Final answer:
Dense packing of DNA in chromosomes prevents gene expression by inhibiting transcription factors due to the compaction caused by DNA methylation and histone hypo-acetylation, which results in tightly packed nucleosomes.
Step-by-step explanation:
In general, dense packing of DNA in chromosomes prevents gene expression because it inhibits transcription factors from accessing the DNA. This compaction usually occurs due to methylation of DNA and hypo-acetylation of histones, leading to tightly packed nucleosomes which in turn repress transcription. The dense packing of DNA makes it difficult for transcription factors to bind to the DNA, thereby preventing the initiation needed for transcription and subsequent gene expression.
Furthermore, epigenetic changes like histone acetylation typically result in a looser packing of nucleosomes, making the DNA more accessible for transcription factors to bind, allowing gene expression to occur. Conversely, histone deacetylation, along with methylation, leads to a tighter packing of nucleosomes, thereby reducing gene expression.