In most prokaryotes and some eukaryotes, gene transcription occurs within a region of the DNA double helix that is partially unwound by the action of the RNA polymerase. This region is called a transcription bubble, and it always corresponds to one of the two DNA strands that act as a template for the growing RNA molecule.
During transcription, the RNA polymerase adds nucleotides one by one to the growing RNA strand. A covalent phosphodiester bond is formed between each new nucleotide and the 3'-OH group of the ribonucleotide that precedes it in the RNA strand.
The resulting single-stranded RNA molecule, referred to as an RNA transcript, acts as a mobile molecular copy of the original DNA sequence. The RNA transcript is transported from the nucleus to the cytoplasm, where it is translated into the proteins it encodes.
The bacterial RNA infrastructure consists of ribosomes and RNA polymerase complexes, which are widely dispersed in the cytoplasm. Other RNA synthesis and processing machinery is concentrated in cytoskeletal structures and associated with membrane-bound organelles (Fig. 1).