DNA and RNA contain four bases each. Which of the following bases is not present in RNA? (i) Adenine (ii) Urac

NCERT Exemplar · Q12

Q. DNA and RNA contain four bases each. Which of the following bases is not present in RNA?

(i) Adenine

(ii) Uracil

(iii) Thymine

(iv) Cytosine

Karnataka PUCMCQ· 1mImportance★★★★★

41% · 45/110 Questions

🔒 Locked · start free trial →

You're viewing a preview — the full solution, concept, methods & PYQ mapping are locked.

Start your 14-day free trial to unlock the full solution →

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: …

Start your 14-day free trial to unlock the full solution →

RNA uses uracil in place of thymine, which is found in DNA. Therefore, thymine is the base absent from RNA.

The key to this question lies in understanding the fundamental chemical difference between DNA and RNA. Both are nucleic acids built from nucleotides, and each nucleotide has three parts: a sugar, a phosphate group, and a nitrogenous base. The bases are divided into two families: purines (adenine and guanine) and pyrimidines (cytosine, thymine, and uracil).

DNA and RNA share three bases: adenine (A), guanine (G), and cytosine (C). The critical difference is in the fourth base. DNA uses thymine (T) as its second pyrimidine, while RNA uses uracil (U) instead. This substitution is a direct consequence of the different sugars in each molecule — deoxyribose in DNA and ribose in RNA — and has important implications for base-pairing and stability.

Let’s walk through the options:

Adenine (A) — This purine is present in both DNA and RNA. It pairs with thymine in DNA and with uracil in RNA. So it is definitely in RNA.
Uracil (U) — This is the pyrimidine that is unique to RNA. It replaces thymine and pairs with adenine during transcription and translation. So it is present in RNA. …

Start your 14-day free trial to unlock the full solution →

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

KCET 2022Set B-31 markMCQ
$Q. Which institute has approved the emergency use of 2-deoxy-D-Glucose as additive therapy for COVID-19 patients? (A) Ministry of Health and Family Welfare (B) Drug Controlled General of India (C) Indian Council of Medical Research (D) World Health Organisation$
›Reveal solutionSolution

The DCGI (Drug Controller General of India), India's national drug regulatory authority, granted Emergency Use Authorisation to 2-Deoxy-D-Glucose (2-DG) on 1 May 2021.

Step 1 — Which body regulates emergency drug approvals in India.

The Drug Controller General of India (DCGI), functioning under the Central Drugs Standard Control Organisation (CDSCO), is the statutory authority responsible for approving new drugs and granting emergency-use permissions in India — not ICMR (which conducts/coordinates medical research), the Ministry of Health and Family Welfare (the policy ministry), or the WHO (an international body with no domestic regulatory authority in India).

Step 2 — The specific approval.

2-Deoxy-D-Glucose (2-DG) is a glucose analogue developed by the Institute of Nuclear Medicine and Allied Sciences (INMAS), a DRDO laboratory, in collaboration with Dr Reddy's Laboratories. It accumulates preferentially in virus-infected cells (which have elevated glucose demand) and interferes with viral energy production and synthesis.

Step 3 — Timeline and use. …
Start your 14-day free trial to see the full solution →

KCET 2022Set B-31 markMCQ

Q. A Nucleic acid, whether DNA or RNA gives on complete hydrolysis, two purines bases, two pyrimidine bases, a pentose sugar and phosphoric acid. Nucleotides which are intermediate products in the hydrolysis contain (A) purine or pyrimidine base and orthophosphoric acid (B) purine or pyrimidine base, a pentose sugar and ortho-phosphoric acid (C) purine or pyrimidine base and pentose sugar (D) a purine base, pentose sugar and orthophosphoric acid

›Reveal solution

A nucleotide = base + pentose sugar + phosphate; only option (B) lists all three components.

1. The hydrolysis ladder

Complete hydrolysis of DNA/RNA gives bases, sugar and phosphoric acid. But hydrolysis is stepwise, and each stage has a name:

\text{Nucleic acid} \xrightarrow{\text{partial}} \textbf{Nucleotides} \xrightarrow{\text{--}H_3PO_4} \textbf{Nucleosides} \xrightarrow{\text{--sugar}} \text{Free bases}

The question asks about the intermediate stage, i.e. the nucleotide.

2. Definitions to keep straight

Species	Components	Linkages
Nucleoside	base + pentose sugar	$\beta$ -N-glycosidic bond (base $\to$ C-1′ of sugar)
Nucleotide	base + pentose sugar + phosphate	glycosidic bond plus an ester bond (phosphate $\to$ C-5′ –OH)

So a nucleotide is literally a nucleoside monophosphate:

\text{Nucleotide} = \text{Base} + \text{Sugar} + H_3PO_4

In the polymer, successive nucleotides are joined by phosphodiester links between the 3′-OH of one sugar and the 5′-phosphate of the next; breaking those links is what liberates the individual nucleotides.

3. Test the options …

Start your 14-day free trial to see the full solution →

KCET 2021Set B-21 markMCQ
$Q. RNA and DNA are chiral molecules, their chirality is due to the presence of (A) D-Sugar component (B) L-Sugar component (C) Chiral bases (D) Chiral phosphate ester unit$
›Reveal solutionSolution
The chirality of RNA and DNA arises entirely from the D-sugar (ribose or deoxyribose) in their backbone; the bases are planar and the phosphate ester unit is not chiral.

The question asks why RNA and DNA are chiral molecules. Chirality means a molecule is not superimposable on its mirror image — it has a handedness. In nucleic acids, the backbone is built from alternating sugar and phosphate units, with a nitrogenous base attached to each sugar. The key is to locate the source of asymmetry.

The sugar component is the only chiral part.

Both ribose (in RNA) and deoxyribose (in DNA) are aldopentoses. In their natural form, they have the D-configuration, meaning the hydroxyl group on the highest-numbered chiral carbon (C-4 in the open chain, or C-5 in the ring) points to the right in a Fischer projection. This D-sugar has multiple chiral centres (C-1, C-2, C-3, C-4 in the ring form), so it is optically active. The entire nucleic acid chain inherits this chirality from the sugar.

The bases are not chiral.

The nitrogenous bases — adenine, guanine, cytosine, thymine (DNA), and uracil (RNA) — are planar heterocyclic molecules. They contain no chiral carbon atoms. Even if a base were attached to the sugar, the base itself contributes no stereocentre.

The phosphate ester unit is not chiral.

In the phosphodiester linkage, the phosphorus atom is bonded to four oxygen atoms. In the physiological pH range, one of these oxygens carries a negative charge, but the phosphorus is not a chiral centre because it has two identical substituents (the two ester oxygens to the sugars) and two other oxygens that are not distinct in a way that creates a stereocentre — the phosphate group is achiral. …
Start your 14-day free trial to see the full solution →
KCET 2021Set B-21 markMCQ
$Q. Primary structure in a nucleic acid chain contains bases as G A T G C _______ The chain which is complementary to this chain is (A) G G T G A _____ (B) T G A A G _____ (C) C T A C G _____ (D) T T T T A G _____$
›Reveal solutionSolution
Apply the complementary base-pairing rules A–T and G–C to each base of the given sequence in turn.

Step 1 — The concept: complementary base pairing

In double-stranded DNA the two chains are held together by hydrogen bonds between specific base pairs (Watson–Crick pairing):

Adenine (A) ≡ Thymine (T) — joined by 2 hydrogen bonds.

Guanine (G) ≡ Cytosine (C) — joined by 3 hydrogen bonds.

The pairing is dictated by geometry (a purine must pair with a pyrimidine to keep the helix width constant) and by which H-bond donors/acceptors line up. Consequently the sequence of one strand completely determines the other — the basis of DNA replication.

Step 2 — Write the given strand and pair base by base

Given strand G A T G C
Rule G→C A→T T→A G→C C→G
Complementary strand C T A C G
$\text{G A T G C} \;\longrightarrow\; \text{C T A C G}$
Step 3 — Match with the options

(A) G G T G A — leaves G opposite G and A opposite A; violates the purine–pyrimidine rule outright. ✗ …
Start your 14-day free trial to see the full solution →
COMEDK 2021Set 2021-B1 markMCQ
$Q. Which one of the following statements is INCORRECT regarding DNA? (A) Process of melting occurs when the 2 strands of DNA separate from each other on heating. (B) Thymine and Adenine are joined by three Hydrogen bonds while Cytosine and Guanine by two Hydrogen bonds. (C) The double helical structure of DNA consists of two right handed helical polynucleotide chains coiled around the same central axis. (D) Total amount of Purines is equal to that of Pyrimidines (ie A+G=C+T)$
›Reveal solutionSolution
Statement (B) has the hydrogen-bond counts swapped, so it is the INCORRECT statement.

Watson–Crick base pairing:

Adenine = Thymine → 2 hydrogen bonds

Guanine ≡ Cytosine → 3 hydrogen bonds

Checking each option:

(A) Melting = strand separation on heating — correct.

(B) Claims A–T have 3 and C–G have 2 H-bonds — this is the reverse of reality, so it is INCORRECT.

(C) DNA is a right-handed double helix of two chains around a common axis — correct. …
Start your 14-day free trial to see the full solution →
KCET 2020Set A-11 markMCQ
$Q. Hypothyroidism is caused by the deficiency of (A) Glucocorticoid (B) Vitamin B-12 (C) Adrenalin (D) Thyroxine$
›Reveal solutionSolution

"Hypo-thyroid-ism" literally means too little thyroid hormone — a deficiency of thyroxine.

Step 1 — Decode the term.

Hypo- = below/under, thyroid = the thyroid gland. Hypothyroidism is therefore the condition in which the thyroid gland secretes too little of its hormone. The principal thyroid hormone is thyroxine ( $T_4$ ), along with triiodothyronine ( $T_3$ ).

Step 2 — Chemistry of thyroxine (why the iodine link).

Thyroxine is an iodinated derivative of the amino acid tyrosine — $T_4$ carries four iodine atoms. The body cannot synthesise it without dietary iodine, which is why an iodine-deficient diet causes hypothyroidism, and why the thyroid enlarges into a goitre trying to compensate. In infants a severe deficiency causes cretinism (stunted growth, mental retardation); in adults, myxoedema — sluggish metabolism, weight gain, cold intolerance. This is also the basis of using iodised salt as a public-health measure.

Step 3 — Its normal role. …
Start your 14-day free trial to see the full solution →
KCET 2019Set A-11 markMCQ
$\text{B(OH)}_3 + 2\text{H}_2\text{O} \longrightarrow [\text{B(OH)}_4]^- + \text{H}_3\text{O}^+$
›Reveal solutionSolution
Boron in $\text{B(OH)}_3$ has an empty $p$ -orbital and only a sextet of electrons — it accepts an electron pair from $\text{H}_2\text{O}$ , so it behaves as a Lewis acid, not a proton donor.

Step 1 — The two acid definitions in play.

Brønsted (protonic) acid = a proton ( $\text{H}^+$ ) donor.

Lewis acid = an electron-pair acceptor.

The question is asking which role $\text{B(OH)}_3$ actually plays, and the trap is that the reaction does produce $\text{H}_3\text{O}^+$ — so it superficially looks like a Brønsted acid.

Step 2 — Why boron is electron-deficient.

Boron has 3 valence electrons. In $\text{B(OH)}_3$ it forms three $\sigma$ -bonds to three $\text{–OH}$ groups, giving it only 6 electrons in its valence shell — an incomplete octet — and leaving an empty $2p$ orbital. A vacant orbital is exactly what makes a species hungry for an electron pair.

Step 3 — Trace the mechanism carefully.
$\text{B(OH)}_3 + 2\text{H}_2\text{O} \longrightarrow [\text{B(OH)}_4]^- + \text{H}_3\text{O}^+$
What actually happens:

A water molecule's oxygen lone pair attacks the empty $p$ -orbital of boron, forming a coordinate (dative) B←O bond. Boron now has 8 electrons and becomes 4-coordinate and tetrahedral.

That coordinated water is now positively polarised; it loses its own $\text{H}^+$ , giving $[\text{B(OH)}_4]^-$ .

The released $\text{H}^+$ is picked up by a second water molecule, producing $\text{H}_3\text{O}^+$ .

Step 4 — Assign the role. …
Start your 14-day free trial to see the full solution →
KCET 2018Set A-11 markMCQ
$Q. Which of the following bases is not present in DNA? (A) Adenine (B) Guanine (C) Cytosine (D) Uracil$
›Reveal solutionSolution
DNA contains four nitrogenous bases: adenine, guanine, cytosine, and thymine. Uracil is found in RNA, not DNA. The base not present in DNA is uracil.

The question tests a fundamental fact from molecular biology: the chemical composition of DNA versus RNA. Both are nucleic acids built from nucleotides, but they differ in two key ways — the sugar (deoxyribose in DNA, ribose in RNA) and one of the four bases. DNA uses thymine; RNA uses uracil in its place. So among the options, three are DNA bases and one is an RNA-specific base.

Let’s check each option:

Adenine (A) — A purine base. Present in both DNA and RNA. So it is in DNA.

Guanine (G) — Another purine base. Also present in both DNA and RNA. So it is in DNA.

Cytosine (C) — A pyrimidine base. Present in both DNA and RNA. So it is in DNA. …
Start your 14-day free trial to see the full solution →

Given strand	G	A	T	G	C
Rule	G→C	A→T	T→A	G→C	C→G
Complementary strand	C	T	A	C	G

←Previous questionQ11 — Each polypeptide in a protein has amino acids linked with each other i…→Next questionQ13 — Which of the following B group vitamins can be stored in our body? (i)…

Q.DNA and RNA contain four bases each. Which of the following bases is not present in RNA?

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: …

Method: Base Composition Comparison (RNA vs DNA)

Steps

The Core Concept First

Common Mistakes Students Make

1. Confusing Uracil and Thymine

2. Forgetting that Adenine and Cytosine are in both

3. Rushing and misreading the question

4. Thinking Uracil is absent from RNA …

1. The hydrolysis ladder

2. Definitions to keep straight

3. Test the options …

Step 1 — The concept: complementary base pairing

Step 2 — Write the given strand and pair base by base

Step 3 — Match with the options

Q.DNA and RNA contain four bases each. Which of the following bases is not present in RNA?

🎓Unlock everything free for 14 days

Unlock everything free for 14 days