Q.Which one is not a pyrimidine?
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From Building Blocks to the Blueprint of Life
Think of a necklace. The necklace itself is a long chain, but if you look closely, it's made of many identical or nearly identical beads strung together. Nucleic acids — DNA and RNA — are exactly like that. They are long chains (polymers) whose repeating units are called nucleotides.
So a nucleotide is simply one bead on that necklace. But unlike a plain bead, each nucleotide has three distinct parts fastened together. Understanding those three parts is the entire story.
The Three-Part Structure
Every nucleotide is built from:
- A nitrogenous base (a ring-shaped molecule containing nitrogen)
- A pentose sugar (a 5-carbon sugar)
- A phosphate group (a phosphorus atom bonded to four oxygen atoms)
These three are covalently bonded in a fixed order: phosphate — sugar — base.
The sugar is the central connector. The base is attached to the sugar's 1' carbon, and the phosphate is attached to the sugar's 5' carbon. This arrangement is universal.
The Two Kinds of Sugar
The pentose sugar determines whether you are looking at DNA or RNA.
- DNA uses deoxyribose — it lacks one oxygen atom (on the 2' carbon) compared to ribose.
- RNA uses ribose — it has a hydroxyl (-OH) group on the 2' carbon.
That single oxygen difference makes RNA chemically more reactive and less stable than DNA — a fact with huge biological consequences.
The Nitrogenous Bases: Two Families
There are five standard bases, divided into two groups based on their ring structure.
| Family | Bases | Rings | Found in |
|---|---|---|---|
| Purines | Adenine (A), Guanine (G) | Two fused rings (a 6-membered and a 5-membered) | Both DNA and RNA |
| Pyrimidines | Cytosine (C), Thymine (T), Uracil (U) | One 6-membered ring | C in both; T in DNA only; U in RNA only |
To remember which bases are purines: Adenine and Guanine are A Giant pair — two rings. Pyrimidines have a single ring and include C, T, and U — think "CUT the pyrimidine pie."
Nucleoside vs. Nucleotide — A Critical Distinction
This is where many students slip. The difference is exactly one phosphate group.
- Nucleoside = nitrogenous base + pentose sugar only. (No phosphate.)
- Nucleotide = nitrogenous base + pentose sugar + phosphate group(s).
So a nucleotide is a nucleoside that has been phosphorylated. In nucleic acids, the nucleotide usually carries one phosphate (called a nucleoside monophosphate), but in cellular energy molecules like ATP, it carries three.
Do not confuse nucleoside with nucleotide. A nucleoside is not the monomer of nucleic acids — the nucleotide is. The nucleoside is just a precursor.
How Nucleotides Join to Form a Chain
Nucleotides link together through a phosphodiester bond. The phosphate group attached to the 5' carbon of one sugar forms a bond with the 3' carbon of the next sugar's hydroxyl group. This creates a sugar-phosphate backbone with the bases sticking out sideways — like the teeth of a zipper. …
Nitrogenous bases in nucleic acids fall into two chemical classes based on their ring structure -- the double-ringed purines and the single-ringed pyrimidines. …
Nitrogenous bases split into purines (double ring) and pyrimidines (single ring); adenine is the odd one out here as a purine.
Nitrogenous bases in nucleic acids are of two chemical classes:
- Purines: double-ring structure -- Adenine (A) and Guanine (G)
- Pyrimidines: single-ring structure -- Cytosine (C), Thymine (T, found only in DNA), and Uracil (U, found only in RNA) …
- CBSE 2026Set ANNUAL1 markQ.Fill in the blank: Nucleic acids are made up of ______.
›Reveal solutionSolution
Nucleic acids (DNA and RNA) are polymers of nucleotide monomers, each nucleotide itself made of a nitrogenous base, a five-carbon (pentose) sugar, and a phosphate group.
Each nucleotide has three components:
- A nitrogenous base — a purine (adenine, guanine) or a pyrimidine (cytosine, thymine in DNA / uracil in RNA).
- A pentose sugar — deoxyribose in DNA, ribose in RNA.
- A phosphate group. …
- CBSE 2026Set ANNUAL1 markMCQQ.Nucleotides are made up of which of the following ?(a) Sugar and base(b) Sugar and phosphate(c) Sugar, phosphate and nitrogenous base(d) Base and phosphate
›Reveal solutionSolution
A nucleotide has three components: sugar, phosphate and nitrogenous base — so the answer is (C).
A nucleotide, the monomer of DNA and RNA, is made of three units:
- A pentose sugar (deoxyribose in DNA, ribose in RNA).
- A phosphate group.
- A nitrogenous base (adenine, guanine, cytosine, thymine or uracil). …
- CBSE 2024Set ANN1 markQ.Name the nucleotide form of guanine.
›Reveal solutionSolution
A nucleotide = nitrogenous base + sugar + phosphate. The nucleotide of guanine is guanylic acid (GMP), formed from the nucleoside guanosine plus a phosphate group.
A nucleic acid is built from nucleotides, each of which has three components: a nitrogenous base, a pentose sugar, and a phosphate group.
- Base + sugar alone (no phosphate) = nucleoside. For guanine this is guanosine (base + ribose/deoxyribose). …
- CBSE 2022Set TERM11 markMCQQ.Which one is not a pyrimidine?(a) Adenine(b) Thymine(c) Uracil(d) Cytosine
›Reveal solutionSolution
Nitrogenous bases split into purines (double ring) and pyrimidines (single ring); adenine is the odd one out here as a purine.
Nitrogenous bases in nucleic acids are of two chemical classes:
- Purines: double-ring structure -- Adenine (A) and Guanine (G)
- Pyrimidines: single-ring structure -- Cytosine (C), Thymine (T, found only in DNA), and Uracil (U, found only in RNA) …
- CBSE 2022Set TERM11 markMCQQ.Ribose and Deoxyribose sugars are(a) Trioses(b) Tetroses(c) Pentoses(d) Hexoses
›Reveal solutionSolution
Ribose and deoxyribose both contain 5 carbon atoms, making them pentose sugars.
Monosaccharides are classified by the number of carbon atoms they contain: trioses (3C), tetroses (4C), pentoses (5C), and hexoses (6C, e.g., glucose, fructose). Ribose (the sugar backbone of RNA) has the formula C5H10O5, and deoxyribose (the sugar backbone of DNA) is ribose with one oxygen removed at the 2' carbon (C5H10O4) -- both retain 5 carbon atoms in th …
- CBSE 2022Set ANNUAL1 markMCQQ.Energy currency of the cells is(a) RNA(b) AMP(c) DNA(d) ATP
›Reveal solutionSolution
ATP stores and releases energy for cellular work → (d).
- ATP (adenosine triphosphate) carries energy in its high-energy phosphate bonds; hydrolysis of ATP → ADP + Pi releases energy for muscle contraction, active transport, biosynthesis, etc. …
- CBSE 2021Set ANNUAL1 markQ.What are the building blocks of nucleic acid?
›Reveal solutionSolution
Nucleic acids (DNA, RNA) are polymers of repeating nucleotide units.
Nucleic acids — DNA and RNA — are polymers built from repeating monomeric units called nucleotides. Each nucleotide is itself made up of three components: a nitrogenous base (a purine like adenine/guanine, or a pyrimidine like cytosine/thymine/uracil), a pentose sugar (deoxyribose in DNA, ribo …
- CBSE 2019Set ANNUAL1 markQ.Name any two types of nitrogenous base found in nucleic acids.
›Reveal solutionSolution
Nucleic acids (DNA and RNA) contain nitrogenous bases belonging to two chemical classes: purines (double-ringed) and pyrimidines (single-ringed).
The nitrogenous bases found in nucleic acids are:
- Purines — Adenine (A) and Guanine (G), found in both DNA and RNA.
- Pyrimidines — Cytosine (C), found in both DNA and RNA; Thymine (T), found only in DNA; and Uracil (U), found only in RNA (replacing thymine). …
- CBSE 2019Set ANNUAL1 markQ.Write the name of the Purine base which is found only in DNA.
›Reveal solutionSolution
The base unique to DNA (absent from RNA) is thymine — but thymine is a pyrimidine base, not a purine; the two purine bases (adenine and guanine) are common to both DNA and RNA.
Nucleic acids contain two classes of nitrogenous bases: purines (double-ringed) — adenine and guanine — and pyrimidines (single-ringed) — cytosine, thymine, and uracil. Adenine and guanine (the purines) are present in both DNA and RNA in equal fashion, so no purine base is exclusive to DNA. Among the pyrimidines, cytosine is common to both, while thymine is found only in DNA and uracil is found only in RNA (RNA uses uracil in place of thymine). Honestly noting this: the question as printed asks for a 'purine base found only in …
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