Final answer:
Histone modification involves the covalent bonding of functional groups to histone proteins, affecting DNA packaging and gene expression. Histone variants are alternative forms of standard histones that also impact chromatin structure and gene regulation.
Step-by-step explanation:
Histone modification refers to the covalent bonding of functional groups, such as phosphate, methyl, or acetyl groups, onto the N-terminal tails of histone proteins. The addition of these groups does not change the DNA base sequence itself, but rather the way DNA is packaged. Particularly, acetyl groups reduce the positive charge on histones, leading to a less tight association with the negatively charged DNA, which can make the DNA more accessible and active in gene expression. Histone variants are alternative versions of the standard histones that can be incorporated into the nucleosome, also affecting chromatin structure and gene expression.
These histone modifications play an important role in chromatin remodeling and, therefore, in regulating gene transcription. Processes such as histone acetylation, methylation, and phosphorylation are crucial for this regulatory mechanism. Enzymes like histone acetyltransferases (HAT enzymes), de-acetylases, and methyl transferases catalyze these reversible chemical modifications, affecting the recruitment of other proteins that alter chromatin conformation.