The Nucleosome

A nucleosome is the basic repeating unit of chromatin, consisting of DNA wrapped around a core of histone proteins. It serves as the fundamental building block for chromatin packaging, allowing approximately 2 meters of DNA to fit into the nucleus of a eukaryotic cell. Nucleosomes enable efficient DNA compaction while maintaining accessibility for gene regulation and replication.

A nucleosome core contains eight histone proteins: two copies each of histones H2A, H2B, H3, and H4. These histones form an octamer around which approximately 147 base pairs of DNA wrap 1.65 times. This protein-DNA complex creates a stable, compact structure that is essential for higher-order chromatin organization.

Histone modification, including acetylation and methylation, alters the chemical properties of histone proteins within nucleosomes, affecting DNA-histone interactions. These modifications regulate chromatin structure and gene accessibility without changing DNA sequence. Modified histones influence whether chromatin remains tightly packed or loosely organized, directly controlling gene expression.

Nucleosome positioning determines whether DNA regulatory regions are accessible to transcription machinery. Nucleosomes positioned over promoters or enhancers block transcription factor binding, silencing genes. Conversely, nucleosome-depleted regions allow transcription factors to access DNA, enabling gene activation. This positioning is a key epigenetic mechanism for controlling gene expression.

The linker histone, typically histone H1, binds to DNA between nucleosomes and stabilizes higher-order chromatin structure. Unlike the eight core histones that form the nucleosome octamer, H1 is a single protein that facilitates the formation of the 30-nanometer chromatin fiber. This distinction makes H1 critical for condensing nucleosomes into more compact chromatin states.

Nucleosomes are the foundational units that assemble into higher-order chromatin structures, ultimately forming chromosomes. Repeated nucleosome arrays wrap around linker histones to create 30-nanometer fibers, which further coil into loops and domains. This hierarchical organization of nucleosomes enables the packaging of an entire genome into discrete chromosome structures.

During DNA replication, nucleosomes are temporarily disrupted as the replication machinery unwinds the DNA double helix. After replication, new histone proteins are incorporated into newly synthesized DNA strands, and nucleosomes are reassembled on both daughter DNA molecules. This process ensures that epigenetic information encoded in histone modifications is maintained across cell divisions.