nukleoidet

The nucleoid is the irregular, membrane-free region within prokaryotic cells where the genome is located. In bacteria and many archaea, it contains the chromosome or chromosomes together with a set of DNA-binding proteins that compact and organize the DNA. The term reflects the distinction from a true nucleus found in eukaryotes.

The core is the DNA molecule, typically a circular chromosome, folded and packaged with nucleoid-associated proteins (NAPs) such as HU, IHF, Fis, H-NS, and others that vary by species. These proteins help bend, bridge, and stabilize DNA and influence gene expression. Enzymes that regulate DNA topology, notably DNA gyrase and topoisomerase I, introduce and relieve supercoiling, contributing to the dynamic compaction and accessibility of the genome.

Organization within the nucleoid is not uniform. The chromosome is condensed into looped domains and, in some bacteria like Escherichia coli, into macrodomains with distinct organization and regulatory properties. Condensation is aided by macromolecular crowding and smaller DNA-binding factors, allowing the nucleoid to occupy a large portion of the cell while remaining accessible for replication, transcription, and repair.

Functions of the nucleoid include compacting the genome to fit within the cell, organizing replication and transcription, and enabling rapid responses to environmental changes. The structure is dynamic, changing with growth phase, stress, and cell-cycle progression, which in turn can affect gene expression patterns. While similar in purpose to the eukaryotic nucleus, the nucleoid differs in being non-membrane-bound and highly dependent on DNA-protein interactions for its architecture.