Final answer:
Histone modification, particularly methylation, results in tight packing of nucleosomes which inhibits transcription factors from accessing the DNA and thus represses gene expression.
Step-by-step explanation:
The tight association of histone proteins with DNA is a key factor in the regulation of gene expression. Histone modifications, such as the addition of methyl or acetyl groups, can alter the tightness of DNA winding around histones. When methylation occurs on histones or DNA, it causes the nucleosomes to pack tightly together, preventing transcription factors from binding to the DNA, thereby repressing gene expression. Conversely, acetylation of histones reduces the positive charge on these proteins, leading to a looser association with DNA and enhanced access for transcription factors, thus promoting gene expression.