All living organisms contain DNA. This amazing macromolecule encodes all of the information needed to program the cell's activities including reproduction, metabolism and other specialized functions. DNA is comprised of two strands of deoxynucleotides. Each deoxynucleotide contains a phosphate, a 5-carbon sugar (2-deoxyribose) and one of four nitrogenous bases: adenine, cytosine, thymine or guanine. The phosphate and sugar make up the backbone of each strand of DNA, while the bases are responsible for holding the two strands together via hydrogen bonds in a structure called the double helix (see figure above). The order of the bases in a DNA strand contains the coded genetic information. All of the DNA found in an organism is collectively referred to as the genome. The human genome is comprised of 23 pairs of linear chromosomes, and approximately 3000 megabases (Mb) of DNA, while thegenome of the bacterium Escherichia coli consists of a single 4.6 Mb circular chromosome. By studying the genomes of bacteria we are able to better understand their metabolic capabilities, their ability to cause disease and also their capacity to survive in extreme environments.Many of the well-studied bacterial model organisms, such as E. coli, have a single circular chromosome. However, advances in molecular genetics have shown that bacteria possess more complex arrangements of their genetic material than just a single circular chromosome per cell. Some bacterial genomes are comprised of multiple chromosomes and/or plasmids and many bacteria harbor multiple copies of their genome per cell. The following are a few examples of bacteria with unusual.

Kb/Mb - A kilobase (Kb) is 1000 bases of DNA, while a megabase (Mb) is 1,000,000 bases.
Circular Chromosome ·The DNA is arranged in a closed circle, which is negatively supercoiled allowing for the compact nature of many bacterial genomes.
Linear Chromosome · A non-closed chromosome, which has inverted repeats at the ends, similar to teleomeres in eukaryotic chromosomes.
Plasmid · Extra-chromosomal DNA which replicates independently of the chromosome and regulates its own replication.
Megaplasmid · A very large plasmid ranging in size from 100- 1700 Kb.