Final answer:
The association of histone proteins with DNA involves the binding of positively charged histones to negatively charged DNA to form nucleosomes. Chemical modifications to histones and DNA can alter chromatin structure and gene expression, influencing whether chromosomal regions are transcriptionally active or repressed.
Step-by-step explanation:
The association of histone proteins and DNA is crucial for the structure and regulation of chromatin in eukaryotic cells. Histones are positively charged proteins that bind to the negatively charged DNA to form nucleosomes, which is referred to as the "beads-on-a-string" structure. The way histone proteins interact with DNA can be altered by the addition or removal of chemical tags such as phosphate, methyl, or acetyl groups to the histones or DNA itself. These modifications do not change the DNA base sequence but affect the chromatin structure by changing how tightly DNA is wound around histones, thereby influencing gene expression and nucleosome spacing. During transcription, these tags can lead to a more open or closed chromosomal region, depending on whether they promote or inhibit the binding of DNA to histones.
Furthermore, the association of histones with DNA plays a role in the compaction of DNA into higher-order structures. The basic histone octamers around which DNA is wrapped eventually fold over to form a 30 nm fiber known as the solenoid, and then into even more condensed chromosomal structures. This is important for processes such as cell division, where chromosomes must be highly compacted.