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Nucleic Acids: DNA vs RNA and Their Sugars

Biology · Biomolecules · NEET

Nucleic acids are long chains of nucleotides. DNA uses the sugar deoxyribose and the base thymine (T), while RNA uses the sugar ribose and the base uracil (U). Both DNA and RNA share the two purines, adenine (A) and guanine (G). Memory hook: "D-D" = DNA has Deoxyribose; "R-R" = RNA has Ribose. Uracil is the U in RNA.
DNA vs RNADNARNASugar: DeoxyriboseSugar: RiboseBase: Thymine (T)Base: Uracil (U)Usually double strandUsually single strandShared: A, G, CShared: A, G, C
DNA has deoxyribose and thymine; RNA has ribose and uracil. Adenine, guanine and cytosine are common to both.

Your doubts, answered

What sugar is present in DNA and what sugar is in RNA?

DNA contains the sugar deoxyribose. RNA contains the sugar ribose. The only chemical difference between these two 5-carbon (pentose) sugars is one oxygen atom: ribose has an -OH group at the 2' carbon, while deoxyribose has only -H there (deoxy means 'missing one oxygen'). This is why the sugar names literally tell you the molecule: Deoxyribose is in DNA, Ribose is in RNA.

Does DNA have thymine or uracil? Does RNA have uracil?

DNA has thymine (T) and does NOT have uracil. RNA has uracil (U) in place of thymine and does NOT have thymine. So the base that differs between the two nucleic acids is: DNA = thymine, RNA = uracil. Cytosine, adenine and guanine are found in both. NCERT states uracil is a pyrimidine base, and it replaces thymine in RNA.

Which nitrogen bases (purines) are common to both DNA and RNA?

Adenine and guanine are the two purines found in both DNA and RNA. NCERT clearly states 'Adenine and Guanine are substituted purines while the rest are substituted pyrimidines.' The pyrimidines are cytosine (in both), thymine (DNA only) and uracil (RNA only). This was asked directly in NEET 2019.

What are the three parts of a nucleotide?

A nucleotide has three chemically distinct parts: (1) a nitrogen base (a heterocyclic compound), (2) a monosaccharide sugar (ribose or deoxyribose), and (3) a phosphate group (phosphoric acid). Base + sugar alone = a nucleoside. Adding the phosphate makes it a nucleotide. Nucleic acids are polynucleotides, meaning long chains of many nucleotides joined by phosphodiester bonds.

Why is DNA double-stranded but RNA usually single-stranded?

For NEET Class 11 Biomolecules, focus on the composition difference: DNA and RNA differ in sugar (deoxyribose vs ribose) and one base (thymine vs uracil). The strand detail (DNA is usually a double helix, RNA is usually single-stranded) is developed fully in Class 12 Molecular Basis of Inheritance. Knowing the sugar and base difference is what the Biomolecules PYQs test.

⚠️ The NEET trap
✗ Adenine and thymine are the purines found in both DNA and RNA.
✓ Adenine and guanine are the two purines, and they are the bases common to both DNA and RNA.
🧠 Thymine is a PYRIMIDINE, not a purine, and it is only in DNA. Remember the purines by 'A and G are Pure As Gold' — only Adenine and Guanine are purines. NEET 2019 asked exactly this and the trap option paired adenine with thymine.

Real NEET questions

2019

Purines found both in DNA and RNA are:

A · Adenine and thymine
B · Adenine and guanine ✓
C · Guanine and cytosine
D · Cytosine and thymine
Solution: Nucleic acid bases are either purines or pyrimidines. NCERT states adenine and guanine are substituted purines, while cytosine, uracil and thymine are substituted pyrimidines. Adenine and guanine occur in both DNA and RNA, so the two purines common to both are adenine and guanine. Thymine is a pyrimidine and is present only in DNA.
2025

Match List-I with List-II. List-I: A. Adenosine; B. Adenylic acid; C. Adenine; D. Alanine. List-II: I. Nitrogen base; II. Nucleotide; III. Nucleoside; IV. Amino acid.

A · A-III, B-II, C-I, D-IV ✓
B · A-II, B-III, C-I, D-IV
C · A-III, B-IV, C-II, D-I
D · A-III, B-II, C-IV, D-I
Solution: A base attached to a sugar is a nucleoside, so adenosine is a nucleoside (A-III). If a phosphate is also esterified, it becomes a nucleotide, so adenylic acid is a nucleotide (B-II). Adenine alone is a nitrogen base/purine (C-I). Alanine is an amino acid (D-IV). This gives A-III, B-II, C-I, D-IV, option A.

Solved Biomolecules NEET PYQs

Try the real previous-year questions from this chapter — each with the answer and a full solution.

See all 51 Biomolecules NEET PYQs ›
Next concept: Primary vs Secondary Metabolites →Keep learning — 2 minFeeling ready? Solve the Biomolecules NEET PYQs ›Or practice on your phone — get the free MedicNEET app ›

Frequently asked

What is the main difference between DNA and RNA?

Two things differ: the sugar and one base. DNA has deoxyribose sugar and thymine; RNA has ribose sugar and uracil. Both share adenine, guanine and cytosine.

Which sugar has more oxygen, ribose or deoxyribose?

Ribose has more oxygen. It carries an -OH group at the 2' carbon. Deoxyribose is 'missing one oxygen' at that position, which is why it is called deoxy-ribose and is found in DNA.

Is uracil a purine or a pyrimidine?

Uracil is a pyrimidine. NCERT lists cytosine, uracil and thymine as pyrimidines, and adenine and guanine as purines. Uracil is found only in RNA.

Are nucleic acids polymers?

Yes. Nucleic acids are polynucleotides, meaning long chains of many nucleotide units. NEET 2017 asked which biomolecule is NOT polymeric, and the answer was lipids; nucleic acids, proteins and polysaccharides are all polymers.

What bond joins nucleotides in a nucleic acid?

Nucleotides are joined by phosphodiester bonds. NEET 2021 and 2023 both tested this by matching nucleic acid to phosphodiester bond.

Related concepts:
Nitrogen Bases: Purines vs PyrimidinesDifference Between Nucleoside and NucleotideNaming Nucleosides and Nucleotides (Adenosine, Adenylic Acid)Bond Types: Peptide, Glycosidic, Ester, PhosphodiesterMicromolecules vs Macromolecules (Biomacromolecules)
Practise Biology for NEET:
Biology MCQs (free, chapter-wise)Biomolecules PYQs