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Histones are highly alkaline proteins found in eukaryotic cell nuclei that package and order DNA into structural units called nucleosomes. They play a crucial role in gene regulation.

There are several key chemical and structural properties of histones that enable them to successfully package eukaryotic DNA: a) Histones have a high proportion of positively charged amino acids such as lysine and arginine, which give them a positive charge. This allows them to bind tightly to the negatively charged DNA phosphate backbone. b) Histones are organized into octameric protein complexes, composed of two copies of each of the four core histone proteins: H2A, H2B, H3, and H4. This octamer forms a histone core around which DNA wraps in a superhelical structure. c) The N-terminal tails of histone proteins extend out from the nucleosome core and can be modified by various post-translational modifications (PTMs) such as acetylation, methylation, and phosphorylation. These PTMs influence the affinity of histones for DNA and other proteins, playing a critical role in gene regulation.

Chromatin remodeling is a process through which the structure of chromatin is altered by protein complexes. This alteration can lead to changes in DNA accessibility, allowing or preventing the binding of proteins (such as transcription factors) to DNA, which in turn controls gene expression.

Chromatin remodeling is controlled by several mechanisms within eukaryotic cells: a) Chromatin remodeling complexes: These are large protein complexes that use energy from ATP hydrolysis to alter the interactions between histones and DNA. They can slide, eject, or restructure nucleosomes, changing the chromatin structure and DNA accessibility. b) Post-translational modifications of histones: As mentioned earlier, histones can be modified by various PTMs, such as acetylation, methylation, and phosphorylation. These modifications affect histone-DNA interactions and the recruitment of remodeling complexes. c) Non-coding RNAs: Certain non-coding RNAs, like small interfering RNAs (siRNAs) and long non-coding RNAs (lncRNAs), can recruit chromatin remodeling complexes to specific genomic regions and influence gene expression. d) DNA methylation: Methylation of cytosine residues in DNA can also affect chromatin structure and accessibility for protein binding. This process can recruit or repel chromatin remodeling complexes, influencing gene expression.