Why do cells with the same DNA work differently?

Almost all human cells contain the same DNA sequence. Yet a skin cell, a neuron, and a liver cell look different and perform very different functions.

The reason is that different cell types use different sets of genes. Some genes are active, while others are largely silent. This determines which proteins a cell produces, how it is structured, and what functions it can perform.

Muscle cells, for example, need proteins involved in contraction, whereas immune cells produce molecules that help defend the body. Their DNA is nearly identical, but they follow different molecular programs.

This selective use of genes is fundamental to development. Hundreds of specialized cell types arise from a single fertilized egg because different genes gradually become active or inactive in different cells.

The same DNA therefore does not make all cells identical. What matters is not only the genetic information itself, but also which parts of it each cell uses.