Assertion (A): Deoxyribose, $\text{C}_5\text{H}_{10}\text{O}_4$ is not a carbohydrate. Reason (R): Carbohydrates are hydrates of carbon so compounds which follow $\text{C}_x(\text{H}_2\text{O})_y$ formula are carbohydrates. (i) Both A and R are true and R is the correct explanation of A. (ii) Both A and R are true but R is not the correct explanation of A. (iii) A is true but R is false. (iv) Both A and R are false.

Assertion (A): Deoxyribose, C₅H₁₀O₄ is not a carbohydrate. Reason (R): Carbohydrates are hydrates of carbon so

NCERT Exemplar · Q69

\text{C}_5\text{H}_{10}\text{O}_4

(i) Both A and R are true and R is the correct explanation of A.

(ii) Both A and R are true but R is not the correct explanation of A.

(iii) A is true but R is false.

(iv) Both A and R are false.

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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: …

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Deoxyribose lacks one oxygen atom relative to the general carbohydrate formula $\text{C}_x(\text{H}_2\text{O})_y$ , so it is not a carbohydrate by that definition. The assertion is true, but the reason is false because the “hydrates of carbon” formula is a useful guide, not a strict rule — many true carbohydrates deviate from it.

The heart of this question lies in understanding what the word “carbohydrate” actually means — and where the classic formula $\text{C}_x(\text{H}_2\text{O})_y$ comes from.

Historically, chemists noticed that many sugars like glucose ( $\text{C}_6\text{H}_{12}\text{O}_6$ ) and sucrose ( $\text{C}_{12}\text{H}_{22}\text{O}_{11}$ ) seemed to be “hydrates of carbon” — that is, they looked like carbon atoms with water molecules attached. That’s where the name carbo-hydrate came from.

But here’s the catch: that formula is a description, not a definition. It’s a pattern that many carbohydrates follow, but it’s not a strict rule. Some true carbohydrates (like rhamnose, $\text{C}_6\text{H}_{12}\text{O}_5$ ) don’t fit it, and some non-carbohydrates (like formaldehyde, $\text{CH}_2\text{O}$ ) do.

Now let’s examine deoxyribose.

Check the formula against the “hydrate” pattern.

Deoxyribose is $\text{C}_5\text{H}_{10}\text{O}_4$ . If it were a “hydrate of carbon,” we’d expect it to be $\text{C}_5(\text{H}_2\text{O})_5 = \text{C}_5\text{H}_{10}\text{O}_5$ . But it has only 4 oxygen atoms — one oxygen short. So by the strict $\text{C}_x(\text{H}_2\text{O})_y$ formula, it doesn’t qualify.
Why is deoxyribose not a carbohydrate?

The real reason is structural: deoxyribose is a modified sugar where one hydroxyl group ( $-\text{OH}$ ) is replaced by a hydrogen atom. That’s why it’s called deoxy — it’s “deoxygenated.” In biochemistry, it’s classified as a deoxy sugar, not a carbohydrate in the strict sense. Carbohydrates are polyhydroxy aldehydes or ketones; deoxyribose lacks one of those hydroxyl groups.
Evaluate the Reason (R). …

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Q.Assertion (A): Deoxyribose, $\text{C}_5\text{H}_{10}\text{O}_4$ is not a carbohydrate.
Reason (R): Carbohydrates are hydrates of carbon so compounds which follow $\text{C}_x(\text{H}_2\text{O})_y$ formula are carbohydrates.

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 ( $T_m$ ) Formula

What it says:

Why this holds:

5. The Central Dogma (Information Flow)

What it says:

Why this holds: …

Step 1: Understand the Assertion (A)

Step 2: Understand the Reason (R)

Step 1: Understand the Assertion (A)

Step 2: Understand the Reason (R)

Step 3: Conclusion

🚩 Common Mistakes Students Make & How to Avoid Them

Mistake 1: Thinking deoxyribose is not a carbohydrate

Mistake 2: Believing the $\text{C}_x(\text{H}_2\text{O})_y$ formula is a strict rule

Q.Assertion (A): Deoxyribose, C5H10O4\text{C}_5\text{H}_{10}\text{O}_4C5​H10​O4​ is not a carbohydrate. Reason (R): Carbohydrates are hydrates of carbon so compounds which follow Cx(H2O)y\text{C}_x(\text{H}_2\text{O})_yCx​(H2​O)y​ formula are carbohydrates.

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Q.Assertion (A): Deoxyribose, $\text{C}_5\text{H}_{10}\text{O}_4$ is not a carbohydrate.
Reason (R): Carbohydrates are hydrates of carbon so compounds which follow $\text{C}_x(\text{H}_2\text{O})_y$ formula are carbohydrates.

Unlock everything free for 14 days