Question

Explain the structure of Nucleic Acid.

Hint:

Describe the nucleotide (sugar + nitrogen base + phosphate), the phosphodiester backbone, and the antiparallel double helix of DNA with A-T and G-C base pairing.

Answer 1

Step 1: What nucleic acids are. Nucleic acids are long-chain biomolecules (biopolymers) that carry and transmit hereditary information. The two types are DNA (deoxyribonucleic acid) and RNA (ribonucleic acid). Their monomer unit is the nucleotide.
Step 2: Components of a nucleotide. Each nucleotide is made of three parts:
(a) A pentose sugar — \(\beta\)-D-2-deoxyribose in DNA and \(\beta\)-D-ribose in RNA.
(b) A nitrogen-containing base — purines (adenine A, guanine G) and pyrimidines (cytosine C, thymine T in DNA; uracil U replaces thymine in RNA).
(c) A phosphate group \((\text{H}_3\text{PO}_4)\).
Step 3: Nucleoside and nucleotide. A base joined to the sugar (through an N-glycosidic bond) is a nucleoside. When a phosphate group is attached to the sugar of a nucleoside (through an ester bond), it becomes a nucleotide.
Step 4: Formation of the chain (primary structure). Nucleotides join together through phosphodiester linkages between the 3' carbon of one sugar and the 5' carbon of the next sugar, forming a long sugar-phosphate backbone with the bases projecting out.
Step 5: Secondary structure of DNA (Watson and Crick). DNA consists of two such polynucleotide strands coiled around each other in a right-handed double helix. The two strands run 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 ≡ C, three H-bonds).
Step 6: RNA structure. RNA is usually a single-stranded molecule containing ribose sugar and uracil in place of thymine.
\[\boxed{\text{Nucleic acid} = \text{repeating nucleotides (sugar + base + phosphate); DNA = double helix}}\]