transcription unit

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The segment of DNA between the sites of initiation and termination of transcription by RNA polymerase; more than one gene may reside in a transcription unit. A polycistronic message (q.v.) may be translated as such and the translational product may be enzymatically cleaved later into two or more functional polypeptide chains. RNA is transcribed in a 5′ to 3′ direction from the template strand of the gene. However, when describing the nucleotide sequence of a specific gene, the convention has been adopted to give it the same nucleotide sequence as the RNA transcript, except that each uridine is replaced by thymidine. Any element to the left of the initiation site is said to be “5′ to” or “upstream of” the gene. Any element to the right is “3′ to” or “downstream of” the gene. Nucleotides are numbered starting at the initiation site and receive positive values to the right and negative values to the left. Thus, in a specific gene the binding site for RNA polymerase II might include nucleotides −80 to −5, and the first intron might contain nucleotides +154 to +688. See Chronology, 1967, Taylor et al.; coding strand, Miller trees, polyprotein, RNA polymerase, strand terminologies.

Subjects: Genetics and Genomics.

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