DNA Structure

Core Concept

DNA (deoxyribonucleic acid) is the molecule of heredity: a double-stranded helix that stores genetic information in the sequence of its four bases. Its structure — two antiparallel strands held together by specific base pairing — is exactly what allows it to be copied accurately and to encode information. Structure and function are inseparable here.

Key Concepts

Nucleotides

Each DNA nucleotide has three parts: a phosphate group, a five-carbon sugar called deoxyribose, and a nitrogenous base. The four bases are adenine (A), thymine (T), guanine (G), and cytosine (C). Nucleotides join by phosphodiester bonds between the phosphate of one and the sugar of the next, forming a sugar–phosphate backbone with the bases pointing inward.

Purines and pyrimidines

The bases come in two chemical classes. Purines (adenine and guanine) are larger, double-ring structures. Pyrimidines (thymine and cytosine) are smaller, single-ring structures. In the double helix, a purine always pairs with a pyrimidine, which keeps the helix a uniform width.

Base pairing

Adenine pairs with thymine via two hydrogen bonds; guanine pairs with cytosine via three hydrogen bonds. This complementarity means the sequence of one strand determines the sequence of the other — the key to faithful replication and to information storage. Because G–C has three bonds, G–C-rich DNA is slightly more stable (harder to separate) than A–T-rich DNA.

The antiparallel double helix

The two strands run in opposite directions: one runs 5′ → 3′, the other 3′ → 5′. The 5′ end has a free phosphate, the 3′ end a free hydroxyl (—OH) on the sugar. The strands wind around each other into a right-handed double helix, with backbones outside and bases stacked inside like rungs of a twisted ladder. Each full turn is about 10 base pairs (~3.4 nm), with a major and a minor groove.

Information storage

The order of bases along a strand is the genetic code. A gene is a specific sequence of bases that encodes a functional product (usually a protein). Because the strands are complementary, either strand can serve as a template for rebuilding the other — the principle behind replication and repair.

How It Works

DNA's information is stored linearly in base sequence, like letters in a sentence. The two strands are held together by hydrogen bonds between complementary bases and by base-stacking interactions between adjacent pairs. When the cell needs to copy or read the DNA, the strands separate (assisted by enzymes), and each exposed strand serves as a template: A attracts T and G attracts C, so a new complementary strand is assembled with exact fidelity.

How it works

DNA's information is stored linearly in base sequence, like letters in a sentence. The two strands are held together by hydrogen bonds between complementary bases and by base-stacking interactions between adjacent pairs. When the cell needs to copy or read the DNA, the strands separate (assisted by enzymes), and each exposed strand serves as a template: A attracts T and G attracts C, so a new complementary strand is assembled with exact fidelity.

Common confusions

• "A pairs with T with three bonds." Wrong — A–T has two hydrogen bonds; G–C has three.
• "Both strands run the same direction." Wrong — they are antiparallel (5′→3′ vs. 3′→5′).
• "Thymine is a purine." Wrong — T and C are pyrimidines (single ring); A and G are purines (double ring).
• "The sequence is stored in the backbone." Wrong — the backbone is constant; the information is in the base sequence.
• "More hydrogen bonds means A–T is stronger." Wrong — G–C pairs have three bonds and are stronger/more stable.

Quick review

• Nucleotide = phosphate + deoxyribose + base.
• Purines (A, G) pair with pyrimidines (T, C).
• A–T = 2 H-bonds; G–C = 3 H-bonds.
• Antiparallel double helix, bases inside, backbone outside.
• Base sequence encodes genetic information.
• Complementarity underlies replication, transcription, and repair.