MP Board · Class 12 · Chemistry · BiomoleculesWhat are nucleic acids? Write down the structural difference between DNA and RNA. Also, explain the double helix model of DNA proposed by Watson and Crick.

Introduction to Nucleic Acids\nNucleic acids are biomolecules found in the nuclei of all living cells, playing a vital role in heredity and protein synthesis. They are long-chain polymers of nucleotides and are mainly of two types: Deoxyribonucleic Acid (DNA) and Ribonucleic Acid (RNA).

Structural Differences Between DNA and RNA

Watson and Crick Model of DNA\nJames Watson and Francis Crick proposed a double-helix model for the structure of DNA based on X-ray diffraction data and chemical composition. The key features of this model are:

1. Two Strands: DNA consists of two polynucleotide chains that wrap around each other in a double helix, spiraling around a central axis.
2. Antiparallel Nature: The two strands run in opposite directions; one strand has a $5' \to 3'$ polarity, while the other has a $3' \to 5'$ polarity.
3. Backbone: The sugar-phosphate backbone forms the outer structural framework of the helix, while the nitrogenous bases project inward.
4. Base Pairing: The nitrogenous bases on the two strands pair through specific hydrogen bonding. Adenine (A) always pairs with Thymine (T) via two hydrogen bonds ($A = T$), and Guanine (G) always pairs with Cytosine (C) via three hydrogen bonds ($G \equiv C$).
5. Complementary and Chargaff's Rule: Because of this specific pairing, the two strands are complementary to each other. Consequently, the total amount of adenine equals thymine, and guanine equals cytosine ($[A] = [T]$ and $[G] = [C]$).
6. Dimensions: The double helix has a uniform diameter of about $2\text{ nm}$ ($20\ \mathring{\text{A}}$), with each complete turn having a length of about $3.4\text{ nm}$ ($34\ \mathring{\text{A}}$) containing approximately 10 base pairs.