Question

Write a short note on the structure of nucleic acid. (2)

Hint:

Nucleic acids are polynucleotides; each nucleotide = base + pentose sugar + phosphate, joined by 3',5'-phosphodiester bonds, and DNA forms a double helix with A-T and G-C base pairing.

Answer 1

Concept: Nucleic acids (DNA and RNA) are long-chain biomolecules (polynucleotides) that store and transfer genetic information. Their structure is best understood at two levels: the building block (nucleotide) and the way these blocks join into a chain.

Step 1: The building unit. The repeating unit of a nucleic acid is a nucleotide, made of three parts joined together:
(a) a nitrogen-containing base, (b) a pentose sugar, and (c) a phosphate group.
A base joined to only the sugar is a nucleoside; adding the phosphate makes it a nucleotide.

Step 2: The components.
Sugar: \(\beta\)-D-2-deoxyribose in DNA and \(\beta\)-D-ribose in RNA.
Bases: purines adenine (A) and guanine (G); pyrimidines cytosine (C), thymine (T, only in DNA) and uracil (U, only in RNA).

Step 3: How the chain is built (primary structure). The nucleotides are linked through phosphodiester bonds: the phosphate connects the 3'-carbon of one sugar to the 5'-carbon of the next sugar, giving a sugar-phosphate backbone with the bases sticking out sideways.

Step 4: The double helix (secondary structure of DNA). In DNA two such strands coil around a common axis to form a right-handed double helix (Watson and Crick model). The two strands run in opposite directions (antiparallel) and are held together by hydrogen bonds between complementary bases: adenine pairs with thymine (A = T, two H-bonds) and guanine pairs with cytosine (G \(\equiv\) C, three H-bonds). RNA is normally single-stranded.

Result: A nucleic acid is a polynucleotide whose sugar-phosphate backbone carries a definite base sequence; in DNA two complementary, antiparallel strands twist into a double helix held by base-pair hydrogen bonds.