摘要: It is a well documented fact that many (if not all) bacteria have but one chromosome (one continuous DNA molecule) forming single linkage group containing all of the essential genes cell. Recently however has realized in contain or more additional structures usually carrying so called facultative are only to cell’s survival under particular circumstances (e.g. antibioticum resistance transfer factors) they can be gained lost without lethal effect for Furthermore these “extrachromosomal” molecules significantly smaller than chromosomal DNA.