Linear chromosome

Linear chromosomes are not limited to eukaryotic organisms; some prokaryotic organisms have linear chromosomes as well. Borrelia burgdorferi was the first bacterium to be found to have a linear chromosome, but new examples, such as various Streptomyces and Coxiella burnetii, have been found. Some bacteria, such as Agrobacterium tumefaciens, even have one linear chromosome and one circular chromosome. These bacteria have also independently invented their own versions of telomeres to protect DNA, though they are not foolproof: Streptomyces has telomeres capped by proteins, yet is known to represent "one of the most spectacular examples of genetic instability among prokaryotes". Experiments in which the circular chromosomes of prokaryotic organisms have been linearized have demonstrated that some prokaryotes can maintain viability even with linear chromosomes. == In organelles ==

The genomes of most eukaryotic mitochondria and plastids are in a single circular chromosome, in line with their bacterial ancestor. However, a good number of eukaryotic species do harbor linear Mitochondrial DNA (mtDNA), some even broken into multiple molecules, across a wide variety of taxa: animals (mammals, medusozoans, sponges), fungi (especially yeast), plants, and Alveolates. In yeast and plants, the shape of mtDNA depends on life-cycle: during some points they may be circular, but during others a linear branched shape is found, consisting of concatenated copies of the original genome. In these genomes, gene conversion help expand the repeats of their telomeres. Similar variations can be found in plastid genomes. Maize seedlings have Chloroplast DNA (cpDNA) mostly in a branched linear form. Acetabularia have a true linear cpDNA. == References ==