Nucleic acids are the polymers of ____. (i) nucleosides (ii) nucleotides (iii) bases (iv) sugars

NCERT Exemplar · Q9

Q. Nucleic acids are the polymers of _______.

(i) nucleosides

(ii) nucleotides

(iii) bases

(iv) sugars

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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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Nucleic acids are polymers of nucleotides, not nucleosides, bases, or sugars alone. The correct option is (ii).

Why This Question Tests a Core Distinction

The question looks simple, but it checks whether you understand the hierarchy of nucleic acid building blocks. Many students memorise "nucleotides are the monomers" without knowing why nucleosides or bases don't fit. Let's break that down.

Nucleic acids (DNA and RNA) are long chains. Each repeating unit must contain three things: a nitrogenous base, a pentose sugar (ribose or deoxyribose), and a phosphate group. That complete unit — base + sugar + phosphate — is called a nucleotide.

If you remove the phosphate, you get a nucleoside (base + sugar only). A nucleoside cannot link to form a polymer chain because it lacks the phosphate that forms the backbone. Similarly, a base alone or a sugar alone cannot polymerise into a nucleic acid.

Watch out

A common mistake is to pick "nucleosides" because they sound similar. Remember: nucleosides are precursors to nucleotides, but they are not the actual monomers. Without the phosphate, no phosphodiester bond can form.

Step-by-Step Reasoning

Identify the monomer unit

A polymer is built from repeating monomeric subunits. For nucleic acids, the monomer must be capable of forming a chain via phosphodiester bonds — a bond between the $3'$ hydroxyl of one sugar and the $5'$ phosphate of the next.
Check each option
- (iv) Sugars: A sugar alone (ribose or deoxyribose) has no base and no phosphate. It cannot form the specific linkages of a nucleic acid.
- (iii) Bases: Purines and pyrimidines are flat, aromatic rings. They attach to sugars but do not themselves polymerise. …

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Here is a breakdown of the common mistakes students make on this specific question, along with the conceptual fixes to avoid them.

The Correct Answer

The correct answer is (B) nucleotides.

Mistake #1: Confusing "Nucleoside" with "Nucleotide"

The Mistake: Students pick (A) nucleosides because they sound similar and are often taught together.

Why it’s wrong:

A nucleoside is just a base + sugar (no phosphate).
A nucleotide is a base + sugar + phosphate.
A polymer is a chain of repeating units joined by specific bonds. The bond in nucleic acids (the phosphodiester bond) forms between the phosphate of one unit and the sugar of the next. Since nucleosides lack a phosphate group, they cannot form this backbone.

How to Avoid:

Visualize the backbone. Draw a small segment of DNA/RNA in your mind. You will see a repeating pattern: Sugar-Phosphate-Sugar-Phosphate. The "Sugar-Phosphate" unit is the nucleotide. The "Sugar-Base" unit (nucleoside) is just a side group hanging off the backbone.
Remember the "P" rule: A polymer needs a Phosphate to make the Phosphodiester bond. Nucleosides have no P.

Mistake #2: Choosing "Bases" (Too Simplistic)

The Mistake: Students think DNA is just made of A, T, G, C, so they pick (C) bases.

Why it’s wrong:

The bases (A, T, G, C, U) are the information-carrying part, but they are not the structural repeating unit.
If you tried to polymerize just bases, you would not get a sugar-phosphate backbone. The molecule would not be a nucleic acid; it would just be a pile of flat rings.

How to Avoid:

Think of a train. The bases are the cargo (the information). The nucleotides are the train cars (the structural unit). The polymer is the whole train, made of cars, not just the cargo.
Definition check: A polymer is a long chain of monomers. The monomer of a nucleic acid is always a nucleotide.

Mistake #3: Picking "Sugars" (Incomplete)

The Mistake: Students know DNA has deoxyribose and RNA has ribose, so they pick (D) sugars.

Why it’s wrong:

A polymer of just sugars would be a polysaccharide (like starch or cellulose), not a nucleic acid.
Nucleic acids require both the sugar and the phosphate to form the backbone, and the bases to carry the genetic code.

How to Avoid:

Compare polymer classes:
- Proteins: Monomer = Amino acids.
- Polysaccharides: Monomer = Sugars (monosaccharides). …

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Q.Nucleic acids are the polymers of _______.

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: …

Why This Question Tests a Core Distinction

Step-by-Step Reasoning

Method: Definition-Based Recall (Direct Fact Retrieval)

Steps:

The Correct Answer

Mistake #1: Confusing "Nucleoside" with "Nucleotide"

Mistake #2: Choosing "Bases" (Too Simplistic)

Mistake #3: Picking "Sugars" (Incomplete)

Q.Nucleic acids are the polymers of _______.

Why This Question Tests a Core Distinction

Step-by-Step Reasoning

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