Final answer:
Histone modifications, such as acetylation, methylation, and phosphorylation, occur on histone tails and regulate gene expression by altering chromatin structure. These modifications change the histone-DNA interactions without altering the DNA sequence. DNA methylation also plays a role in regulating gene expression by influencing chromatin structure.
Step-by-step explanation:
The core histones undergo several types of chemical modifications, affecting the structure of chromatin and consequently gene expression. Common modifications include histone acetylation, histone methylation, and histone phosphorylation. These modifications typically occur on the amino acid residues of the N-terminal 'tails' of the histone proteins and are catalyzed by specific enzymes such as histone acetyltransferases, histone methyltransferases, and histone kinases. For instance, acetylation of lysine residues by histone acetyltransferases can relax DNA binding to histones, making DNA accessible and genes active. In contrast, deacetylation leads to tighter DNA-histone binding and reduced gene expression. Methylation can either activate or repress transcription depending on which residues are modified, and phosphorylation affects chromatin condensation during certain cellular processes.
These modifications do not change the DNA sequence but alter the physical structure of the chromatin, influencing the binding of DNA to histones. Chemical groups such as acetyl, methyl, or phosphate groups are attached to histones, changing their charge and influencing how tightly DNA is bound. This can either open up chromatin for transcription or close it, thus silencing gene expression. Additionally, DNA itself can be modified through methylation of cytosines in CpG islands, which affects how DNA interacts with histone proteins and other transcriptional machinery, further regulating gene expression.