Final answer:
Chemical modifications of histone proteins and DNA can direct local alterations in chromatin structures, opening or closing chromosomal regions and influencing gene transcription.
Step-by-step explanation:
Local alterations in chromatin structures can be directed by chemical modifications of histone proteins and DNA. These modifications include acetylation, methylation, and phosphorylation. For example, acetylation of histones H2B and H4 tends to unwind nucleosomes and open the underlying DNA for transcription, while methylation of histones H3 and H4 can also open DNA for transcription. Additionally, DNA can be chemically modified through methylation of certain cytosine nucleotides, which can influence gene expression. These modifications of histones and DNA determine whether a chromosomal region should be open or closed, affecting gene transcription.