Bacterial Chromosome and Plasmids

A bacterium typically carries most of its genes on a single, usually circular chromosome that is compacted into a dense region called the nucleoid, and it may also carry plasmids, smaller DNA molecules that replicate independently. The chromosome holds the core genes a cell needs to live, while plasmids often carry accessory genes, including those for antibiotic resistance and virulence, that can move between cells.

Definition

The bacterial chromosome is the cell's principal genome, usually a single circular DNA molecule condensed into the nucleoid; a plasmid is a smaller, autonomously replicating DNA molecule that carries accessory genes and is distinct from the chromosome.

Scope

This topic covers how the bacterial genome is organized and packaged, the distinction between chromosomal and extrachromosomal (plasmid) DNA, plasmid replication and maintenance, and the role of plasmids as mobile, accessory genetic elements. It is a reference overview of structure and function, not clinical guidance.

Core questions

• How is a bacterial chromosome organized and compacted into the nucleoid?
• What distinguishes chromosomal DNA from plasmid DNA?
• How do plasmids replicate and remain stably maintained in a dividing population?
• What kinds of accessory functions do plasmids commonly carry?

Key concepts

• Circular bacterial chromosome
• Nucleoid and chromosome compaction
• Supercoiling and nucleoid-associated proteins
• Plasmid and replicon
• Origin of replication
• Plasmid copy number and incompatibility
• Toxin-antitoxin maintenance systems
• Accessory (resistance and virulence) genes

Mechanisms

Most bacterial genomes are organized as a single circular chromosome that is several orders of magnitude longer than the cell, so it is folded into supercoiled loops and organized by nucleoid-associated proteins and condensin-like machinery into the nucleoid, as reviewed by Wang and colleagues. Plasmids are separate replicons: each carries its own origin and replicates independently of the chromosome, with copy number and incompatibility groups determined by the replication control system. Stable inheritance of low-copy plasmids is reinforced by partitioning systems and by post-segregational killing through toxin-antitoxin loci, which Gerdes and colleagues describe as a mechanism that selectively kills daughter cells that lose the plasmid. Plasmids and related mobile elements act as accessory genomes that can spread genes such as antibiotic-resistance determinants across populations.

Clinical relevance

Plasmids are a principal vehicle for the spread of antimicrobial resistance and certain virulence genes among bacteria, which is why their biology is central to understanding resistance epidemiology. This entry explains chromosome and plasmid biology at a mechanistic level and is not a basis for diagnostic or treatment decisions.

History

The term plasmid was introduced by Joshua Lederberg in 1952 to denote extrachromosomal hereditary determinants, and subsequent work established plasmids as autonomous replicons. The recognition that plasmids carry transferable resistance genes, and later genome-scale and imaging studies of nucleoid organization reviewed by Wang and colleagues, refined the modern picture of a structured chromosome accompanied by a mobile accessory genome.

Key figures

• Joshua Lederberg
• Stanley Cohen
• Kenn Gerdes

Seminal works

• wang-2013
• smillie-2010
• frost-2005

Frequently asked questions

What is the difference between a chromosome and a plasmid?

The chromosome carries the core genes essential for life and is usually a single large circular molecule, whereas a plasmid is a smaller, independently replicating DNA molecule that carries accessory, often transferable, genes.

Why are plasmids important for antibiotic resistance?

Many resistance genes are located on plasmids that can be copied and transferred between bacteria, allowing resistance to spread rapidly through and between bacterial populations.