Term from genetics. The term initiation refers to the first step in the process of transcription, in which the RNA polymerase binds to the DNA and begins the synthesis of an RNA copy of a specific gene. This phase is crucial as it sets the starting point for RNA synthesis.
Initiation
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Initiation begins when the RNA polymerase binds to a specific DNA sequence, the promoter. The promoter is usually upstream of the gene to be transcribed and contains specific recognition sequences that are recognized by the RNA polymerase and other transcription factors. In eukaryotes, additional transcription factors are required to recruit the RNA polymerase to the promoter.
Formation of the transcription complex: In eukaryotes, the general transcription factors bind to the promoter first. These factors help to bring the RNA polymerase II to the correct position. The transcription factor TFIID binds to the TATA box (a common sequence in the promoter) and recruits other factors (such as TFIIB) that bring RNA polymerase II to the promoter.
Opening of the DNA double helix: After the RNA polymerase binds to the promoter, the DNA double strand opens near the transcription start site. This creates a transcription bubble in which the DNA strands are separated and the template strand becomes accessible.
Start of RNA synthesis: The RNA polymerase begins to incorporate RNA nucleotides using the complementary sequence of the template strand. Initially, some RNA nucleotides can be synthesized without a specific template (a kind of "check") before the polymerase enters the elongation phase (see elongation below) and continues the synthesis.