nucleosomenucleosome

A nucleosome is a fundamental unit of DNA packaging in eukaryotes, consisting of a segment of DNA wound around a core of histone proteins. This structure plays a crucial role in the organization and regulation of genetic material within the cell nucleus. The core of a nucleosome is made up of an octamer of histone proteins, comprising two copies each of the histones H2A, H2B, H3, and H4. Approximately 146 base pairs of DNA are wrapped around this histone core in about 1.65 left-handed superhelical turns. The DNA enters and exits the nucleosome at specific points, creating a structure that resembles beads on a string when viewed under an electron microscope.

Nucleosomes are not static structures; they undergo dynamic modifications that influence gene expression and DNA replication. Histone proteins can be chemically modified through processes such as acetylation, methylation, and phosphorylation, which alter the interaction between histones and DNA. These modifications can either tighten or loosen the DNA's association with the histones, thereby regulating access to the genetic information. For instance, acetylation of histones generally leads to a more open chromatin structure, facilitating transcription, while deacetylation tends to compact the chromatin, repressing gene expression.

The organization of nucleosomes is essential for the compaction of DNA within the nucleus. In higher-order chromatin structures, nucleosomes are further folded and packaged into more compact forms, allowing the long strands of DNA to fit within the confined space of the cell nucleus. This hierarchical packaging is vital for maintaining the integrity and functionality of the genome. Additionally, nucleosomes play a role in various cellular processes, including DNA repair, replication, and the regulation of gene expression, making them a cornerstone of genetic regulation and cellular function.