Final answer:
The accessibility of genes to RNA polymerase II and the transcription machinery is modulated by transcription factors, chemical modifications, and nucleosome positioning.
Step-by-step explanation:
The accessibility of genes to RNA polymerase II and the transcription machinery is modulated by several mechanisms:
- Transcription factors: These proteins alter the specificity of RNA polymerase for a promoter or set of promoters, making it more or less likely to bind to the promoter and begin transcription.
- Chemical modifications: DNA and histone proteins can undergo chemical modifications that signal the cell to open or close a chromosomal region, controlling the accessibility of RNA polymerase and its transcription factors.
- Nucleosome positioning: The movement of nucleosomes on DNA determines whether a specific chromosomal region is accessible or silenced, influencing the binding of RNA polymerase and other transcription factors.