Q.When you take cells or tissue pieces and grind them with an acid in a mortar and pestle, all the small biomolecules dissolve in the acid. Proteins, polysaccharides and nucleic acids are insoluble in mineral acid and get precipitated. The acid soluble compounds include aminoacids, nucleosides, small sugars etc. When one adds a phosphate group to a nucleoside, one gets another acid soluble biomolecule called

Nucleic Acid Structure: From Building Blocks to the Double Helix

You already know that DNA stores genetic information. But how does a molecule physically hold that information? The answer lies in its structure — a chain of repeating units that can fold into a precise, stable shape.

The Intuition: A Zipper Made of Letters

Imagine a long, flexible chain. Each link in the chain is a nucleotide — a small molecule with three parts: a sugar (deoxyribose in DNA, ribose in RNA), a phosphate group, and a nitrogenous base (A, T, G, C in DNA; A, U, G, C in RNA). The sugar and phosphate form the backbone of the chain, while the bases stick out to the side like teeth on a zipper.

Now, take two such chains and line them up so their bases face each other. The bases have a natural pairing rule: A always pairs with T (or U in RNA), and G always pairs with C. This is complementary base pairing — the zipper's teeth fit together perfectly only in one specific way. When the two chains twist around each other, you get the famous double helix.

The Precise Statement: Phosphodiester-Linked Chains

A nucleotide consists of:

• A pentose sugar (deoxyribose or ribose)
• A phosphate group attached to the 5' carbon of the sugar
• A nitrogenous base attached to the 1' carbon of the sugar

Nucleotides join together through phosphodiester bonds — a phosphate group links the 3' carbon of one sugar to the 5' carbon of the next. This creates a sugar-phosphate backbone with a direction: one end has a free 5' phosphate (the 5' end), the other has a free 3' hydroxyl (the 3' end). The sequence is always read 5' → 3'.

The Double Helix: Watson-Crick Model

Two antiparallel polynucleotide chains (one running 5'→3', the other 3'→5') wind around a common axis. The bases face inward, forming hydrogen bonds:

• A pairs with T (2 hydrogen bonds)
• G pairs with C (3 hydrogen bonds)

The sugar-phosphate backbones are on the outside, exposed to water. The base pairs stack inside, like a spiral staircase. This is the secondary structure of DNA — a right-handed double helix with a diameter of 2 nm and a rise of 0.34 nm per base pair.

Why This Structure Matters

The double helix is not just a pretty shape. It explains: …