Final answer:
Basal transcription factors help to form a preinitiation complex, which recruits RNA polymerase II for transcription initiation. Other transcription factors bind to enhancers and silencers to regulate the frequency of pre-mRNA synthesis. Enhancers and silencers affect transcription efficiency but are not essential for transcription to occur.
Step-by-step explanation:
Basal transcription factors are crucial in the formation of a preinitiation complex on the DNA template that subsequently recruits RNA polymerase II for transcription initiation. The complexity of eukaryotic transcription does not end with the polymerases and promoters.
An army of other transcription factors, which bind to upstream enhancers and silencers, also help to regulate the frequency with which pre-mRNA is synthesized from a gene. Enhancers and silencers affect the efficiency of transcription but are not necessary for transcription to proceed.