Final answer:
Heterochromatin represses transcription by preventing transcriptional activators from accessing their enhancer regions, which in turn makes RNA polymerase II initiation more difficult. Option b.
Step-by-step explanation:
The role of heterochromatin in repressing transcription is mainly due to its interference with the accessibility of transcription factors to the DNA.
Heterochromatin is a tightly packed form of DNA, which creates a barrier to transcriptional machinery, therebyreducing the likelihood of transcription initiation. As a result, all factors involved in transcription initiation can be affected, including RNA polymerase II, general transcription factors, transcriptional activators, and coactivators.
Among these, the most direct interference by heterochromatin is with transcriptional activators, which cannot bind to their enhancer regions if the DNA is highly condensed.
This lack of activator binding impedes the attraction and stabilization of the transcription complex, thereby making it difficult for RNA polymerase II to initiate transcription. Option b.