Nukleosomer

Nukleosomer (nucleosomes) are the fundamental units of chromatin, the complex that packages eukaryotic DNA in the nucleus. A nucleosome consists of a histone protein core around which about 147 base pairs of DNA are wrapped in roughly two turns. The core particle is an octamer built from two copies each of the histones H2A, H2B, H3 and H4. A linker histone, H1, binds the DNA between nucleosomes and helps compact chromatin into higher-order fibers, producing the chromatosome with a total DNA length per unit of about 180–200 base pairs.

Nucleosomes organize DNA into a compact, accessible structure. Their positioning along the genome influences which regions are accessible to transcription factors, polymerases and other enzymes. In regions where chromatin is open, genes are more likely to be expressed; in more condensed regions, expression is reduced. The tails of histones can be chemically modified, for example by acetylation, methylation or phosphorylation. Such post-translational marks act as epigenetic signals that regulate chromatin structure and recruitment of chromatin-modifying enzymes, influencing gene activity without changing the DNA sequence.

Nucleosomes are dynamic. They are assembled and disassembled during DNA replication and repair, and their composition is reshaped by chromatin remodeling complexes (such as SWI/SNF, ISWI, CHD, INO80) and histone chaperones (CAF-1, HIRA). These factors slide or eject nucleosomes, replace histones, and help restore chromatin after disruptions, thereby controlling access to the genome.