNA has four bases i.e. adenine, guanine, cytosine and thymine. In RNA instead of thymine, the bases is uracil and other three bases are same. Base attached to sugar forms nucleoside, while, sugar molecule joined to phosphate group and base forms nucleotide.$
(i) Name the bases present in RNA.
›Reveal solutionSolution

RNA's four bases are adenine, guanine, cytosine and uracil.

Nucleic acids are built from purine and pyrimidine bases attached to a sugar-phosphate backbone. The purines (adenine and guanine) are common to both DNA and RNA. Among the pyrimidines, DNA uses cytosine and thymine, while RNA uses cytosine and uracil in place …

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CBSE 2026Set ANNUAL1 mark
$Q. Read the following paragraph and answer the question given below: Complete hydrolysis of DNA/RNA gives a pentane sugar, phosphoric acid and nitrogen containing heterolysis base. In DNA, sugar is β-D-2-deoxyribose and in RNA, sugar is β-D-ribose. DNA has four bases i.e. adenine, guanine, cytosine and thymine. In RNA instead of thymine, the bases is uracil and other three bases are same. Base attached to sugar forms nucleoside, while, sugar molecule joined to phosphate group and base forms nucleotide.$
(ii) What is nucleotide?
›Reveal solutionSolution

A nucleotide = nitrogenous base + pentose sugar + phosphate group.

When a nitrogenous base (purine or pyrimidine) is attached to the pentose sugar (ribose or deoxyribose) through a $\beta$ -N-glycosidic bond, the unit formed is called a nucleoside. When the 5'-OH of this sugar is further esterified with phosphoric acid, the resulting base-sugar-phosphate unit is called a nucleotide. Nucleotides are the repeating monomer units that are joined by phosphodiester bond …

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CBSE 2026Set ANNUAL1 mark
$Q. Read the following paragraph and answer the question given below: Complete hydrolysis of DNA/RNA gives a pentane sugar, phosphoric acid and nitrogen containing heterolysis base. In DNA, sugar is β-D-2-deoxyribose and in RNA, sugar is β-D-ribose. DNA has four bases i.e. adenine, guanine, cytosine and thymine. In RNA instead of thymine, the bases is uracil and other three bases are same. Base attached to sugar forms nucleoside, while, sugar molecule joined to phosphate group and base forms nucleotide.$
(iii) Name the sugar present in DNA.
›Reveal solutionSolution

DNA's sugar is $\beta$ -D-2-deoxyribose (lacks an -OH at the 2' carbon).

DNA and RNA differ in the pentose sugar of their backbone. DNA contains $\beta$ -D-2-deoxyribose, in which the 2'-carbon of the ribose ring bears only hydrogen atoms (no -OH group) — hence the name 'deoxy'. This lack of a 2'-OH makes DNA che …

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CBSE 2026Set ANNUAL1 mark
$Q. Read the following paragraph and answer the question given below: Complete hydrolysis of DNA/RNA gives a pentane sugar, phosphoric acid and nitrogen containing heterolysis base. In DNA, sugar is β-D-2-deoxyribose and in RNA, sugar is β-D-ribose. DNA has four bases i.e. adenine, guanine, cytosine and thymine. In RNA instead of thymine, the bases is uracil and other three bases are same. Base attached to sugar forms nucleoside, while, sugar molecule joined to phosphate group and base forms nucleotide.$
(iv) Name the sugar present in RNA.
›Reveal solutionSolution

RNA's sugar is $\beta$ -D-ribose (has a free -OH at the 2' carbon).

Unlike DNA, RNA retains a hydroxyl group at the 2'-carbon of its pentose sugar, so its sugar is called $\beta$ -D-ribose (not 'deoxy'). This extra -OH group makes the RNA backbone more reactive/less stable than DNA's, which is consistent with RNA's shor …

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CBSE 2026Set ANNUAL1 mark
$Q. Read the following paragraph and answer the question given below: Complete hydrolysis of DNA/RNA gives a pentane sugar, phosphoric acid and nitrogen containing heterolysis base. In DNA, sugar is β-D-2-deoxyribose and in RNA, sugar is β-D-ribose. DNA has four bases i.e. adenine, guanine, cytosine and thymine. In RNA instead of thymine, the bases is uracil and other three bases are same. Base attached to sugar forms nucleoside, while, sugar molecule joined to phosphate group and base forms nucleotide.$
(v) Name the bases present in DNA.
›Reveal solutionSolution

DNA's four bases are adenine, guanine, cytosine and thymine.

DNA carries two purine bases, adenine (A) and guanine (G), and two pyrimidine bases, cytosine (C) and thymine (T). In the DNA double helix these bases pair specifically by hydrogen bonding — A pairs with T (two H-bonds) and G pairs with C (three H-bonds) — wh …

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CBSE 2025Set D1 markMCQ
$Q. The purine base present in RNA is$
(a) Guanine
(b) Thymine
(c) Cytosine
(d) Uracil
›Reveal solutionSolution
Nucleic acid bases are purines (adenine, guanine) or pyrimidines (cytosine, thymine, uracil). The purine in RNA among the options is guanine.

The nitrogenous bases in nucleic acids are of two types:

Purines (two fused rings): Adenine (A) and Guanine (G).

Pyrimidines (single ring): Cytosine (C), Thymine (T) and Uracil (U). …
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CBSE 2025Set A1 mark
$Q. Fill in the blank: If third amino acid combines to a dipeptide, the product is called a ______.$
›Reveal solutionSolution

A peptide is named by how many amino acid residues it contains: two = dipeptide, three = tripeptide, and so on.

When two amino acids join through a peptide (amide) bond, formed by condensation between the –COOH of one amino acid and the –NH2 of another (with loss of a water molecule), the product is a dipeptide. If a third amino acid molecule now condenses onto this dipeptide (forming another peptide bond), the r …

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CBSE 2025Set ANNUAL1 mark
$Q. Draw a diagram of double strand helix structure for DNA.$
›Reveal solutionSolution
Figure — Labelled DNA double helix (Watson-Crick model). Two ANTIPARALLEL sugar-phosphate backbones coiled RI

The Watson-Crick double helix model of DNA: two antiparallel polynucleotide strands wound around a common axis, sugar-phosphate backbone outside, complementary bases paired by hydrogen bonds on the inside.

A labelled diagram of the DNA double helix would show the following key features, described here in place of a drawing:

TWO POLYNUCLEOTIDE STRANDS: each strand is a chain of alternating deoxyribose sugar and phosphate units (the sugar-phosphate backbone), drawn as two continuous ribbons running along the outside of the structure.

ANTIPARALLEL ORIENTATION: the two strands run in opposite directions - one strand 5' -> 3' from top to bottom, the other 3' -> 5' from top to bottom.

RIGHT-HANDED HELIX: the two backbones are coiled around a common central axis in a right-handed spiral (like a twisted ladder).

…
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Concept understanding — Nucleic Acid Components

Nucleic Acid Components: The Alphabet of Life

The Big Picture: What Are Nucleic Acids?

The Three Parts of a Nucleotide

The Two Families of Bases

DNA vs. RNA: The Key Differences

Why This Matters

Why this formula?

Nucleic Acid Components: Understanding the "Why" Behind the Key Relationships

1. The Basic Composition Formula

What it says:

Why this holds:

2. Chargaff’s Rules (for DNA)

What it says:

Why this holds:

3. The Phosphodiester Bond Energy Formula

What it says:

Why this holds:

4. The Melting Temperature (TmT_mTm​) Formula

What it says:

Why this holds:

5. The Central Dogma (Information Flow)

What it says:

Why this holds: …

Concept understanding — Biochemical Bonds

Biochemical Bonds: The Glue That Holds Life Together

The Core Idea

1. Covalent Bonds — The Strong, Permanent LEGO Clicks

2. Ionic Bonds — The Static Cling of Opposites

3. Hydrogen Bonds — The Gentle, Reversible Magnets

4. Van der Waals Interactions — The Fleeting, Accidental Touches

Putting It All Together: A Biological Example

Why this formula?

Biochemical Bonds: Why the Key Formulas Hold

1. Ionic Bond Energy: Coulomb's Law

2. Covalent Bond Energy: The Morse Potential (Approximation)

3. Hydrogen Bond Energy: Dipole-Dipole Interaction

4. Van der Waals Interaction: Lennard-Jones Potential

(b) Nucleoside vs nucleotide

Concept understanding — this question

(c)(i) Two functions of nucleic acids

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4. The Melting Temperature ( $T_m$ ) Formula

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