A gene is a segment of DNA that specifies, or codes for, the sequence of a protein*. A gene also contains one or more regulatory sequences that either increase or decrease the rate of its transcription.
Shown above is a picture of a long piece of DNA. The portion inside the blue rectangle represents a typical gene. Only the coding strand of the gene is shown. Various regions of the gene are indicated by different colored outlines:
Major features of a typical gene
Red: a region that encodes a protein sequence
Black: a non-coding region (a single gene usually contains more than one)
Green: a regulatory sequence
The orange double slash marks represent sequences of the gene that are not pictured; a gene is actually much longer than can be shown here.
*There are some exceptions to this definition which will be discussed later.