Final answer:
The three methods of histone modification at the N-terminal ends of core histones are acetylation, methylation, and phosphorylation. These modifications alter chromatin structure, influencing gene expression by making chromosomal regions more or less accessible for transcription.
Step-by-step explanation:
The question concerns the three methods by which core histones may be modified at their N-terminal ends. These histone modifications are critical for regulating gene expression by affecting chromatin structure and dynamics. The core histone proteins, including Histone H2A, Histone H2B.1, and others, have their N-terminal 'tails' protruding from the nucleosome, and these tails are subject to various reversible chemical modifications. The three main methods are:
- Acetylation: Addition of acetyl groups, usually by enzymatic activity of histone acetyltransferases (HAT), neutralizes the positive charge of lysine residues. This loosens the tight binding of DNA to histones, thereby making the chromosomal region more accessible for transcription.
- Methylation: The addition of methyl groups, catalyzed by histone methyl transferases, to lysine or arginine residues can either activate or repress transcription, depending on the specific sites of methylation.
- Phosphorylation: The addition of phosphate groups to serine or threonine residues by histone kinases can influence processes such as chromosome condensation and segregation during mitosis and transcriptional regulation.
This action is part of chromatin remodeling which is essential for the regulation of gene transcription in non-dividing and interphase cells. These histone modifications, such as histone methylation and histone phosphorylation, often work in concert with DNA modifications to control the accessibility of genetic information and are thus vital for proper cellular function, including the regulation of housekeeping genes.