Q.Nucleic acids are the polymers of _______.
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Nucleic Acid Components: The Alphabet of Life
Imagine you want to write a book that contains all the instructions for building and running a living organism. You'd need an alphabet — a set of letters that can be combined in endless ways to form words, sentences, and chapters. In living cells, that alphabet is made of nucleic acids (DNA and RNA), and each "letter" is called a nucleotide.
The Big Picture: What Are Nucleic Acids?
Nucleic acids are long, chain-like molecules that store and transmit genetic information. DNA holds the master blueprint; RNA helps execute it. But both are built from the same basic building block: the nucleotide.
Think of a nucleotide as a single Lego brick. DNA and RNA are long chains of these bricks, each brick slightly different from the next.
The Three Parts of a Nucleotide
Every nucleotide has three components, like a three-part key:
- A phosphate group — a small, negatively charged group (PO43−). It acts like the "handle" that links nucleotides together.
- A sugar — either ribose (in RNA) or deoxyribose (in DNA). This is the "body" of the brick.
- A nitrogenous base — a ring-shaped molecule containing nitrogen. This is the "colored part" that carries the actual information.
The sugar and base together form a nucleoside. When you add the phosphate, you get a nucleotide.
Nucleoside = Sugar + Base
Nucleotide = Sugar + Base + Phosphate
The Two Families of Bases
The bases come in two structural types:
- Purines (double-ring structures): Adenine (A) and Guanine (G)
- Pyrimidines (single-ring structures): Cytosine (C), Thymine (T) (only in DNA), and Uracil (U) (only in RNA)
A mnemonic: Purines are Pure as All Gold (A and G). Pyrimidines are CUT (C, U, T).
DNA vs. RNA: The Key Differences
| Feature | DNA | RNA |
|---|---|---|
| Sugar | Deoxyribose (missing one oxygen) | Ribose (has that oxygen) |
| Bases | A, G, C, T | A, G, C, U |
| Structure | Double-stranded helix | Usually single-stranded |
| Function | Stores genetic information | Carries and executes instructions |
A common mistake: thinking "nucleoside" and "nucleotide" are the same. Remember: nucleotide has the phosphate; nucleoside does not. ATP (adenosine triphosphate) is a nucleotide — it's the energy currency of the cell.
Why This Matters
The sequence of bases along a DNA strand spells out the genetic code. A change in even one base (a mutation) can alter a protein, sometimes with dramatic consequences — like sickle cell anemia, where a single base change in the hemoglobin gene causes red blood cells to deform. …
Why this formula?
Nucleic Acid Components: Understanding the "Why" Behind the Key Relationships
Let’s start with the big picture: Nucleic acids (DNA and RNA) are polymers made of nucleotides. Each nucleotide has three parts: a nitrogenous base, a pentose sugar, and a phosphate group. The key formulas and relationships in this topic arise from how these parts are linked and how they behave chemically.
1. The Basic Composition Formula
What it says:
A nucleotide = Base + Sugar + Phosphate
Why this holds:
- Chemical necessity: The sugar (ribose in RNA, deoxyribose in DNA) has a 5-carbon ring. The base attaches to the 1' carbon (via a glycosidic bond), and the phosphate attaches to the 5' carbon (via an ester bond).
- Directionality: This creates a 5' → 3' linkage in the polymer. The phosphate of one nucleotide bonds to the 3' carbon of the next sugar.
- Reasoning: Without the phosphate, you have a nucleoside (base + sugar). Adding the phosphate makes it a nucleotide — the monomer that can polymerize.
Key takeaway: The formula isn’t arbitrary — it reflects the specific carbon positions on the sugar that allow for stable, directional chain formation.
2. Chargaff’s Rules (for DNA)
What it says:
In double-stranded DNA:
- [A]=[T]
- [G]=[C]
- [A]+[G]=[T]+[C]
Why this holds:
- Base pairing: Adenine (A) forms two hydrogen bonds with Thymine (T). Guanine (G) forms three hydrogen bonds with Cytosine (C).
- Structural constraint: The DNA double helix has a constant width (20 Å). A purine (A or G) always pairs with a pyrimidine (T or C) — otherwise the helix would bulge or narrow.
- Derivation: If every A on one strand must pair with a T on the opposite strand, then the number of A equals number of T in the whole molecule. Same for G and C.
- Consequence: The sum of purines equals sum of pyrimidines (A+G=T+C).
Why it’s not just a rule: It’s a geometric and energetic necessity — hydrogen bonding and helix stability force this equality.
3. The Phosphodiester Bond Energy Formula
What it says:
Formation of a phosphodiester bond requires ~30 kJ/mol of energy (from ATP).
Why this holds:
- Mechanism: The 3' hydroxyl of one nucleotide attacks the α-phosphate of a nucleotide triphosphate (e.g., ATP). This releases pyrophosphate (PPi).
- Energy source: The hydrolysis of PPi to two inorganic phosphates (PPi→2Pi) is highly exergonic (ΔG ≈ -30 kJ/mol). This drives the reaction forward.
- Reasoning: The bond itself is a covalent ester linkage — strong but not spontaneously formed. The energy comes from breaking a high-energy phosphate bond in the triphosphate.
Key insight: The formula isn’t about the bond’s strength — it’s about the thermodynamic cost of making it in a cell.
4. The Melting Temperature (Tm) Formula
What it says:
Tm (in °C) ≈ 4(G+C)+2(A+T) for short oligonucleotides.
Why this holds:
- Hydrogen bonds: G-C pairs have 3 H-bonds, A-T pairs have 2 H-bonds. More H-bonds = more energy needed to separate strands.
- Stacking interactions: G-C base pairs also have stronger π-stacking (aromatic ring overlap) than A-T.
- Derivation: The formula is empirical — it comes from measuring Tm for many sequences. The coefficients (4 and 2) reflect the relative stability contributed by each base pair.
- Limitation: For long DNA, this simple formula fails because nearest-neighbor interactions matter more.
Why it works: It’s a linear approximation of the free energy needed to break all base pairs, weighted by H-bond count.
5. The Central Dogma (Information Flow)
What it says:
DNA → RNA → Protein
Why this holds: …
The key idea is that nucleic acids (DNA and RNA) are built from monomeric units called nucleotides, not from nucleosides or isolated bases.
- A nucleotide consists of three components: a nitrogenous base, a pentose sugar, and a phosphate group.
- Nucleotides link together via phosphodiester bonds (between the sugar of one nucleotide and the phosphate of the next) to form the long polynucleotide chain of a nucleic acid. …
Nucleic acids are polymers of nucleotides, not nucleosides, bases, or sugars alone. The correct option is (ii).
Why This Question Tests a Core Distinction
The question looks simple, but it checks whether you understand the hierarchy of nucleic acid building blocks. Many students memorise "nucleotides are the monomers" without knowing why nucleosides or bases don't fit. Let's break that down.
Nucleic acids (DNA and RNA) are long chains. Each repeating unit must contain three things: a nitrogenous base, a pentose sugar (ribose or deoxyribose), and a phosphate group. That complete unit — base + sugar + phosphate — is called a nucleotide.
If you remove the phosphate, you get a nucleoside (base + sugar only). A nucleoside cannot link to form a polymer chain because it lacks the phosphate that forms the backbone. Similarly, a base alone or a sugar alone cannot polymerise into a nucleic acid.
A common mistake is to pick "nucleosides" because they sound similar. Remember: nucleosides are precursors to nucleotides, but they are not the actual monomers. Without the phosphate, no phosphodiester bond can form.
Step-by-Step Reasoning
-
Identify the monomer unit
A polymer is built from repeating monomeric subunits. For nucleic acids, the monomer must be capable of forming a chain via phosphodiester bonds — a bond between the 3′ hydroxyl of one sugar and the 5′ phosphate of the next.
-
Check each option
- (iv) Sugars: A sugar alone (ribose or deoxyribose) has no base and no phosphate. It cannot form the specific linkages of a nucleic acid.
- (iii) Bases: Purines and pyrimidines are flat, aromatic rings. They attach to sugars but do not themselves polymerise. …
Method: Definition-Based Recall (Direct Fact Retrieval)
This method relies on knowing the precise structural definition of nucleic acids as per NCERT and standard exam syllabi.
Steps:
-
Recall the monomer unit
Nucleic acids (DNA and RNA) are long-chain polymers. The repeating monomeric unit is the nucleotide, not the nucleoside.
-
Distinguish between nucleoside and nucleotide
- A nucleoside = nitrogenous base + sugar (no phosphate).
- A nucleotide = nitrogenous base + sugar + phosphate group. …
Here is a breakdown of the common mistakes students make on this specific question, along with the conceptual fixes to avoid them.
The Correct Answer
The correct answer is (B) nucleotides.
Mistake #1: Confusing "Nucleoside" with "Nucleotide"
The Mistake: Students pick (A) nucleosides because they sound similar and are often taught together.
Why it’s wrong:
- A nucleoside is just a base + sugar (no phosphate).
- A nucleotide is a base + sugar + phosphate.
- A polymer is a chain of repeating units joined by specific bonds. The bond in nucleic acids (the phosphodiester bond) forms between the phosphate of one unit and the sugar of the next. Since nucleosides lack a phosphate group, they cannot form this backbone.
How to Avoid:
- Visualize the backbone. Draw a small segment of DNA/RNA in your mind. You will see a repeating pattern: Sugar-Phosphate-Sugar-Phosphate. The "Sugar-Phosphate" unit is the nucleotide. The "Sugar-Base" unit (nucleoside) is just a side group hanging off the backbone.
- Remember the "P" rule: A polymer needs a Phosphate to make the Phosphodiester bond. Nucleosides have no P.
Mistake #2: Choosing "Bases" (Too Simplistic)
The Mistake: Students think DNA is just made of A, T, G, C, so they pick (C) bases.
Why it’s wrong:
- The bases (A, T, G, C, U) are the information-carrying part, but they are not the structural repeating unit.
- If you tried to polymerize just bases, you would not get a sugar-phosphate backbone. The molecule would not be a nucleic acid; it would just be a pile of flat rings.
How to Avoid:
- Think of a train. The bases are the cargo (the information). The nucleotides are the train cars (the structural unit). The polymer is the whole train, made of cars, not just the cargo.
- Definition check: A polymer is a long chain of monomers. The monomer of a nucleic acid is always a nucleotide.
Mistake #3: Picking "Sugars" (Incomplete)
The Mistake: Students know DNA has deoxyribose and RNA has ribose, so they pick (D) sugars.
Why it’s wrong:
- A polymer of just sugars would be a polysaccharide (like starch or cellulose), not a nucleic acid.
- Nucleic acids require both the sugar and the phosphate to form the backbone, and the bases to carry the genetic code.
How to Avoid:
- Compare polymer classes:
- Proteins: Monomer = Amino acids.
- Polysaccharides: Monomer = Sugars (monosaccharides). …
- GSEB Higher Secondary Certificate (HSC) Examination 2026Set ANNUAL1 markMCQQ.By which linkage are nucleotides joined together between 5' and 3' carbon atoms of pentose sugar?(a) Glycosidic(b) Polypeptide(c) Phosphodiester(d) Hydrogen bond
›Reveal solutionSolution
The backbone of DNA/RNA is built from repeating phosphodiester bonds linking the 3' carbon of one sugar to the 5' carbon of the next, via a phosphate group.
Each nucleotide consists of a nitrogenous base, a pentose sugar, and a phosphate group. To build a nucleic acid chain, the phosphate group forms TWO ester linkages: one to the 3'-OH of the preceding sugar and one to the 5'-OH of the next sugar — hence phosphodiester linkage ("di" = two ester bonds from one phosphate).
…
- GUJCET 2025Set 031 markMCQQ.______ nucleotide is not present in RNA. (A) Uracil containing (B) Adenine containing (C) Cytosine containing (D) Thymine containing
›Reveal solutionSolution
RNA bases are A, G, C, U — thymine (T) is a DNA base.
Concept — nucleic acid bases. RNA uses adenine, guanine, cytosine and uracil; DNA uses thymine instead of uracil. …
- GSEB Higher Secondary Certificate (HSC) Examination 2025Set ANNUAL1 markMCQQ.By which linkage, Nucleotides are Joined together between 5' and 3' carbon atoms of pentose sugar.(a) Poly peptide(b) Phosphodiester(c) Glycosidic(d) Polyamide
›Reveal solutionSolution
Successive nucleotides in a nucleic acid strand are joined by a phosphate group forming two ester bonds - one to the 3' carbon of one sugar and one to the 5' carbon of the next - hence 'phosphodiester' linkage.
In a nucleic acid (DNA/RNA), each nucleotide consists of a nitrogenous base, a pentose sugar, and a phosphate group. Adjacent nucleotides are linked together by a phosphate group that forms an ester bond with the 3'-OH of one sugar and another ester bond with the 5'-OH of the next sugar's phosphate.
…
- GUJCET 2024Set 131 markMCQQ.Which of the following statement is incorrect for the structure of Nucleic acid? (A) Nucleotides are joined together by phosphodiester linkage (B) In DNA molecule, the sugar moiety is β-D-2-deoxyribose (C) A unit formed by the attachment of a base 1' position of sugar is known as nucleoside (D) RNA contains four bases adenine, guanine, cytosine and thymine
›Reveal solutionSolution
Thymine is a DNA base; RNA uses uracil in its place — so the statement naming thymine in RNA is the incorrect one.
Concept — why: In nucleic acids, nucleotides are linked through 3′−5′ phosphodiester bonds. DNA sugar is β-D-2-deoxyribose; a base joined to the 1′-carbon of sugar is a nucleoside. DNA bases: A, G, C, T. RNA bases: A, G, C and uracil (U) — thymine is replaced by uracil. …
- GSEB Higher Secondary Certificate (HSC) Examination 2024Set ANNUAL1 markMCQQ.Hydrogen bond is present in which two pair of bases in double helix structure of DNA?(a) Guanine and Thymine(b) Adenine and Thymine(c) Adenine and cytosine(d) Cytosine and Thymine
›Reveal solutionSolution
In DNA's double helix, purine bases pair with pyrimidine bases via specific hydrogen bonding: Adenine pairs with Thymine (2 H-bonds), and Guanine pairs with Cytosine (3 H-bonds).
Of the listed pairs, Adenine-Thymine is a genuine complementary base pair held by hydrogen bonding across the two DNA strands. The other listed combinations (Guanine-Thymine, Adenine-Cytosine, Cytosine-Thymine) are NOT the standard Watson- …
- GUJCET 2023Set 091 markMCQQ.Which base is not present in the DNA structure? (A) Uracil (B) Adenine (C) Guanine (D) Cytosine
›Reveal solutionSolution
[!TLDR]
Uracil belongs to RNA (replacing thymine); DNA contains adenine, guanine, cytosine and thymine, so uracil is the base absent from DNA.
Concept
Nucleic acid bases: purines (adenine, guanine) and pyrimidines (cytosine, thymine, uracil). DNA contains A, G, C and T; RNA contains A, G, C and U, with uracil substituting for thymine.
Solution
Check each option against DNA's base set (A, G, C, T): …
- GUJCET 2022Set 171 markMCQQ.Select proper statement from following True (T) and False (F) statements. (I) Pentose sugar + base → Nucleotide (II) Nucleotide + Phosphate → Nucleoside (III) DNA contains four bases A, G, C and T (IV) RNA contains four bases A, G, C and U (A) FTFT (B) FTTT (C) FFTT (D) TTTT
›Reveal solutionSolution
Correct order = FFTT.
Concept. The nucleic-acid building sequence is: pentose + base → nucleoside; nucleoside + phosphate → nucleotide.
- (I) "Pentose + base → Nucleotide" — wrong (that gives a nucleoside) → F. …
- GSEB Higher Secondary Certificate (HSC) Examination 2022Set ANNUAL1 markMCQQ.Which is a bicyclic (purine) nitrogenous base?(a) A (Adenine)(b) T (Thymine)(c) C (Cytosine)(d) U (Uracil)
›Reveal solutionSolution
Nitrogenous bases split into two purines (fused bicyclic rings) and three pyrimidines (single six-membered ring).
Adenine and Guanine are purines - built from a fused imidazole + pyrimidine ring system (bicyclic). Thymine, Cytosine and Uracil are pyrimidines, each a single six-membered ring …
- GUJCET 2021Set 151 markMCQQ.In DNA, which bases is not present of following? (A) Thymine (B) Guanine (C) Uracil (D) Adenine
›Reveal solutionSolution
Uracil is an RNA base; DNA uses thymine in its place.
Concept: DNA bases are A, G, C, T. RNA replaces thymine with uracil. So uraci …
- GSEB Higher Secondary Certificate (HSC) Examination 2020Set ANNUAL1 markMCQQ.Which of the following base is not present in DNA?(a) Guanine(b) Adenine(c) Uracil(d) Thymine
›Reveal solutionSolution
DNA's four bases are A, G, C, T; RNA uses Uracil (U) in place of Thymine (T) - so Uracil is the one base of the four options that is NOT found in DNA.
DNA (deoxyribonucleic acid) contains the purine bases adenine and guanine, and the pyrimidine bases cytosine and thymine. RNA (ribonucleic acid) contains the same purines and cytosine, but replaces thymine with uracil (which lacks the 5-met …
- GUJCET 2019Set 131 markMCQQ.Giving 'T' symbol for true statement and 'F' symbol for false statement, select suitable option from the given options for following statements.(i) Cytosine base is the derivative of pyrimidine(ii) β- D Ribose sugar is present in DNA.(iii) The message for the synthesis of a specific protein in present in RNA.(iv) DNA is responsible for maintaining the identity of different species of organisms for one century (A) FFTF (B) FTFF (C) FFFT (D) TFFT
›Reveal solutionSolution
The statements evaluate to T F F T.
Reasoning:
- (i) Cytosine is a pyrimidine derivative → True. (This alone rules out every option that begins with F.)
- (ii) β-D-Ribose is present in DNA → False (DNA contains 2′-deoxyribose; ribose is in RNA). …
- GSEB Higher Secondary Certificate (HSC) Examination 2019Set ANNUAL1 markMCQQ.Which linkage is proper for a nucleotide?(a) Base - phosphate(b) Sugar - base(c) Sugar - base - phosphate(d) Phosphate - sugar - base
›Reveal solutionSolution
A nucleotide is built by first joining a nitrogenous base to a pentose sugar (forming a nucleoside), and then attaching a phosphate group to that sugar -- so the correct chain of linkage is phosphate-sugar-base.
Structure of a nucleotide: A NUCLEOSIDE is formed first, by linking a nitrogenous base to the 1' carbon of a pentose sugar (ribose or deoxyribose) via a beta-N-glycosidic bond. A NUCLEOTIDE is then formed when a phosphate group is esterified onto the 5' (or occasionally 3') carbon of that sugar via a phosphoester bond. So structurally the base is attached to the sugar at one position (C …
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