Secondary chromosomes (recently renamed chromids) are a class of bacterial replicons (replicating DNA molecules). These replicons are called "chromids" because they have characteristic features of both chromosomes and plasmids. Early on, it was thought that all core genes could be found on the main chromosome of the bacteria. However, in 1989 a replicon (now known as a chromid) was discovered containing core genes outside of the main chromosome. These core genes make the chromid indispensable to the organism. Chromids are large replicons, although not as large as the main chromosome. However, chromids are almost always larger than a plasmid (or megaplasmid). Chromids also share many genomic signatures of the chromosome, including their G + C content and their codon usage bias. On the other
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| - Secondary chromosomes (recently renamed chromids) are a class of bacterial replicons (replicating DNA molecules). These replicons are called "chromids" because they have characteristic features of both chromosomes and plasmids. Early on, it was thought that all core genes could be found on the main chromosome of the bacteria. However, in 1989 a replicon (now known as a chromid) was discovered containing core genes outside of the main chromosome. These core genes make the chromid indispensable to the organism. Chromids are large replicons, although not as large as the main chromosome. However, chromids are almost always larger than a plasmid (or megaplasmid). Chromids also share many genomic signatures of the chromosome, including their G + C content and their codon usage bias. On the other
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| - Secondary chromosomes (recently renamed chromids) are a class of bacterial replicons (replicating DNA molecules). These replicons are called "chromids" because they have characteristic features of both chromosomes and plasmids. Early on, it was thought that all core genes could be found on the main chromosome of the bacteria. However, in 1989 a replicon (now known as a chromid) was discovered containing core genes outside of the main chromosome. These core genes make the chromid indispensable to the organism. Chromids are large replicons, although not as large as the main chromosome. However, chromids are almost always larger than a plasmid (or megaplasmid). Chromids also share many genomic signatures of the chromosome, including their G + C content and their codon usage bias. On the other hand, chromids do not share the replication systems of chromosomes. Instead, they use the replication system of plasmids. Chromids are present in 10% of bacteria. Bacterial genomes divided between a main chromosome and one or more chromids (and / or megaplasmids) are said to be divided or multipartite genomes. The vast majority of chromid-encoding bacteria only have a single chromid, although 9% have more than one (compared with 12% of megaplasmid-encoding bacteria containing multiple megaplasmids). The genus Azospirillum contains three species which have up to five chromids, the most chromids known in a single species to date. Chromids also appear to be more common in bacteria which have a symbiotic or pathogenic relationship with eukaryotes and with organisms with high tolerance to abiotic stressors. Chromids were discovered in 1989, in a species of Alphaproteobacteria known as Rhodobacter sphaeroides. However, the formalization of the concept of a "chromid" as an independent type of replicon only came about in 2010. Several classifications further distinguish between chromids depending on conditions of their essentiality, their replication system, and more. The two hypotheses for the origins of chromids are the "plasmid" and "schism" hypotheses. According to the plasmid hypothesis, chromids originate from plasmids which have acquired core genes over evolutionary time and so stabilized in their respective lineages. According to the schism hypothesis, chromids as well as the main chromosome originate from a schism of a larger, earlier chromosome. The plasmid hypothesis is presently widely accepted, although there may be rare cases where large replicons originate from a chromosomal schism. One finding holds that chromids originated 45 times across bacterial phylogenies and were lost twice.
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