1. Wprowadzenie. 2. Nazewnictwo replikonów niezbędnych. 3. Koncepcja chromidu. 4. Identyfikacja i klasyfikacja chromidów. 5. Chromidy w genomach bakterii. 6. Ewolucyjne korzyści wynikające z obecności chromidów. 7. Podsumowanie

Abstract: Extrachromosomal replicons are common components of bacterial genomes. While the genetic information essential for growth and division of bacterial cells is located within the chromosome, the extrachromosomal replicons, usually classified as plasmids, can provide functions which are critical for the survival of a bacterium in a specific environment; however, they are not indispensable for the viability of the host cells. Comparative genomic studies revealed that in many bacterial genomes some chromosomal genes had been transferred into the co-occurring plasmids. This phenomenon has led to the generation of essential extrachromosomal replicons, called chromids, sharing features of both chromosomes and plasmids. The prevalence of chromids in bacteria and their conserved character within certain taxonomic groups suggest an important role for these replicons in the evolution of bacteria.

1. Introduction. 2. Nomenclature of essential replicons. 3. The concept of chromid. 4. Identification and classification of chromids. 5. Chromids in bacterial genomes. 6. Evolutionary significance of chromids. 7. Summary