Transcription

Transcription is the first step of gene expression, where the information from a gene, which is composed of DNA, is copied into RNA by the enzyme RNA polymerase. Both DNA and RNA are nucleic acids, which use base pairs of nucleotides as a complementary language that can be converted back and forth from DNA to RNA by the action of the correct enzymes. During transcription, a DNA sequence is read by an RNA polymerase, which produces a complementary, antiparallel RNA strand. As opposed to DNA replication, transcription results in an RNA complement that includes uracil (U) in all instances where the DNA had thymine (T).

Process

Transcription is divided into initiation, promoter escape, elongation, and termination.

Initiation

Transcription begins with the binding of RNA polymerase to the promoter in DNA. RNA polymerase is a core enzyme consisting of five subunits: 2 α subunits, 1 β subunit, 1 β' subunit, and 1 ω subunit. At the start of initiation, the core enzyme is associated with a sigma factor that aids in finding the appropriate -35 and -10 base pairs downstream of promoter sequences.

Promoter escape

After the first bond is synthesized, the RNA polymerase must clear the promoter. During this time there is a tendency to release the RNA transcript and produce truncated transcripts. This is called abortive initiation and is common for both eukaryotes and prokaryotes.

Elongation

Once promoter clearance has occurred, the RNA polymerase is considered to be in the elongation phase. During this stage, the RNA polymerase traverses the template strand and synthesizes an RNA copy of the gene.

Termination

Termination of transcription occurs when RNA polymerase crosses a termination sequence in the gene. The RNA product is then released from the RNA polymerase and the polymerase is then free to leave the DNA and find another gene to transcribe.

See also

• DNA replication
• Translation (biology)
• Gene expression
• RNA polymerase

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