Not all DNA is transcribed, only DNA corresponding to genes. In addition, gene expression is regulated according to stage of development, the cell type, the cellular environment, etc.
Thus, a component must enter into play to serve as a starting point for DNA transcription. This component is the reminder. (To learn more, see Chapter 2: Gene expression, 3.3 Initiation of transcription)
The reminder corresponds to an untranslated region of DNA. Given reminder sequences (called "boxes") have a specific role in the transcription process, mainly because their sequences are specifically recognized by different proteins in the initiation complex: The "TATA box", the most frequently encountered, rich in thymine and adenine, is located -25 to -30 nt upstream from the transcription start site (noted +1).
Example: The deletion of a few nucleotides in the TATA box of the promoter of an eucaryotic gene fully stops transcription.