The Bacterial Chromosome and Plasmid

While eukaryotes have two or more chromosomes, prokaryotes such as bacteria possess a single chromosome composed of double‐stranded DNA in a loop. The DNA is located in the nucleoid of the cell and is not associated with protein. In Escherichia coli, the length of the chromosome, when open, is many times the length of the cell.

Many bacteria (and some yeasts or other fungi) also possess looped bits of DNA known as plasmids, which exist and replicate independently of the chromosome. Plasmids have relatively few genes (fewer than 30). The genetic information of the plasmid is usually not essential to survival of the host bacteria.

Plasmids can be removed from the host cell in the process of curing. Curing may occur spontaneously or may be induced by treatments such as ultraviolet light. Certain plasmids, called episomes, may be integrated into the bacterial chromosome. Others contain genes for certain types of pili and are able to transfer copies of themselves to other bacteria. Such plasmids are referred to as conjugative plasmids.

A special plasmid called a fertility (F) factor plays an important role in conjugation. The F factor contains genes that encourage cellular attachment during conjugation and accelerate plasmid transfer between conjugating bacterial cells. Those cells contributing DNA are called F+ (donor) cells , while those receiving DNA are the F- (recipient) cells. The F factor can exist outside the bacterial chromosome or may be integrated into the chromosome.

Plasmids contain genes that impart antibiotic resistance. Up to eight genes for resisting eight different antibiotics may be found on a single plasmid. Genes that encode a series of bacteriocins are also found on plasmids. Bacteriocins are bacterial proteins capable of destroying other bacteria. Still other plasmids increase the pathogenicity of their host bacteria because the plasmid contains genes for toxin synthesis.

Transposable elements. Transposable elements, also known as transposons, are segments of DNA that move about within the chromosome and establish new genetic sequences. First discovered by Barbara McClintock in the 1940s, transposons behave somewhat like lysogenic viruses except that they cannot exist apart from the chromosome or reproduce themselves.

The simplest transposons, insertion sequences, are short sequences of DNA bounded at both ends by identical sequences of nucleotides in reverse orientation (inverted repeats). Insertion sequences can insert within a gene and cause a rearrangement mutation of the genetic material. If the sequence carries a stop codon, it may block transcription of the DNA during protein synthesis. Insertion sequences may also encourage the movement of drug-resistance genes between plasmids and chromosomes.