CHROMOSOMES, PROKARYOTIC

The genetic material of microorganisms, be they prokaryotic or eukaryotic, is arranged in an organized fashion. The arrangement in both cases is referred to as a chromosome.

The chromosomes of prokaryotic microorganisms are different from that of eukaryotic microorganisms, such as yeast, in terms of the organization and arrangement of the genetic material. Prokaryotic DNA tends to be more closely packed together, in terms of the stretches that actually code for something, than is the DNA of eukaryotic cells. Also, the shape of the chromosome differs between many prokaryotes and eukaryotes. For example, the deoxyribonucleic acid of yeast (a eukaryotic microorganism) is arranged in a number of linear arms, which are known as chromosomes. In contrast, bacteria (the prototypical prokaryotic microorganism) lack chromosomes. Rather, in many bacteria the DNA is arranged in a circle.

The chromosomal material of viruses is can adopt different structures. Viral nucleic acid, whether DNA or ribonucleic acid (RNA) tends to adopt the circular arrangement when packaged inside the virus particle. Different types of virus can have different arrangements of the nucleic acid. However, viral DNA can behave differently inside the host, where it might remain autonomous or integrating into the host's nucleic acid. The changing behavior of the viral chromosome makes it more suitable to a separate discussion.

The circular arrangement of DNA was the first form discovered in bacteria. Indeed, for many years after this discovery the idea of any other arrangement of bacterial DNA was not seriously entertained. In bacteria, the circular bacterial chromosome consists of the double helix of DNA. Thus, the two strands of DNA are intertwined while at the same time being oriented in a circle. The circular arrangement of the DNA allows for the replication of the genetic material. Typically, the copying of both strands of DNA begins at a certain point, which is called the origin of replication. From this point, the replication of one strand of DNA proceeds in one direction, while the replication of the other strand proceeds in the opposite direction. Each newly made strand also helically coils around the template strand. The effect is to generate two new circles, each consisting of the intertwined double helix.

The circular arrangement of the so-called chromosomal DNA is mimicked by plasmids. Plasmids exist in the cytoplasm and are not part of the chromosome. The DNA of plasmids tends to be coiled extremely tightly, much more so than the chromosomal DNA. This feature of plasmid DNA is often described as supercoiling. Depending of the type of plasmid, replication may involve integration into the bacterial chromosome or can be independent. Those that replicate independently are considered to be minichromosomes.

Plasmids allow the genes they harbor to be transferred from bacterium to bacterium quickly. Often, such genes encode proteins that are involved in resistance to antibacterial agents or other compounds that are a threat to bacterial survival, or proteins that aid the bacteria in establishing an infection (such as a toxin).

The circular arrangement of bacterial DNA was first demonstrated by electron microscopy of Escherichia coli and Bacillus subtilus bacteria in which the DNA had been delicately released from the bacteria. The microscopic images clearly established the circular nature of the released DNA. In the aftermath of these experiments, the assumption was that the bacterial chromosome consisted of one large circle of DNA. However, since these experiments, some bacteria have been found to have a number of circular pieces of DNA, and even to have linear chromosomes and sometimes even linear plasmids. Examples of bacteria with more than one circular piece of DNA include Brucella species, Deinococcus radiodurans, Leptospira interrogans, Paracoccus denitrificans, Rhodobacter sphaerodes, and Vibrio species. Examples of bacteria with linear forms of chromosomal DNA are Agrobacterium tumefaciens, Streptomyces species, and Borrelia species.

The linear arrangement of the bacterial chromosome was not discovered until the late 1970s, and was not definitively proven until the advent of the technique of pulsed field gel electrophoresis a decade later. The first bacterium shown to possess a linear chromosome was Borrelia burgdorferi.

The linear chromosomes of bacteria are similar to those of eukaryotes such as yeast in that they have specialized regions of DNA at the end of each double strand of DNA. These regions are known as telomeres, and serve as boundaries to bracket the coding stretches of DNA. Telomeres also retard the double strands of DNA from uncoiling by essentially pinning the ends of each strand together with the complimentary strand.

There are two types of telomeres in bacteria. One type is called a hairpin telomere. As its name implies, the telomers bends around from the end of one DNA strand to the end of the complimentary strand. The other type of telomere is known as an invertron telomere. This type acts to allow an overlap between the ends of the complimentary DNA strands.

Replication of a linear bacterial chromosome proceeds from one end, much like the operation of a zipper. As replication moves down the double helix, two tails of the daughter double helices form behind the point of replication.

Research on bacterial chromosome structure and function has tended to focus on Escherichia coli as the model microorganism. This bacterium is an excellent system for such studies. However, as the diversity of bacterial life has become more apparent in beginning in the 1970s, the limitations of extrapolating the findings from the Escherichia coli chromosome to bacteria in general has also more apparent. Very little is known, for example, of the chromosome structure of the Archae, the primitive life forms that share features with prokaryotes and eukaryotes, and of those bacteria that can live in environments previously thought to be completely inhospitable for bacterial growth.