nukleoids

Nukleoids are the dense, irregular regions inside prokaryotic cells where most of the genetic material resides. Unlike eukaryotic nuclei, nucleoids are not bounded by a membrane; they are a dynamic DNA-protein complex that organizes the bacterial chromosome. In many bacteria the chromosome is a single circular DNA molecule that is highly supercoiled and compacted by nucleoid-associated proteins, topoisomerases, and structural maintenance proteins.

Nucleoid organization is heterogeneous and changes with growth, metabolism, and stress. DNA is arranged in loops and domains, with macrodomains and local interactions stabilized by proteins such as HU, IHF, FIS, H-NS, and Dps. Archaeal and some bacterial systems use histone-like proteins to shape the DNA as well. Structural maintenance of chromosomes (SMC) complexes, such as MukBEF in many Gram-negative bacteria or SMC-ScpAB in others, help condense and segregate the chromosome during replication.

Functionally, the nucleoid influences genome accessibility, replication timing, transcription, and chromosome segregation. Its architecture affects gene expression, while changes in supercoiling and protein composition respond to environmental conditions.

During the cell cycle, nucleoids condense to facilitate replication and then decondense to allow transcription and segregation into daughter cells. The study of nucleoids integrates molecular biology, genetics, and biophysics to understand how prokaryotic genomes are organized without a surrounding nucleus.

In summary, nucleoids describe the membrane-less DNA-protein networks that harbor bacterial genomes and coordinate their maintenance and expression.