In cellular organisms, a gene is a region of DNA whose information contributes to a functional molecular product. Expression of a gene begins by transcribing DNA into RNA (ribonucleic acid), another nucleic-acid polymer. Some RNA products function directly, while others act as messenger molecules used later in protein synthesis.

Regulatory DNA that controls when and where a gene is transcribed can lie next to the transcribed region or at a distance from it. A promoter, for example, helps position transcription machinery but is conceptually distinct from the RNA sequence that is copied from DNA.

The physical position of a gene on a DNA molecule is its locus.

A chromosome is a long DNA molecule together with associated material that organizes it. A chromosome can contain many genes separated by DNA with structural, regulatory or other functions.

A genome is the complete genetic material of a cell or organism. A cellular genome can consist of one chromosome or many chromosomes, depending on the organism and cellular compartment.

These levels should not be confused:

DNA sequence region → gene
many sequence regions → chromosome
all chromosomal genetic material → genome

Not every DNA sequence is a gene, and one chromosome is not generally one gene.

Different genes can have different kinds of products

For many genes, the transcribed RNA is used as an intermediate in protein production. For other genes, the functional product is the RNA itself.

This prevents a common oversimplification: a gene is not simply 'a piece of DNA that makes a protein.' Genes encode biological information, and gene expression can produce either functional RNA or, through additional steps, protein.