DNA Structure: Chemical Composition & The Double Helix
Deoxyribonucleic Acid (DNA) is the molecule of life. Understanding its chemical makeup—from nucleosides to the double helix—is fundamental for Class 12 Boards, JEE, and NEET.
1. Chemical Composition of Nucleic Acids
DNA and RNA are polymers (polynucleotides) composed of monomeric units called Nucleotides. Each nucleotide has three components:
- Pentose Sugar: $\beta$-D-2-deoxyribose in DNA; $\beta$-D-ribose in RNA.
- Nitrogenous Base: Heterocyclic compounds.
- Phosphate Group: Phosphoric acid ($H_3PO_4$) responsible for the acidic nature.
Linked at C-1' of sugar.
Linked at C-5' of sugar.
Nitrogenous Bases Classification
- Adenine (A)
- Guanine (G)
- Cytosine (C)
- Thymine (T) [DNA only]
- Uracil (U) [RNA only]
Figure: The Watson-Crick Double Helix Model of DNA
2. Watson-Crick Double Helix Model
Proposed by James Watson and Francis Crick in 1953. The backbone consists of alternating sugar-phosphate chains joined by 3'-5' phosphodiester linkages.
Base Pairing Rules (Chargaff's Rule)
For double-stranded DNA:
- Adenine pairs with Thymine via 2 Hydrogen Bonds ($A=T$).
- Guanine pairs with Cytosine via 3 Hydrogen Bonds ($G \equiv C$).
- Molar Ratio: Total Purines = Total Pyrimidines ($A+G = T+C$).
- Strand Polarity: The two strands run anti-parallel ($5' \rightarrow 3'$ and $3' \rightarrow 5'$).
Test Your Knowledge (10 Questions)
Select the best answer. Explanations will appear after submission.
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