Final answer:
Substitution leading to a stop signal that prevents mRNA formation is describing transcription, a process where DNA is copied into mRNA. The stop codon ends translation by signaling the release of the polypeptide chain, and in bacterial transcription, the σ subunit of RNA polymerase is involved in initiation.
Step-by-step explanation:
The process being described, where substitution leads to a stop signal so that no mRNA is formed, is transcription. During transcription, a particular segment of DNA is copied into RNA (mRNA) by the enzyme RNA polymerase. If an incorrect substitution occurs, such as the insertion of a stop codon, it can cause the premature termination of the transcription process, preventing the formation of a complete mRNA strand. The stop codon also plays a critical role in translation, as it is necessary for ending translation, which allows the polypeptide strand to be released.
In transcription of bacteria, the σ subunit of the RNA polymerase is involved in the initiation stage. Upon termination in prokaryotes, transcription is complete, and the transcript can be immediately used for protein synthesis, unlike in eukaryotic cells where transcription and translation are separated by the nuclear membrane.