Transcription in E. coli. Coloured transmission electron micrograph of deoxyribonucleic acid (DNA blue horizontal line) transcription in the bacterium Escherichia coli. During transcription, complementary messenger ribonucleic acid (mRNA) strands (blue vertical lines) are synthesised. The enzyme RNA polymerase recognises a start sign on the DNA strand and moves along the strand building the mRNA. mRNA is the intermediary between DNA and its protein product. Bacteria are prokaryotic organisms, they have a simple cell structure with no organelles or nucleus, so transcription occurs in the cytoplasm (orange).