the prokaryotic nucleoid and plasmids

A bacterium has no nucleus, no neat 46-chromosome portrait, and almost no introns. Its genome is organized in a strikingly different, leaner style. Picture a single rubber band of DNA coiled tightly in the open cytoplasm, with a few small extra loops floating alongside that can be swapped between cells.

Most bacteria and archaea carry their main genome as a single, usually circular chromosome packed into a region called the nucleoid — not membrane-bound like a true nucleus, but a dense, organized clump kept compact by supercoiling and by architectural proteins (analogous in role to histones, though largely different molecules). The genome is compact and gene-dense: little non-coding DNA, few or no introns, and genes that share a job are often strung together under one control region as an operon, transcribed into a single messenger RNA. Alongside the chromosome, many bacteria carry plasmids — small, independent circles of DNA that replicate on their own and carry useful extras, such as antibiotic-resistance genes, rather than essentials.

This organization matters in two big ways. First, plasmids can be passed from one bacterium to another (horizontal gene transfer), which is how antibiotic resistance spreads so fast and why this is a serious medical problem. Second, plasmids are the workhorse tool of molecular biology and biotechnology: their easy uptake and replication make them the standard vehicle for carrying engineered genes into cells. The contrast with the sprawling, chromatin-packaged, intron-rich eukaryotic genome is one of the deepest divides in the living world.