On this page
Nucleoside and nucleotide differ by one letter and one phosphate group, and exam questions are built on exactly that difference. They appear twice in the NEET and JEE books, once in chemistry and once in biology, with slightly different emphasis. This post puts the two together, shows how each unit is assembled from its parts, and uses molar masses to check that the bookkeeping is right.
Where is this in the syllabus?
The JEE (Main) 2026 syllabus and the NEET (UG) 2026 syllabus both list "Nucleic acids – chemical constitution of DNA and RNA" and their biological functions in Chemistry Unit 19, Biomolecules. NEET Biology adds "Structure of DNA and RNA" under Molecular basis of Inheritance. The matching NCERT texts are Class 12 Chemistry, Unit 10 Biomolecules, and Class 12 Biology, Chapter 5 Molecular Basis of Inheritance, both in the 2026-27 reprint. Check your own year's syllabus.
What is a nucleoside?
NCERT Chemistry: complete hydrolysis of DNA or RNA gives "a pentose sugar, phosphoric acid and nitrogen containing heterocyclic compounds (called bases)". In DNA the sugar is β-D-2-deoxyribose; in RNA it is β-D-ribose. DNA uses adenine, guanine, cytosine and thymine; RNA uses uracil in place of thymine.
"A unit formed by the attachment of a base to 1′ position of sugar is known as nucleoside." The primes number the sugar carbons, 1′ to 5′, so they are not confused with the atoms of the base. NCERT Biology adds that the link is an N-glycosidic linkage, through a nitrogen of the base, and names examples: adenosine or deoxyadenosine, guanosine or deoxyguanosine, cytidine or deoxycytidine, and uridine or deoxythymidine.
What turns it into a nucleotide?
"When nucleoside is linked to phosphoric acid at 5′-position of sugar moiety, we get a nucleotide," says NCERT Chemistry. NCERT Biology calls this a phosphoester linkage. Nucleotides then join through a 3′–5′ phosphodiester linkage, and the chain has a free phosphate at its 5′ end and a free 3′–OH at the other. In RNA every sugar also carries an –OH at 2′.
Worked example 1: build adenosine and AMP from their parts
Each new bond here is a condensation: two –OH groups (or an –OH and an N–H) join and one water molecule leaves. Use PubChem molar masses in g/mol: adenine 135.13, D-ribose 150.13, phosphoric acid 97.995 and water 18.015.
Adenosine = adenine + ribose − water = 135.13 + 150.13 − 18.015 = 267.24 g/mol. PubChem lists adenosine, C₁₀H₁₃N₅O₄, as 267.24.
AMP = adenosine + phosphoric acid − water = 267.24 + 97.995 − 18.015 = 347.22 g/mol. PubChem lists adenosine monophosphate, C₁₀H₁₄N₅O₇P, as 347.22.
The match confirms two things: each link loses exactly one water, and the nucleotide has one phosphorus.
Worked example 2: the DNA versions
Swap ribose for 2-deoxyribose, 134.13 g/mol, which has one oxygen fewer at 2′.
Deoxyadenosine = 135.13 + 134.13 − 18.015 = 251.24 g/mol, matching PubChem's 2′-deoxyadenosine.
dAMP = 251.24 + 97.995 − 18.015 = 331.22 g/mol, matching PubChem's deoxyadenosine monophosphate.
The difference between AMP and dAMP is 16.00 g/mol, one oxygen atom: the 2′–OH that RNA has and DNA lacks. Do the same with thymine, 126.11: thymine + deoxyribose − water = 242.23 g/mol, which is PubChem's thymidine.
Why does the 2′–OH matter?
It is the one chemical difference in the sugar. NCERT Biology says the 2′–OH present at every RNA nucleotide "is a reactive group and makes RNA labile and easily degradable", so DNA, "chemically less reactive and structurally more stable", is the better genetic material. DNA is the genetic material in most organisms. The double helix itself was proposed by Watson and Crick in 1953; our post on biomolecules in chemistry covers base pairing numbers, and our guide to DNA replication and transcription takes the biology further.
Which mistakes cost marks?
Calling adenine a nucleoside. Adenine is the base; adenosine is the nucleoside; AMP is the nucleotide.
Attaching the base at 5′ or the phosphate at 1′. Base at 1′, phosphate at 5′.
Calling the base-sugar link a phosphodiester bond. It is N-glycosidic.
Writing "thymidine" in RNA. RNA has uracil, and its nucleoside is uridine.
Forgetting the water. Every link in a nucleotide or chain is a condensation.
A practice routine
Draw a nucleotide as three boxes, labelled with the bond type between each pair and the carbon number at each joint.
Answer NCERT Chemistry Exercise 10.7: what forms when a DNA nucleotide containing thymine is hydrolysed?
Check uridine with the condensation method: uracil (112.09) plus ribose minus water should give PubChem's 244.20.
Explain to a friend, without notes, why AMP and dAMP differ by 16 g/mol.
Repeat the drill in three days; Dunlosky and colleagues (2013) rated practice testing and spaced practice as high utility.
If the sugar numbering or bond names keep slipping, the Fermi Concept Library covers every concept in physics, chemistry, biology, mathematics and computing, each with its own lesson.
Today, write adenine, adenosine, AMP and dAMP in a row with their molar masses, and check each step against this page.
