Final answer:
Histone tails can be modified by acetylation, methylation, phosphorylation, and ubiquitination, influenced by enzymes like HATs and methyltransferases. These modifications affect chromatin structure and gene expression patterns. Histone acetylation generally activates transcription, whereas methylation can both activate and repress genes.
Step-by-step explanation:
Histone tails can be modified through several types of chemical modifications: acetylation, methylation, phosphorylation, and ubiquitination. These modifications are carried out by a set of enzyme families such as histone acetyltransferases (HATs), histone methyltransferases, kinases, and ubiquitin enzymes.
Acetylation of histone tails by HATs generally leads to a more open chromatin structure and active gene expression since the addition of acetyl groups reduces the positive charge of histones, loosening their interaction with the negatively charged DNA. In contrast, histone deacetylases (HDACs) remove acetyl groups, leading to a closed chromatin structure and reduced gene expression. Methylation can activate or repress transcription depending on the site of methylation. For instance, trimethylation of H3 lysine 4 is associated with active gene expression while methylation of H3 lysine 27 is linked with gene repression.
These modifications play a critical role in chromatin remodeling, which is essential for the regulation of gene expression during processes such as development, differentiation, and response to environmental signals. Alterations in histone modifications can lead to changes in gene expression patterns, which can have profound effects on cell function and can sometimes contribute to diseases like cancer.