JEE Mains Chemistry · Biomolecules

Nucleic Acids

A nucleotide is a base on C-1′ of a pentose sugar with a phosphate on C-5′, and nucleotides join through phosphodiester links; DNA uses 2-deoxyribose and the bases A, G, C, T, RNA uses ribose and A, G, C, U, and in the DNA double helix A pairs with T by two hydrogen bonds and G with C by three.

All Biomolecules notes JEE Mains Chemistry strategy

Why this matters

Twelve PYQs, nine multiple choice and three asking for a number, one from 2026. Eight are on the parts of a nucleotide: which sugar and which bases each nucleic acid has, the link between nucleotides, where the chirality comes from, the structures of the bases, and counting the oxygen atoms in a nucleotide. Four are on base pairing, from writing the complementary strand and counting hydrogen bonds to what DNA and RNA each do.

Concept 1 of 2: Nucleosides, nucleotides and the bases and sugars of DNA and RNA

A nucleic acid is a long chain of nucleotides. Each nucleotide has three parts: a nitrogen base, a five-carbon sugar and a phosphate. The base sits on C-1′ of the sugar and the phosphate on C-5′. The phosphate of one nucleotide then bonds to C-3′ of the next sugar, so the backbone is sugar–phosphate–sugar and the bases hang off it.

Definition

Nucleoside = base + sugar, the base attached at C-1′. Nucleotide = nucleoside + phosphoric acid at C-5′. Sugar carbons are numbered 1′, 2′, 3′ to keep them apart from the base atoms.
Nucleotides are joined by phosphodiester links between C-5′ of one sugar and C-3′ of the next, not by glycosidic links.
Sugars: β-D-2-deoxyribose in DNA, β-D-ribose in RNA. Both are five-membered (furanose) rings in the D series; the D-sugar is the source of the chirality of DNA and RNA. 2-Deoxyribose has no oxygen at C-2′. Free ribose, like any monosaccharide, is a reducing sugar.
Bases: purines (two fused rings) adenine and guanine; pyrimidines (one ring) cytosine, thymine and uracil. Thymine is 5-methyluracil. Uracil in RNA is in its diketo form.
Oxygen count of a nucleotide = oxygens in the base (A 0, G 1, C 1, T 2, U 2) + oxygens left on the sugar after both links (ribose 3: the ring O and the 2′ and 3′ OH; deoxyribose 2) + 4 in the phosphate.
RNA comes in three kinds: messenger (m-RNA), ribosomal (r-RNA) and transfer (t-RNA). DNA is a double helix; RNA is single stranded.

Component	Kind	In DNA	In RNA
Adenine (A)	Purine base, two rings	Yes	Yes
Guanine (G)	Purine base, two rings	Yes	Yes
Cytosine (C)	Pyrimidine base, one ring	Yes	Yes
Thymine (T)	Pyrimidine base, 5-methyluracil	Yes	No
Uracil (U)	Pyrimidine base	No	Yes
β-D-2-Deoxyribose	Pentose sugar, furanose ring	Yes	No
β-D-Ribose	Pentose sugar, furanose ring	No	Yes
Phosphate	Joins C-5′ of one sugar to C-3′ of the next	Yes	Yes

Thymine and deoxyribose mark DNA; uracil and ribose mark RNA.

Practice this conceptself-check · 4 quick reps

The same idea in a real exam question:

JEE Mains · 2022 · 26 June 2022 · Q60Moderate

Example 1 · Biomolecules · Nucleic Acids

The number of oxygens present in a nucleotide formed from a base, that is present only in RNA is

The nucleic-acid sugar is a β furanose

Ribose and 2-deoxyribose sit in nucleic acids as five-membered β-D rings. A statement that the sugar is a pyranose, or the α anomer, is false.

Nucleotides are joined by phosphodiester links

The glycosidic link joins the base to C-1′ of its own sugar. Nucleotide to nucleotide is a phosphodiester link between C-5′ and C-3′, not a C-1 to C-4 glycosidic link.

Chirality comes from the sugar

The bases are flat and the phosphate is not a stereocentre. DNA and RNA are chiral because every unit carries a D-sugar.

Drill 7 more on nucleosides, nucleotides and the bases and sugars of dna and rna

Concept 2 of 2: Base pairing and counting hydrogen bonds in DNA

The two strands of DNA are held by hydrogen bonds between bases, and only two pairings fit: A with T, and G with C. So one strand fixes the other. The A–T pair has two hydrogen bonds and the G–C pair three, so from one strand you can count every hydrogen bond in the helix.

Definition

Complementary pairs: A with T and G with C in DNA; in RNA, A pairs with U.
The strands are antiparallel: write the complement of a 5′ → 3′ strand from 3′ to 5′ under it.
A–T is held by two hydrogen bonds and G–C by three (NCERT Biology XII).
Each base of one strand is part of exactly one pair, so count on one strand only: every A or T gives an A–T pair, every G or C a G–C pair.
Functions (NCERT): DNA is the reserve of genetic information. It self-duplicates during cell division, and identical strands pass to the daughter cells. The message for making each protein is in DNA, but the proteins are made by RNA.

Hydrogen bonds in a DNA double helix

N_{\text{H-bonds}} = 2\,n_{\mathrm{A=T}} + 3\,n_{\mathrm{G\equiv C}}

Worked example

One strand of a DNA helix is

5'\text{-ATTGCAGC-}3'

. Write the complementary strand and count the hydrogen bonds in the helix.

Practice this conceptself-check · 4 quick reps

The same idea in a real exam question:

JEE Mains · 2025 · 4 April 2025 · Q50Moderate

Example 2 · Biomolecules · Nucleic Acids

The total number of hydrogen bonds of a DNA double helix whose one strand has the following sequence of bases is ____.

5'-G-G-C-A-A-A-T-C-G-G-C-T-A-3'

Count one strand, not both

Each base of the given strand already stands for one base pair. Counting the complementary strand as well doubles the answer.

RNA makes the proteins

DNA carries the message for protein synthesis, but the proteins are made by RNA. Statements that DNA synthesises proteins, or that RNA is the reserve of genetic information, are false.

Drill 3 more on base pairing and counting hydrogen bonds in dna

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Formulas (1)

Base pairing and counting hydrogen bonds in DNA
Hydrogen bonds in a DNA double helix
$N_{\text{H-bonds}} = 2\,n_{\mathrm{A=T}} + 3\,n_{\mathrm{G\equiv C}}$

Reference tables (1)

Nucleosides, nucleotides and the bases and sugars of DNA and RNA8 rows

Component	Kind	In DNA	In RNA
Adenine (A)	Purine base, two rings	Yes	Yes
Guanine (G)	Purine base, two rings	Yes	Yes
Cytosine (C)	Pyrimidine base, one ring	Yes	Yes
Thymine (T)	Pyrimidine base, 5-methyluracil	Yes	No
Uracil (U)	Pyrimidine base	No	Yes
β-D-2-Deoxyribose	Pentose sugar, furanose ring	Yes	No
β-D-Ribose	Pentose sugar, furanose ring	No	Yes
Phosphate	Joins C-5′ of one sugar to C-3′ of the next	Yes	Yes

Thymine and deoxyribose mark DNA; uracil and ribose mark RNA.

Watch out for (5)

The nucleic-acid sugar is a β furanose
→ Nucleosides, nucleotides and the bases and sugars of DNA and RNA
Nucleotides are joined by phosphodiester links
→ Nucleosides, nucleotides and the bases and sugars of DNA and RNA
Chirality comes from the sugar
→ Nucleosides, nucleotides and the bases and sugars of DNA and RNA
Count one strand, not both
→ Base pairing and counting hydrogen bonds in DNA
RNA makes the proteins
→ Base pairing and counting hydrogen bonds in DNA

Test yourself on Biomolecules

20 past JEE Mains questions from this chapter, timed at 48 minutes and marked the way the exam marks it. You see your score and every answer the moment you finish. Free to start.

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