The Double Helix and the Four Bases

A Double Helix Structure

The DNA molecule is made up of two long strands, wound around each other in a double helix, like a twisted rope ladder. This structure was first described in 1953 by James Watson and Francis Crick, drawing notably on the work of Rosalind Franklin.

Four Chemical Letters

Each strand of the helix is made up of a chain of small molecules called nitrogenous bases. There are only four types, designated by their initial:

• A for Adenine
• T for Thymine
• G for Guanine
• C for Cytosine

A Strict Pairing Between the Two Strands

The two strands of the double helix are linked by bonds between the bases, but not just any way: each base can only pair with one other.

• A always pairs with T
• G always pairs with C

This rule, called base complementarity, means that if you know the sequence of one strand, you can immediately deduce that of the complementary strand.

(strand 1)                : A - T - G - C - C - A
(strand 2, complementary) : T - A - C - G - G - T

Why This Rule Is Essential

This strict complementarity (A-T, G-C) makes possible the duplication of DNA before each cell division: the two strands separate, and each serves as a template to rebuild a complementary strand identical to the original one. This is how genetic information is faithfully copied into the daughter cells.

A Common Mistake to Avoid

Don't confuse the bases with each other: A pairs with T, never with G or C. Likewise, G pairs exclusively with C. It is this simple rule that guarantees the fidelity of DNA copying.