Nucleosome

A nucleosome is a fundamental structural unit of chromatin, the substance that makes up eukaryotic chromatin, and is a crucial component of eukaryotic DNA. It is formed when DNA is wrapped around a core of histone proteins, creating a bead-like structure that serves as a scaffold for the packing of chromatin into higher-order structures.

Nucleosomes consist of a segment of DNA that is coiled around an octamer of histone proteins. The eight histones are arranged in two copies each of the histone proteins H2A, H2B, H3, and H4. The histones are connected by specific protein-DNA interaction sites, forming a tight cylinder that the DNA is wrapped around.

The arrangement of DNA and histone proteins in a nucleosome creates a steric hindrance, which prevents all of the DNA sequences on the same strand from being densely packed at the same location on the histone surface. This steric hindrance has a significant effect on the ability of nucleosomes to be acetylated or deacetylated and also influences the migration of chromatin-bound proteins.

Nucleosomes are major points of chromatin modification and remodeling. Such mutations are extensively studied in biology and ecology for their potential in cancer treatments and epigenetic disorders research. By cutting the DNA chain at a nucleosome, nucleases, a class of endonucleases, provide space for the picking of genetic materials during the gene transcription process.

Studies of the dynamics of nucleosome repositioning show that the transcription factor movement has specific preferred nucleosome positions. However, the interaction between chromatin and transcription factors is highly diverse and difficult to predict for many systems.