chromatin remodeling

Picture a long necklace of beads, where each bead is DNA wrapped around histone proteins. Chromatin remodeling is the act of sliding, ejecting, or rearranging those beads so a stretch of DNA becomes exposed or hidden. Where the beads sit decides whether the cell's machinery can reach a gene.

Mechanically, this is done by molecular machines called chromatin-remodeling complexes that use the energy of ATP to reposition nucleosomes, the bead-like units of DNA and histones. By opening up a promoter or clearing a path, remodeling lets transcription factors and polymerases bind; by packing nucleosomes tightly, it can shut a region down.

Remodeling cooperates with histone modifications and DNA methylation. Marks on histones can recruit remodeling complexes, and the resulting open or closed state can in turn make further marks more or less likely, forming self-reinforcing patterns.

Because access to DNA is the gatekeeper of nearly every genomic process, remodeling matters not only for transcription but also for DNA replication and repair. Faulty remodeling machinery is linked to developmental disorders and cancer.