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A transcription unit is the segment of DNA between the sites of initiation and termination of transcription by RNA polymerase.

Any element to the left of the initiation site is said to be "5' to " or "upstream from" the gene. Any element to the right is "3' to " or "downstream from" the genes.

RNA is transcribed in a 5' to 3' direction from the 3'->5' DNA "template" strand of a gene.