Why does G-C base pair have 3 hydrogen bonds while A-T has only 2?

Guanine and Cytosine each have three groups capable of forming hydrogen bonds in their complementary positions, creating 3 H-bonds between them. Adenine and Thymine have only two complementary H-bonding groups. This difference has functional significance: DNA regions rich in G-C pairs are harder to denature (need higher temperature to separate strands) than A-T rich regions, because 3 H-bonds per pair vs 2.

How do enzymes increase the rate of a reaction?

Enzymes lower the activation energy (Ea) of a reaction by: (1) Providing an alternative reaction pathway with lower Ea. (2) Positioning substrates in optimal orientation at the active site. (3) Creating a microenvironment (acid/base groups in active site) that stabilises the transition state. (4) Forming enzyme-substrate complex (ES), which then releases product, regenerating the enzyme. The enzyme is not consumed — it can catalyse millions of reactions.

Nucleic Acids, DNA Structure, and Enzyme Kinetics

Biomolecules • Class 12 Chemistry • NCERT • CBSE

Nucleotide = nitrogenous base + pentose sugar + phosphate. DNA double helix: antiparallel strands, A-T (2 H-bonds), G-C (3 H-bonds). mRNA carries genetic code; tRNA carries amino acids; rRNA forms ribosomes.

Key Formulas

Nucleotide = base + pentose sugar + phosphate
Chargaff's rule: A=T, G=C (in DNA)
A-T: 2 H-bonds; G-C: 3 H-bonds

Frequently Asked Questions

Why does G-C base pair have 3 hydrogen bonds while A-T has only 2?: Guanine and Cytosine each have three groups capable of forming hydrogen bonds in their complementary positions, creating 3 H-bonds between them. Adenine and Thymine have only two complementary H-bonding groups. This difference has functional significance: DNA regions rich in G-C pairs are harder to denature (need higher temperature to separate strands) than A-T rich regions, because 3 H-bonds per pair vs 2.
How do enzymes increase the rate of a reaction?: Enzymes lower the activation energy (Ea) of a reaction by: (1) Providing an alternative reaction pathway with lower Ea. (2) Positioning substrates in optimal orientation at the active site. (3) Creating a microenvironment (acid/base groups in active site) that stabilises the transition state. (4) Forming enzyme-substrate complex (ES), which then releases product, regenerating the enzyme. The enzyme is not consumed — it can catalyse millions of reactions.

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