Transcription unit. Definition. noun. A stretch of DNA being transcribed into an RNA molecule. Supplement. Transcription units are mapped by kinetic studies of rRNA synthesis. It can also be visualized by electron microscopy.ranscription unit. Definition. noun. A stretch of DNA being transcribed into an RNA molecule. Supplement. Transcription units are mapped by kinetic studies of rRNA synthesis. It can also be visualized by electron microscopy.