By definition, a gene includes the entire nucleic acid sequence necessary for the expression of its product (peptide or RNA). Such sequence may be divided into regulatory region and transcriptional region. The regulatory region could be near or far from the transcriptional region. The transcriptional region consists of exons and introns. Exons encode a peptide or functional RNA. Introns will be removed after transcription (see Chapter 5 Section A).
As shown in the following figure, a typical DNA molecule consists of genes, pseudogenes and extragenic region. Pseudogenes are nonfunctional genes. They often originate from mutation of duplicated genes. Because duplicated genes have several copies, the organism can still survive even if a couple of them become nonfunctional.