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RNA polymerase (RNAP) carries out DNA-dependent RNA synthesis in both bacteria and eukaryotes. Bacteria do not have a membrane-bound nucleus. So, transcription and translation occur simultaneously, on the same DNA template.

Transcription can be divided into three main stages, each involving distinct DNA sequences to guide the polymerase. These are:

  1. Initiation, which involves two specific sequences 10 and 35 base pairs upstream of the gene, which are called promoters.
  2. Elongation, where the polymerase proceeds along the DNA template, synthesizing mRNA in the 5'to 3'direction.
  3. Termination, in which the polymerase encounters a region rich in C-G nucleotides and stops mRNA synthesis.

Bacterial RNAP carries out all three steps in conjunction with other accessory proteins. Transcription starts at a DNA base pair that is conventionally numbered +1. Base pairs along the direction of transcription are termed to be downstream of the start site and are denoted with positive numbers. Basepairs in the opposite direction are said to be upstream and are denoted with negative numbers. Transcription begins when the RNAP binds the promoter sequences forming a closed complex. At this stage, the template DNA strands are still base-paired. To transition to the elongation stage, the RNAP breaks the hydrogen bonds between the base pairs and binds tightly to the single-stranded DNA at the start site. This is called an open complex, which can now carry out mRNA synthesis.

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