Q.DNA differs from RNA in having
🔒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 — Nucleic Acid Structure
Nucleic Acid Structure: From Building Blocks to the Double Helix
You already know that DNA stores genetic information. But how does a molecule physically hold that information? The answer lies in its structure — a chain of repeating units that can fold into a precise, stable shape.
The Intuition: A Zipper Made of Letters
Imagine a long, flexible chain. Each link in the chain is a nucleotide — a small molecule with three parts: a sugar (deoxyribose in DNA, ribose in RNA), a phosphate group, and a nitrogenous base (A, T, G, C in DNA; A, U, G, C in RNA). The sugar and phosphate form the backbone of the chain, while the bases stick out to the side like teeth on a zipper.
Now, take two such chains and line them up so their bases face each other. The bases have a natural pairing rule: A always pairs with T (or U in RNA), and G always pairs with C. This is complementary base pairing — the zipper's teeth fit together perfectly only in one specific way. When the two chains twist around each other, you get the famous double helix.
The key insight: the sequence of bases along one chain determines the sequence along the other. This is how DNA replication works — each strand serves as a template for making its partner.
The Precise Statement: Phosphodiester-Linked Chains
A nucleotide consists of:
- A pentose sugar (deoxyribose or ribose)
- A phosphate group attached to the 5' carbon of the sugar
- A nitrogenous base attached to the 1' carbon of the sugar
Nucleotides join together through phosphodiester bonds — a phosphate group links the 3' carbon of one sugar to the 5' carbon of the next. This creates a sugar-phosphate backbone with a direction: one end has a free 5' phosphate (the 5' end), the other has a free 3' hydroxyl (the 3' end). The sequence is always read 5' → 3'.
5’ — phosphate — sugar — base — phosphate — sugar — base — 3’
The Double Helix: Watson-Crick Model
Two antiparallel polynucleotide chains (one running 5'→3', the other 3'→5') wind around a common axis. The bases face inward, forming hydrogen bonds:
- A pairs with T (2 hydrogen bonds)
- G pairs with C (3 hydrogen bonds)
The sugar-phosphate backbones are on the outside, exposed to water. The base pairs stack inside, like a spiral staircase. This is the secondary structure of DNA — a right-handed double helix with a diameter of 2 nm and a rise of 0.34 nm per base pair.
A common mistake: thinking the two strands are identical. They are complementary, not identical. If one strand is 5'-ATGC-3', the other is 3'-TACG-5'.
Why This Structure Matters
The double helix is not just a pretty shape. It explains: …
DNA and RNA share three of their four nitrogenous bases, differing in only one, and this single difference in base composition distinguishes the two nucleic acids. …
Thymine is the base found in DNA but replaced by uracil in RNA.
Both DNA and RNA are nucleic acids built from nucleotides, and they share three nitrogenous bases: adenine (A), guanine (G), and cytosine (C). The key difference lies in the fourth pyrimidine base: DNA uses THYMINE (T), which pairs with adenine, whereas RNA uses URACIL (U) instead of thymine, which also pairs with adenine. (Additionally, DNA's sugar is deoxyribose while RNA's is ribose, …
- CBSE 2026Set ANNUAL1 markMCQQ.Which of the following statements regarding DNA is true?(i) The bond between the phosphate and hydroxyl group of sugar is a glycosidic bond.(ii) The backbone is formed by the sugar-phosphate-sugar chain.(iii) The nitrogenous bases are projected more or less perpendicular to the backbone.(iv) The two strands of polynucleotides are antiparallel.(a)(i) only(b)(i) and(ii)(c) (ii),(iii) and(iv)(d) (i),(iii) and (iv)
›Reveal solutionSolution
Of the four statements, (ii), (iii) and (iv) correctly describe DNA structure; (i) wrongly names the sugar-phosphate linkage.
Evaluating each statement against the Watson-Crick double-helix model of DNA: (i) is FALSE — the bond between the phosphate group and the 3'/5'-hydroxyl groups of adjacent sugars is a PHOSPHODIESTER bond, not a glycosidic bond (a glycosidic bond, by contrast, links the nitrogenous base to the sugar, i.e. an N-glycosidic bond). (ii) is TRUE — alternating sugar and phosphate groups linked by phosphodiester bonds form the continuous backbone of each polynucleotide strand. (iii) is TRUE — the nitrogenous bases are attached to the sugars and project inward, oriented more or less perpendicular to the sugar-phosphate backbone, pairing with a complementary base on the opposite strand (A with T, G with C, via hydrogen bonds). (iv) is TRUE — the two polynucleotide strands of the DNA do …
- CBSE 2024Set HALF_YEARLY1 markQ.Fill in the blank: The distance between the two strands of DNA is __________.
›Reveal solutionSolution
The two strands of the DNA double helix are a constant 20 A (2 nm) apart, a spacing maintained by the regular geometry of complementary base-pairing.
DNA is a double helix made of two polynucleotide chains coiled around a common axis, running in antiparallel directions and held together by hydrogen bonds between complementary nitrogenous bases (adenine pairs with thymine via two hydrogen bonds; guanine pairs with cytosine via three hydrogen bonds). Because every base pair (whether A-T or G-C) has essentially the same width, the two sugar-phosphate backbones stay a uniform distance apart …
- CBSE 2022Set TERM11 markMCQQ.DNA differs from RNA in having(a) Adenine(b) Thymine(c) Cytosine(d) Guanine
›Reveal solutionSolution
Thymine is the base found in DNA but replaced by uracil in RNA.
Both DNA and RNA are nucleic acids built from nucleotides, and they share three nitrogenous bases: adenine (A), guanine (G), and cytosine (C). The key difference lies in the fourth pyrimidine base: DNA uses THYMINE (T), which pairs with adenine, whereas RNA uses URACIL (U) instead of thymine, which also pairs with adenine. (Additionally, DNA's sugar is deoxyribose while RNA's is ribose, …
- CBSE 2019Set ANNUAL1 markMCQQ.How many base pairs are present in one full turn of DNA helical strand ?(a) 10(b) 12(c) 8(d) 34
›Reveal solutionSolution
In the Watson–Crick B-form DNA double helix, one complete turn measures 3.4 nm and each stacked base pair contributes 0.34 nm of rise, so 3.4 nm ÷ 0.34 nm = 10 base pairs per turn.
Watson and Crick's 1953 double helix model describes DNA as two antiparallel polynucleotide chains coiled around a common axis, held together by hydrogen bonds between complementary bases (A=T, G≡C).
Key helical parameters of B-DNA:
- Diameter of the helix: 2 nm
- Distance between two consecutive base pairs (rise per bp): 0.34 nm …
🎓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.