Gene expression is the process by which information in a gene is used to produce its functional molecular product.

For a protein-coding gene, the core flow is

DNA → RNA → polypeptide → functional protein

DNA is transcribed into RNA, and an appropriate messenger RNA is translated into a polypeptide. The polypeptide can then fold, assemble with partners, move to a cellular location or undergo chemical modification before becoming fully functional.

For a gene whose functional product is RNA, expression ends without translation:

DNA → functional RNA

Examples include RNAs that become parts of ribosomes or transfer amino acids during translation.

Expression is a quantity, not merely an on/off label

A gene can be expressed at different levels. The amount of functional product depends on several rates, including

• how often transcription initiates;
• how rapidly the RNA product is degraded;
• for protein-coding genes, how efficiently the RNA is translated;
• how rapidly the protein product is degraded.

Additional RNA-processing steps can create further control points in organisms where the initial transcript must be matured before use.

Thus two cells containing the same gene can contain very different amounts of its RNA or protein product.

The DNA sequence can remain unchanged while expression changes

Different cell types in the same multicellular organism can carry essentially the same genome yet use different subsets of genes and produce different quantities of their products. Cellular identity therefore depends not only on which genes exist but also on which genes are expressed, when, where and how strongly.

The familiar DNA→RNA→protein scheme describes an important direction of sequence-information transfer, but it is not a claim that every RNA is translated or that regulation occurs only at one step. Gene expression is a multi-stage process whose individual stages can be independently controlled.