MP Board · Class 12 · Chemistry · BiomoleculesWhat are nucleic acids? Name the two types of nucleic acids found in living systems. Describe their chemical composition, structural organization (including the double-helix model of DNA), and their primary biological functions.

Introduction to Nucleic Acids\nNucleic acids are long-chain polymers of nucleotides that are primarily responsible for the storage and transmission of genetic information from one generation to the next. They are found in the nucleus of living cells and are broadly classified into two types:

1. DNA (Deoxyribonucleic Acid): Contains deoxyribose sugar and stores genetic information.
2. RNA (Ribonucleic Acid): Contains ribose sugar and is primarily involved in protein synthesis.

Chemical Composition\nChemically, a nucleic acid is composed of repeating units called nucleotides. Each nucleotide consists of three fundamental components:

• Pentose Sugar: A 5-carbon monosaccharide; either $\beta$-D-2-deoxyribose (in DNA) or $\beta$-D-ribose (in RNA).
• Nitrogenous Base: Heterocyclic organic compounds containing nitrogen. They are divided into two categories:
  • Purines: Adenine (A) and Guanine (G).
  • Pyrimidines: Cytosine (C), Thymine (T - found in DNA only), and Uracil (U - found in RNA only).
• Phosphoric Acid ($H_3PO_4$): Links the pentose sugars of successive nucleotides through phosphodiester bonds.

Structural Organization and DNA Double Helix Model

• Nucleoside & Nucleotide Formation: A base linked to a pentose sugar forms a nucleoside. When a nucleoside is phosphorylated at the 5'-hydroxyl group of the sugar, it forms a nucleotide.
• Watson and Crick Double Helix Model of DNA: DNA consists of two polynucleotide chains that coil around a common axis in a double helix. The two chains are antiparallel (run in opposite directions, i.e., 5'$\to$3' and 3'$\to$5'). The sugar-phosphate backbone forms the outer framework, while the nitrogenous bases point inward, pairing specifically via hydrogen bonds: Adenine pairs with Thymine ($A=T$ via two hydrogen bonds) and Guanine pairs with Cytosine ($G\equiv C$ via three hydrogen bonds).

Biological Functions

• DNA is the chemical basis of heredity and is uniquely responsible for maintaining and passing genetic traits across generations.
• RNA molecules perform specific roles in protein biosynthesis, categorized into messenger RNA (mRNA), ribosomal RNA (rRNA), and transfer RNA (tRNA).