MP Board · Class 12 · Chemistry · BiomoleculesWhat are nucleic acids? Differentiate between DNA and RNA in terms of their chemical composition, structural conformation, and biological functions. Also, briefly explain the double-helix model of DNA.

Introduction to Nucleic Acids\nNucleic acids are biopolymers, macromolecules essential for all known forms of life. They are primarily responsible for the storage, transmission, and expression of genetic information from generation to generation. Nucleic acids are of two types: Deoxyribonucleic acid (DNA) and Ribonucleic acid (RNA).

Chemical Composition of DNA and RNA

• Pentose Sugar: DNA contains $\beta$-D-2-deoxyribose, whereas RNA contains $\beta$-D-ribose.
• Nitrogenous Bases:
  • DNA contains Purines (Adenine, Guanine) and Pyrimidines (Cytosine, Thymine).
  • RNA contains Purines (Adenine, Guanine) and Pyrimidines (Cytosine, Uracil). Thymine is absent in RNA and replaced by Uracil.
• Phosphoric Acid: Both DNA and RNA contain phosphoric acid ($\text{H}_3\text{PO}_4$) linked through phosphodiester bonds.

Differences between DNA and RNA

Double-Helix Model of DNA\nProposed by James Watson and Francis Crick in 1953:

• It consists of two polynucleotide chains that wrap around each other in a double helix.
• The two strands are anti-parallel; one runs in the $5' \rightarrow 3'$ direction and the other in the $3' \rightarrow 5'$ direction.
• The backbones are made up of alternating sugar and phosphate groups, while the nitrogenous bases project inward.
• The bases form specific complementary hydrogen bonds: Adenine (A) pairs with Thymine (T) via two hydrogen bonds ($\text{A}=\text{T}$), and Guanine (G) pairs with Cytosine (C) via three hydrogen bonds ($\text{G}\equiv\text{C}$).