Final answer:
Chemical modifications on histone tails alter nucleosome spacing and gene expression, influencing whether DNA is accessible for transcription and ultimately regulating cellular response to environmental stimuli.
Step-by-step explanation:
Chemical modifications on the exposed tails of histone proteins influence the structure and function of chromatin, particularly gene expression and nucleosome spacing. These modifications can include the addition of methyl, acetyl, or phosphate groups, which affect how tightly DNA is wound around histones. For instance, acetylation generally makes the histones less positively charged, loosening their grip on the negatively charged DNA and making it more accessible for transcription. Conversely, other modifications like methylation can tighten this interaction and reduce gene expression.
This epigenetic regulation does not change the underlying DNA sequence, but it does affect the physical accessibility of DNA to the transcription machinery, essentially turning genes "on" or "off." The reversibility of these modifications allows cells to respond to environmental stimuli in a dynamic way by altering gene expression patterns. These changes can affect everything from individual cell function to the development of an organism and can in some cases be inherited.