A gene is just a stretch of nucleotides—a sequence of words. If you look at a gene as just a piece of data tape and stretch it out, you’ll see that at the 3’  end there’s a region called a promoter. The promoter is basically a little stretch of DNA that says: “Yo, polymerase! Bind  here.” Different genes have different promoters, which are recognized by different transcription factors. A transcription factor is a protein that lives to recognize and bind to a promoter or other regulatory element on DNA. When the transcription factor binds to the promoter, it guides polymerase onto the DNA strand, in more or less the right place and in more or less the right configuration to begin the process of transcription.

Figure 17. The promoter and transcription. A promoter element guides RNA polymerase into the proper configuration to begin transcription of mRNA from the coding region of a gene.