For a protein-coding gene to be expressed, it must first be transcribed. In transcription, the code in the gene's DNA is converted into a complementary code in an RNA molecule. The RNA molecule then participates in the second phase of gene expression: translation. In translation, the code in the RNA is converted into an amino acid sequence in a protein. Transcription and translation are the main events of gene expression.
In the accompanying animation, we focus on transcription, which occurs in three phases: initiation, elongation, and termination.
Transcription begins with initiation, which requires a promoter, a special sequence of DNA to which the RNA polymerase binds very tightly. A promoter orients the RNA polymerase and thus "aims" it at the correct strand to use as a template. Part of each promoter is the initiation site, where transcription begins.
Once RNA polymerase has bound to the promoter, it begins the process of elongation. RNA polymerase unwinds the DNA about 10 base pairs at a time and reads the template strand in the 3′-to-5′ direction while it forms RNA in the opposite (5′-to-3′) direction. As in the process of DNA replication, base-pairing rules apply. However, uracil is used in RNA instead of the base thymine, which is used in DNA.
Just as initiation sites in the DNA template strand specify the starting point for transcription, particular base sequences specify its termination. For some genes, the newly formed transcript falls away from the DNA template and the RNA polymerase. For others, a helper protein pulls the transcript away.
Not only mRNA is produced by transcription. The same process is responsible for the synthesis of tRNA and ribosomal RNA (rRNA), which have important roles in protein synthesis. Like polypeptides, these RNAs are encoded by specific genes.
Textbook Reference: Concept 10.2 DNA Expression Begins with Its Transcription to RNA