Bacteria are prokaryotic cells with no membrane-enclosed nucleus or other cell organelles. The bacterial genome consists of a single circular chromosome in which the genes are tightly packed, with about one gene per kilobase pair.
During transcription in E. coli, different strands of DNA serve as the template for different genes, with the most frequently transcribed genes oriented in the direction of replication fork movement. Replication begins at the origin of chromosomal replication (oriC) and proceeds bidirectionally around the circular genome to the termination region (terC).
In addition to their chromosome, most bacteria carry plasmids: small circles of double-stranded DNA. Plasmids may include genes that benefit the bacterial host under certain conditions. One important group of plasmids promotes conjugative gene transfer between two bacteria.
Bacterial genomes contain IS and Tn elements, transposons that can move between sites on any DNA molecule in the cell.
Transformation is a form of gene transfer in which donor DNA that is floating free in the growth medium enters a recipient cell. Some bacteria have cellular machinery that supports efficient natural transformation. Species that do not undergo natural transformation can be induced to take up DNA by treatments that disrupt their cell walls in a process known as artificial transformation.
Conjugation is a second form of gene transfer. It depends on direct cell-to-cell contact between a donor carrying a conjugative plasmid (the F plasmid is one example) and a recipient lacking such a plasmid. In crosses between F1 and F2 strains, only the plasmid is transferred. In crosses between Hfr and F2 strains or between F' and F2 strains, the plasmid also transfers chromosomal genes.
Transduction is a third form of gene transfer in bacteria. It depends on the packaging of bacterial donor DNA in the protein coat of a bacteriophage. In generalized transduction, phages can package any part of the donor genome. Specialized transduction is a property of lysogenic bacteriophages, which can package only host genes adjacent to the integrated prophage genome.
It is possible to map bacterial genes using any of the three forms of gene transfer. Interrupted-mating experiments can map the approximate positions of genes. Measures of frequencies of cotransformation or cotransduction provide better resolution: The closer two genes are, the more likely they are to appear on the same short DNA fragment. Sequences of complete bacterial genomes are becoming increasingly important as mapping resources.
