Transcription in E. coli. Coloured transmission electron micrograph of deoxyribonucleic acid, (DNA pink), transcription coupled with translation in the bacterium Escherichia coli. During transcription, complementary messenger ribonucleic acid (mRNA) strands (green) are synthesised and immediately translated by ribosomes (blue). 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 and translation occur in the cytoplasm (black).