Class 12 Biology Molecular Basis of Inheritance Structure of Nucleic Acids

Structure of Nucleic Acids

Nucleic acids are the biomolecules which play a very important role in the process of Inheritance.

Two types of nucleic acids exist: DNA (Deoxyribo Nucleic Acid) and RNA (Ribo Nucleic Acid).

DNA has a double-stranded structure. It is a polynucleotide whose monomer units are deoxyribonucleotide. Length of DNA is determined by number of nucleotides in it.

RNA, on the other hand, has a single-stranded structure. It is also a polymer whose monomer units are ribonucleotide.


A nucleotide has 3 components:                     

  • Pentose sugar
    • Monosaccharide with 5 Carbon atoms
    • Ribose sugar in RNA
    • Deoxyribose sugar in DNA


  • Nitrogenous base
    • Nitrogen containing compound with properties of a base
    • 2 types: Purines, Pyrimidines
      • Purine
        • Heterocyclic aromatic organic compound
        • 9-membered ring
        • Examples: Adenine, Guanine
    •  Class_12_Molecular_Basis_Of_Inheritance_Adenine_&_Guanine
      • Pyrimidine
        • Heterocyclic aromatic organic compound
        • 6-membered ring
        • Examples: Cytosine, Uracil, Thymine


  • Phosphate group
    • Inorganic salt of phosphorus
    • Forms backbone of polynucleotide chain along with the sugar


Formation of a polynucleotide takes place using the following linkages:


  1. Nitrogenous base is linked to the pentose sugar through a N-glycosidic bond to form a nucleoside
  2. A phosphate group is linked to 5'-OH of a nucleoside through phosphoester bond to form a nucleotide
  3. Multiple nucleotides are joined together through 3'-5' phosphodiester bond to form a polynucleotide




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