A gene then is a segment of DNA that when converted to RNA specifies a protein.
Each gene has two discrete areas: a coding region and a regulatory region. The regulatory region acts as a switch capable of turning the coding region on or off.
Transcription is carried out by a very large and complex enzyme called RNA polymerase.
RNA polymerase can only combine with DNA and begin transcribing RNA within a special regulatory region of the gene called the promoter.
Except for the substitution of the nucleotide uracil for thymine the base pairing rules and chemical aspects of RNA synthesis are the same as those of DNA replication.