2.25 Watson and Crick Model of DNA

This model describes the three-dimensional structure of deoxyribonucleic acid (DNA), the molecule that carries genetic information in all living organisms. DNA is a polymer of nucleotides — the building blocks of life.

Historical Context and Foundational Discoveries

Chargaff's Rule (1951)

Erwin Chargaff discovered that in any DNA molecule:

• The amount of Adenine (A) always equals the amount of Thymine (T)
• The amount of Guanine (G) always equals the amount of Cytosine (C)
• Therefore: total Purines (A + G) = total Pyrimidines (T + C) → ratio 1:1

This rule provided the biochemical basis for complementary base pairing in the Watson-Crick model.

X-ray Diffraction by Wilkins and Franklin

Maurice Wilkins and Rosalind Franklin used X-ray diffraction to show that DNA is a duplex (double-stranded) helical molecule with key dimensions:

• Width: 2 nm
• Length of one complete turn: 3.4 nm

The Watson and Crick Model (1953)

James Watson and Francis Crick integrated the findings of Chargaff, Wilkins, and Franklin into a comprehensive physical model of DNA structure.

Double Helix Structure

DNA consists of two polynucleotide chains coiled around each other to form a right-handed double helix.

Antiparallel Strands

The two polynucleotide chains run in opposite directions:

• One strand runs 5′ → 3′
• The complementary strand runs 3′ → 5′

The backbone of each strand is composed of alternating deoxyribose sugar and phosphate groups linked by phosphodiester bonds. The nitrogenous bases project inward toward the centre of the helix.

Complementary Base Pairing

The two strands are held together by specific hydrogen bonding between nitrogenous bases:

• A always pairs with T ($A=T$)
• G always pairs with C ($G\equiv C$)
• This specific pairing maintains the constant 2 nm width of the helix
• G–C pairs are more thermally stable than A–T pairs due to the extra hydrogen bond

Dimensions of the DNA Helix

Genetic Information and Base Sequence

The sequence of nucleotides (A, T, C, G) along a strand is not restricted and can vary enormously. This sequence is:

• Highly specific for different species, organisms, and individuals
• The basis of the genetic code
• The source of virtually unlimited information storage capacity

What is a Gene?

A gene is a specific segment of DNA made up of a sequence of nucleotides. It is the fundamental physical and functional unit of heredity. Each gene contains the instructions (genetic code) to build a specific polypeptide (protein), which determines characteristics such as eye colour and hair colour.

Protein Synthesis — The Central Dogma

Genetic information flows from DNA → RNA → Protein:

1. Transcription: The gene's DNA sequence is copied into a messenger RNA (mRNA) molecule in the nucleus.
2. Processing: The mRNA moves from the nucleus to the cytoplasm.
3. Translation: Ribosomes read the mRNA sequence and assemble a polypeptide chain (string of amino acids), which folds into a functional protein.

$\text{DNA}\stackrel{\text{Transcription}}{\to }\text{mRNA}\stackrel{\text{Translation}}{\to }\text{Protein}$