Final answer:
Histone modifications and insertion of variants regulate gene expression by altering the chromosomal structure. When they result in acetylation, nucleosomes are loosely packed, making DNA accessible for gene activity. These reversible epigenetic changes enable cells to control which genes are activated and when.
Step-by-step explanation:
Histone modifications, such as the addition of chemical groups like phosphate, methyl, or acetyl groups, and the insertion of histone variants produce a chromosomal region that can be either open or closed. When modifications such as histone acetylation occur, they result in a less positive charge on the histones and therefore looser packing of nucleosomes, making DNA accessible and allowing gene expression because transcription factors can bind more easily to the DNA. Conversely, different modifications can cause the chromatin structure to be closed, preventing transcription factors from accessing the DNA, thus keeping the gene inactive.
These epigenetic changes, although reversible, can persist through multiple rounds of cell division and even cross generational lines. The alterations in chromatin remodeling are critical for gene regulation, allowing cells to respond to environmental signals and developmental cues by making specific genes available for transcription as needed.