Bacterial genetics

Bacterial heredity is encoded chiefly in a chromosome, supplemented in many species by independently replicating plasmids. The chromosome is usually a circular DNA molecule located in the nucleoid rather than a membrane-bound nucleus, although exceptions and unusual architectures occur. Genes are commonly arranged in operons, allowing several functionally related proteins to be transcribed from one regulatory region.¹ Transcription produces messenger RNA, and translation occurs on bacterial ribosomes, often while the RNA is still being synthesized. Regulatory proteins, small RNAs, DNA supercoiling, and environmental signals coordinate expression. In Escherichia coli, the lac operon remains a classic example of inducible control, while the trp operon illustrates repression by a biosynthetic end product.

Bacterial variation arises through mutation, recombination, and acquisition of DNA from other cells. Mutations include base substitutions, insertions, deletions, and larger rearrangements; they may be neutral, harmful, or advantageous depending on the environment.² DNA can spread horizontally by transformation, in which a cell takes up free DNA; by transduction, in which a bacteriophage carries bacterial DNA; or by conjugation, which commonly uses cell-to-cell contact and a plasmid-encoded transfer apparatus. These routes can move genes between distantly related bacteria and need not follow reproduction. Homologous recombination may then integrate incoming DNA into the chromosome, while mobile elements such as transposons can relocate genes within or between DNA molecules.³

Horizontal gene transfer makes bacterial evolution partly network-like rather than strictly tree-like. Genes for antibiotic resistance, toxin production, metabolism, and host interaction may spread rapidly when plasmids, phages, or other mobile elements carry them. This process complicates clinical treatment because resistance can accumulate in a single strain and move through microbial communities. Bacterial genetics also underpins recombinant DNA technology: researchers use bacterial hosts to clone genes, produce proteins such as human insulin, and test regulatory systems. Laboratory methods include selectable markers, reporter genes, targeted recombination, transposon libraries, and CRISPR-Cas editing. Genome sequencing and comparative genomics now reveal both stable core genes shared by a species and a variable accessory genome that reflects ecological specialization and past gene exchange.¹

Bacterial genetics includes several less obvious forms of inheritance and regulation. Some bacteria maintain multiple chromosomes, linear chromosomes, or large secondary replicons rather than one standard circular chromosome. Plasmids may impose a metabolic cost, yet persist through addiction systems, partition mechanisms, or selection for the traits they carry. A bacterium can also enter a transient competent state for transformation, and some phages follow lysogenic cycles in which viral DNA remains integrated as a prophage. CRISPR-Cas systems provide an adaptive immune memory against nucleic acids from invading phages and plasmids, although mobile elements can evolve countermeasures.⁴ Finally, bacterial populations may exchange genes through extracellular vesicles, membrane bridges, or gene-transfer agents, processes that are real but more specialized and less universal than the three classical routes.

Operon

A cluster of functionally related bacterial genes controlled by shared regulatory DNA and transcribed together.

Plasmid

A usually circular, extrachromosomal DNA molecule capable of independent replication in a suitable host.

Transformation

The uptake and possible incorporation of DNA from the surrounding environment.

Transduction

The transfer of bacterial DNA by a bacteriophage.

Conjugation

Direct cell-associated transfer of DNA, often mediated by a conjugative plasmid or integrative element.

Accessory genome

Genes present in some members of a species or population but absent from others.