Figure 1. The circular genetic map of the Escherichia coli chromosome. The map is divided into 100 map units, and representative genes are shown. The lac operon is at approximately 8 minutes. The origin of replication (not shown) is at 83.5 minutes. Based on Ingraham, 2000.

such as Vibrio cholerae (the agent that causes the disease cholera) and Deinococ-cus radiodurans, that have two different chromosomes.

It is also quite common for bacterial species to possess extrachromosomal genetic elements called plasmids. These are small, circular DNA molecules which, when present, vary in number from one to about thirty identical copies per cell. Plasmids include the fertility factor (F+ plasmid), described below, as well as plasmids that carry drug-resistance genes. Indeed, these drug-resistance plasmids may be passed from species to species and are a major problem in the spread of antibiotic resistance. Whereas most bacteria that contain plasmids have just a single kind of plasmid, some bacterial species simultaneously possess a number of different plasmids, each of which, in turn, is present in varying numbers within the bacterial cell.

The bacterial chromosome is condensed into chromosomal domains.

The bacterial chromosome must be tightly packed to fit into the small volume of the bacterial cell. Figure 3 shows the relative sizes of the unfolded chromosome and the E. coli cell. During the 1980s, techniques were developed to isolate intact bacterial nucleoids by gentle lysis, under conditions that prevented the DNA of the chromosome from uncoiling. These isolated

Figure 2. A transmission electron microscope image of Escherichia coli magnified approximately 20,000 times.

Figure 2. A transmission electron microscope image of Escherichia coli magnified approximately 20,000 times.

nucleoids were highly condensed into a very compact structure, as shown in Figure 4.

Compacting the DNA involves supercoiling, or further twisting the twisted chromosome. The chromosome's fifty or so DNA domains are held together by a scaffold of RNA and protein, and the entire nucleoid is attached to the cell membrane. This membrane attachment aids in the segregation of the chromosomes after they replicate in preparation for cell division. Bacteria lack the histone proteins that are found bound to the DNA and that form the nucleosomes of eukaryotic chromosomes. However, it is believed that polyamines (organic molecules with multiple — NH2 amine groups) such as spermidine, as well as some basic proteins, aid in compacting the bacterial chromosome. These basic proteins have a net positive charge that bind them to the negative charge of the phosphates in the DNA backbone.

Replication of the circular chromosome begins at a single point, called OriC, and proceeds in both directions around the circle, until the two replication forks meet up. The result is two identical loops. Replication takes approximately forty minutes.

The E. coli genetic chromosome. The field of bacterial genetics began in 1946, even before the structure of DNA was determined, with the discovery by the geneticists Joshua Lederberg and Edward Tatum at the University of Wisconsin of sex in bacteria, in the form of conjugational genetic exchange between E. coli bacteria. In the conjugation process, a fertility

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