Gene

Expression of protein-coding genes begins with the process of transcription. During transcription, the helix is unwound, and an enzyme (RNA polymerase) binds to the DNA. It then moves along the DNA, and beginning slightly "downstream" at the so-called initiation site, it copies one of the strands to form a molecule of RNA. Transcription ceases when the polymerase reaches a special DNA sequence called the termination site, usually a region high in G-Cs followed by A-Ts.

In prokaryotes, this RNA product is ready to use for protein synthesis, and is called messenger RNA (mRNA). After the mRNA of a gene is formed, it is used by the cell in protein synthesis (translation) at the ribosomes.

Thus, the prokaryotic gene consists of an RNA binding site (called the "promoter"), a transcription initiation site, the coding region, and a termination signal. The initiation site should not be confused with the start signal for protein synthesis, nor the termination site with the stop signal in protein synthesis. Each of the translation signals is within the coding region, or "open reading frame," of the gene.