Final answer:
The preinitiation complex is a key component in eukaryotic transcription initiation, composed of RNA polymerase II and general transcription factors, leading to the unwinding of DNA and beginning of RNA synthesis.
Step-by-step explanation:
The preinitiation complex is a crucial assembly that consists of RNA polymerase II and five general transcription factors. This complex is responsible for the initiation of transcription in eukaryotic cells. It begins with the assembly of the basal transcription factors, known as TFIIs (Transcription Factor for RNA polymerase II), which include TFIID with its TATA-binding protein (TBP) component.
These factors bind to the promoter region of a gene, stabilizing each other and facilitating the recruitment of RNA polymerase II. Once RNA polymerase II is bound, the DNA double helix unwinds to form a transcription bubble where RNA synthesis can begin. After binding, RNA polymerase II also undergoes phosphorylation, causing some of the transcription factors to dissociate, leaving a complex that can initiate mRNA synthesis.