How four chemical letters store biological instructions
DNA’s letters are the names of four chemical bases: A, T, C, and G. They form a long molecule in two matching strands. The bases pair in a specific way—A with T, and C with G—so the sequence on one strand determines the sequence on the other.
The important information lies in the order of the bases, much as the same letters can form different words in different arrangements. A cell does not treat the entire DNA molecule as one uninterrupted instruction. It uses particular stretches of sequence as genes, with nearby DNA helping regulate when those stretches are used.
From a DNA sequence to a working molecule
To use a gene, a cell first makes an RNA copy of its sequence. This step is called transcription. RNA is similar to DNA, but it uses U in place of T. The cell’s protein-making machinery then reads the RNA in groups of three letters. Each group, called a codon, corresponds to an amino acid—the small building block used to assemble a protein—or to a signal to stop.
That conversion is why a tiny change in DNA can matter: it may alter a codon, change an amino acid, or affect whether a gene is read at all. But not every change has a noticeable effect; some leave the resulting protein unchanged, and others occur in DNA that does not directly specify a protein.
Copying preserves the sequence
Before a cell divides, it copies its DNA. The two strands separate, and each serves as a pattern for building a new partner: a T is added opposite A, and a G opposite C. This pairing makes accurate copying possible, though occasional copying errors can still occur.
The four-letter alphabet is simple; the biological complexity comes from the sequence, the way cells interpret it, and the controls that determine when an instruction is used.
