Q.In RNA molecule thymine is replaced by
🔒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 — Nitrogenous Bases vs Nucleosides
Let’s start with something you already know: the alphabet. The English alphabet has 26 letters. You can string them together to make words, sentences, whole books. But a letter by itself is just a symbol — it has no meaning until it’s part of a word.
In the world of biology, nitrogenous bases are like those letters. They are the fundamental chemical units that carry genetic information. A nucleoside is like a single letter that has been given a “handle” — a sugar molecule attached to it — so it can be picked up and used to build the real genetic material.
What is a Nitrogenous Base?
A nitrogenous base is a nitrogen-containing molecule that acts as the “information-carrying” part of DNA and RNA. There are five main ones, but you only need to remember two families:
- Purines (double-ringed): Adenine (A) and Guanine (G)
- Pyrimidines (single-ringed): Cytosine (C), Thymine (T), and Uracil (U)
Think of purines as the larger, heavier letters (like ‘A’ and ‘G’ in a font), and pyrimidines as the smaller ones (C, T, U). In DNA, A pairs with T, and G pairs with C. In RNA, U replaces T.
The NCERT textbook (Class 11, Chapter 9) defines nitrogenous bases as “nitrogen-containing heterocyclic compounds.” That’s a fancy way of saying they are ring-shaped molecules that contain nitrogen atoms. You don’t need to memorise the rings — just know they are the “letters” of the genetic code.
What is a Nucleoside?
A nucleoside is simply a nitrogenous base plus a sugar molecule (ribose in RNA, deoxyribose in DNA). The sugar acts like a handle or a backbone attachment point.
So:
Nucleoside = Nitrogenous base + Sugar
For example:
- Adenine + ribose = Adenosine
- Guanine + deoxyribose = Deoxyguanosine
- Cytosine + ribose = Cytidine
Notice the naming: purine bases end in “-osine” (adenosine, guanosine), and pyrimidine bases end in “-idine” (cytidine, thymidine, uridine). That’s a small but useful pattern.
A nucleoside is not the same as a nucleotide. A nucleotide is a nucleoside plus a phosphate group. That phosphate is what allows nucleotides to link together into long chains — the actual DNA or RNA strand. So:
Base → add sugar → nucleoside → add phosphate → nucleotide → link together → nucleic acid (DNA/RNA)
Why Does This Distinction Matter?
If you’re studying this for a commerce or humanities exam, you won’t be asked to draw chemical structures. But you will be asked to differentiate between these terms, and to understand their roles.
Here’s the key takeaway:
- Nitrogenous bases are the “letters” — they store the genetic information.
- Nucleosides are the “letters with a handle” — they are the building blocks that get activated (by adding phosphate) to become nucleotides.
- Nucleotides are the actual “bricks” that build DNA and RNA.
In NCERT, the distinction is clearly stated: “A nucleoside is composed of a nitrogenous base and a pentose sugar. A nucleotide is a nucleoside with a phosphate group.” That’s the exact line you should remember.
A Quick Comparison
| Feature | Nitrogenous Base | Nucleoside |
|---|---|---|
| What it contains | Only the base (A, G, C, T, U) | Base + sugar (ribose or deoxyribose) |
| Role | Carries genetic information | Intermediate building block |
| Example | Adenine | Adenosine |
| Found freely in cells? | Yes, as free bases | Rarely; usually converted to nucleotides |
One Common Mistake to Avoid …
RNA contains the same bases as DNA except that thymine is replaced by another pyrimidine. …
In RNA, thymine is replaced by uracil — option (b).
DNA has four bases: adenine, guanine, cytosine and thymine. RNA also has adenine, guanine and cytosine, but instead of thymine it contains uracil. Uracil pairs with adenin …
Showing the 12 most recent of 14 on this concept.
- CBSE 2026Set V11 markMCQQ.Statement I : A nitrogenous base is linked to the -OH of IC pentose sugar through N-glycosidic linkage to form a nucleoside. Statement II : Two nucleosides are linked through 3'-5' phosphodiester linkage to form a dinucleoside.(a) Both statement I and statement II are correct(b) Both statement I and statement II are incorrect(c) Statement I is correct, but statement II is incorrect(d) Statement I is incorrect but statement II is correct
›Reveal solutionSolution
A nucleoside forms by an N-glycosidic bond (I is correct); but a phosphodiester bond joins nucleotides to give a dinucleotide, not nucleosides to give a dinucleoside (II is wrong).
Statement I is correct: a nitrogenous base linked to the -OH of the 1'C of the pentose sugar by an N-glycosidic linkage forms a nucleoside. …
- CBSE 2026Set ANNUAL1 markMCQQ.Which nitrogenous base is found in both DNA and RNA?(a) Cytosine(b) Thymine(c) Uracil(d) All of the above
›Reveal solutionSolution
DNA and RNA share adenine, guanine and cytosine; thymine is unique to DNA and uracil is unique to RNA.
Both DNA and RNA contain the purines adenine and guanine, and the pyrimidine cytosine. The other pyrimidine differs between the two: DNA uses thymine, while RNA use …
- CBSE 2026Set ANNUAL1 markQ.State whether True or False: Nitrogenous base Uracil present in DNA.
›Reveal solutionSolution
False - DNA contains thymine, not uracil; uracil occurs in RNA.
Both DNA and RNA contain the bases adenine, guanine and cytosine. The difference is in the fourth base: DNA contains thymine (T), whereas RNA contains uracil (U) in place of thymine. The …
- CBSE 2025Set KH1 markQ.What is nucleoside?
›Reveal solutionSolution
Nucleoside = nitrogenous base + pentose sugar (no phosphate); adding phosphate gives a nucleotide.
Concept. The building blocks of nucleic acids are nucleotides, which have three parts: a nitrogenous base, a pentose sugar, and a phosphate group.
Definition. When only a nitrogenous base links to a pentose sugar (ribose in RNA, deoxyribose in DNA) by an N-glycosidic bond, the unit is called a nucleoside. Examples: adenosine, guanosine, cytidine, uridine/thymidine.
…
- CBSE 2024Set ANNUAL1 markQ.Write the name of bond between phosphate group and nucleoside in DNA.
›Reveal solutionSolution
Successive nucleotides in a DNA strand are joined by phosphodiester bonds linking the sugar of one nucleotide to the phosphate of the next.
A nucleotide is formed when a phosphate group is attached, via an ester (phosphoester) linkage, to the 5'-OH of the sugar of a nucleoside (base + sugar). Successive nucleotides are then joined together to form the DNA strand's backbone by a bond between the 5'-phosphate of one nucleotide and the 3'-OH of the sugar of the next nucleotide — since thi …
- CBSE 2023Set ANNUAL1 markQ.Write name of components of nucleoside.
›Reveal solutionSolution
A nucleoside = nitrogenous base + pentose sugar (joined by an N-glycosidic bond); adding a phosphate group to a nucleoside makes it a nucleotide.
A nucleoside is formed by the attachment of a nitrogenous base (adenine, guanine, cytosine, thymine or uracil) to the 1' carbon of a pentose sugar (ribose in RNA, deoxyribose in DNA) through an N-glycosidic linkage.
…
- CBSE 2022Set ANNUAL1 markMCQQ.Which of the following nitrogenous bases is not present in DNA?(a) Thymine(b) Adenine(c) Guanine(d) Uracil
›Reveal solutionSolution
Uracil is the pyrimidine base found in RNA, not DNA, which uses thymine instead.
DNA is built from four nitrogenous bases: the purines adenine (A) and guanine (G), and the pyrimidines cytosine (C) and thymine (T). RNA carries the same bases except that thymine is replaced by uracil (U), which pairs with adenine in the same way thymine does but lacks t …
- CBSE 2022Set ANNUAL1 markMCQQ.Purine nitrogenous base is -(a) Cytosine(b) Adenine(c) Uracil(d) Thymine
›Reveal solutionSolution
Purines have a fused double-ring structure; adenine is a purine, the other three options are pyrimidines.
Nitrogenous bases in nucleic acids are of two chemical classes:
- Purines (double fused ring): Adenine (A) and Guanine (G). …
- CBSE 2021Set D1 markMCQQ.Pyrimidines present in RNA are(a) Cytosine and Thymine(b) Adenine and Guanine(c) Cytosine and Uracil(d) Thymine and Uracil
›Reveal solutionSolution
RNA contains the pyrimidines cytosine and uracil (uracil replaces DNA's thymine), so the answer is (C).
Nitrogenous bases are of two types: purines (adenine and guanine, each with a double ring) and pyrimidines (single ring).
…
- CBSE 2021Set D1 markMCQQ.Nucleoside is(a) Sugar + a nitrogenous base(b) Sugar + Phosphate(c) Nitrogenous base + Phosphate(d) None of these
›Reveal solutionSolution
Nucleoside = sugar + nitrogenous base; adding a phosphate makes it a nucleotide.
A nucleoside is formed when a nitrogenous base (purine or pyrimidine) links to a pentose sugar (ribose in RNA, deoxyribose in DNA) through an N-glycosidic bond. When a phosphate group is further attached to the sugar, the molecule becomes a nucleotide. Therefore 'sugar …
- CBSE 2021Set D1 markMCQQ.Purine bases of DNA are(a) Adenine and Cytosine(b) Cytosine and Thymine(c) Adenine and Guanine(d) None of these
›Reveal solutionSolution
Purines in DNA are adenine and guanine; the pyrimidines are cytosine and thymine.
Nitrogenous bases are of two types. The double-ringed purines are adenine (A) and guanine (G), and the single-ringed pyrimidines are cytosine (C) and thymine (T) (uracil in RNA). Therefore the purine bases of DNA are adenine …
- CBSE 2019Set 57/3/11 markQ.Differentiate between a DNA and a RNA nucleotide.
›Reveal solutionSolution
DNA and RNA nucleotides differ primarily in their pentose sugar (deoxyribose in DNA, ribose in RNA) and one nitrogenous base (thymine in DNA, uracil in RNA), leading to distinct structural and functional roles.
A nucleotide is the fundamental building block of nucleic acids, DNA (deoxyribonucleic acid) and RNA (ribonucleic acid). Understanding the differences between a DNA nucleotide and an RNA nucleotide is crucial because these subtle variations dictate the unique structures, stabilities, and functions of DNA and RNA within a cell. Essentially, the type of nucleotide determines the type of genetic information carrier or messenger.
Every nucleotide is composed of three main parts:
- A phosphate group (or groups).
- A pentose sugar (a five-carbon sugar).
- A nitrogenous base.
The distinctions between DNA and RNA nucleotides arise from differences in the pentose sugar and the nitrogenous bases.
Differentiating DNA and RNA Nucleotides
-
The Pentose Sugar:
The most significant difference lies in the sugar component.
- DNA nucleotide: Contains deoxyribose sugar. The key feature of deoxyribose is the absence of a hydroxyl group (-OH) at the 2' (two-prime) carbon position. Instead, it has a hydrogen atom (-H) at this position. This "de-oxy" (lacking oxygen) characteristic makes DNA more stable and less reactive, which is essential for its role as the long-term genetic information storage molecule.
- RNA nucleotide: Contains ribose sugar. Ribose has a hydroxyl group (-OH) at both the 2' and 3' carbon positions. The presence of the 2'-hydroxyl group makes RNA more reactive and less stable than DNA, contributing to its diverse and often transient roles in gene expression, such as mRNA (messenger RNA), tRNA (transfer RNA), and rRNA (ribosomal RNA).
ImportantThe presence or absence of the hydroxyl group at the 2' carbon of the pentose sugar is the defining difference between ribose and deoxyribose, and consequently, between RNA and DNA nucleotides.
-
The Nitrogenous Bases:
Both DNA and RNA nucleotides contain nitrogenous bases, which are heterocyclic compounds. These bases are categorized into two types: purines (double-ring structures) and pyrimidines (single-ring structures).
- Common Bases: Both DNA and RNA nucleotides share three common bases:
- Adenine (A) - a purine
- Guanine (G) - a purine
- Cytosine (C) - a pyrimidine
- Unique Bases: The fourth base differs:
- DNA nucleotide: Contains Thymine (T) - a pyrimidine. Thymine pairs specifically with Adenine.
- RNA nucleotide: Contains Uracil (U) - a pyrimidine. Uracil replaces Thymine in RNA and also pairs with Adenine.
Watch outA common misconception is that DNA contains Uracil or RNA contains Thymine. Remember: DNA has Thymine, RNA has Uracil. …
- Common Bases: Both DNA and RNA nucleotides share three common bases:
🎓Unlock everything free for 14 days
- ✓Full step-by-step solutions
- ✓Concept-first explanations
- ✓Methods, shortcuts & mistakes
- ✓PYQ mapping + timed mock tests
Full access for 14 days. No credit card required.