Final answer:
The elongation stage of transcription in bacteria begins when the sigma subunit dissociates from RNA polymerase, allowing the mRNA synthesis process to proceed.
Step-by-step explanation:
The transition to the elongation stage of transcription in bacteria is marked by the release of the sigma (σ) subunit from the RNA polymerase. Once the σ subunit is released, the core enzyme of the RNA polymerase can then proceed along the DNA template, synthesizing mRNA in the 5' to 3' direction at a rate of about 40 nucleotides per second. This stage is characterized by the continuous unwinding of the DNA ahead of the core enzyme and rewinding behind it.