Transcription is the synthesis of an RNA strand using DNA as a template. It copies selected genetic information into RNA without copying the entire genome.

A transcription unit contains a DNA region whose sequence is transcribed and nearby sequence information that helps position the transcription machinery. A promoter is a DNA region where RNA polymerase and associated factors assemble to initiate transcription.

Template and coding strands

Only one DNA strand is used as the template for a particular RNA transcript. RNA polymerase reads that template $3'\rightarrow5'$ while synthesizing RNA $5'\rightarrow3'$.

The opposite DNA strand is often called the coding strand because, except that DNA contains T where RNA contains U, its sequence matches the RNA product.

For example:

coding DNA:   5'-A T G C C A-3'
template DNA: 3'-T A C G G T-5'
RNA:          5'-A U G C C A-3'

Initiation, elongation and termination

Transcription can be divided into three phases.

1. Initiation: RNA polymerase is positioned at a promoter, locally opens the DNA and begins RNA synthesis.
2. Elongation: the polymerase moves along the template, extending the RNA by complementary ribonucleotide addition.
3. Termination: sequence- and factor-dependent signals cause transcription to stop and release the RNA product.

Unlike the major replicative DNA polymerases, RNA polymerases can initiate an RNA chain without requiring a pre-existing nucleic-acid primer.

Transcription is selective

A cell does not normally transcribe every DNA region at the same rate. Which promoters recruit active transcription machinery is a major control point in gene expression.

The immediate product of transcription is RNA. Depending on the gene, that RNA may itself become functional or may undergo processing and, for messenger RNA, later serve as a template for protein synthesis.