Question

Write short notes on the following: (i) Denaturation of proteins (ii) Name and structural formula of bases present in the nucleic acid. (2 + 2)

Hint:

Denaturation breaks the hydrogen bonds of the secondary and tertiary structure (primary stays intact), e.g. boiling an egg. Bases: purines A and G; pyrimidines C, T (DNA) and U (RNA).

Answer 1

(i) Denaturation of proteins
Step 1 (Meaning): A protein found in a living system with a unique three-dimensional shape and biological activity is called a native protein. When this native protein is subjected to a physical change (such as a rise in temperature) or a chemical change (such as a change in pH, or adding acid, alcohol or heavy-metal salts), it loses its natural shape and its biological activity. This process is called denaturation.
Step 2 (What happens at the molecular level): During denaturation the hydrogen bonds and other weak forces that hold the secondary and tertiary structure are broken. The coiled and folded chains (the helix uncoils and the globules unfold), so the secondary and tertiary structures are destroyed, but the primary structure (the sequence of amino acids joined by peptide bonds) remains intact.
Step 3 (Examples): Coagulation of egg white on boiling, and curdling of milk (due to the formation of lactic acid by bacteria) are common examples of denaturation.

(ii) Bases present in nucleic acids
Step 4 (Two families): The nitrogen-containing bases in nucleic acids are of two types:
• Purines (fused double-ring): Adenine (A) and Guanine (G).
• Pyrimidines (single ring): Cytosine (C), Thymine (T) and Uracil (U).
Step 5 (Where they occur): DNA contains A, G, C and T. RNA contains A, G, C and U (thymine is replaced by uracil).
Step 6 (Names and structural description / formulae):
• Adenine (6-aminopurine), \(C_5H_5N_5\): a purine ring bearing an –NH₂ group at position 6.
• Guanine (2-amino-6-oxopurine), \(C_5H_5N_5O\): a purine ring with an –NH₂ group at position 2 and a keto (=O) group at position 6.
• Cytosine (4-amino-2-oxopyrimidine), \(C_4H_5N_3O\): a pyrimidine ring with an –NH₂ group at position 4 and a keto group at position 2.
• Thymine (5-methyl-2,4-dioxopyrimidine), \(C_5H_6N_2O_2\): a pyrimidine ring with keto groups at positions 2 and 4 and a –CH₃ group at position 5.
• Uracil (2,4-dioxopyrimidine), \(C_4H_4N_2O_2\): a pyrimidine ring with keto groups at positions 2 and 4 (same as thymine but without the –CH₃).
\[\boxed{\text{Purines: A, G;} \quad \text{Pyrimidines: C, T, U}}\]