Plasmids are extrachromosomal hereditary determinants, that is, chromosome-independent double-stranded ring-shaped DNA molecules of various molecular weights that have replicon properties, which are the ability for independent replication. Plasmids are not the obligatory genetic material of bacteria necessary for the manifestation of their vital activity.
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At the same time, plasmids can determine important properties of bacteria, for example, F-plasmid allows bacteria to transfer genetic material from donor F+ cells to recipient F cells during conjugation (Stranahan et al. 274). R-plasmids also provide resistance to antibiotics and sulfa drugs. In addition, Ent-plasmids can give bacteria an ability to synthesize toxins and form of fimbriae by which enterobacteria are attached to the intestinal epithelium.
All known plasmids are divided into conjugative and non-conjugative types. Conjugative plasmids transfer their DNA from a donor cell to a recipient cell during conjugation. Non-conjugative plasmids do not have the ability to conjugate transfer from one cell to another. The molecular weight of the conjugative plasmids can range from 26 Da to 75 Da, and the non-conjugative are not more than 10-106 Da (Sun et al. 619).
Some plasmids, such as the F-plasmid, have the ability to exist in bacterial cells in two states, which are physically independent of the chromosome and integrated with the chromosome (Sun et al. 620). Other types of plasmids can also integrate into the chromosome of bacteria, but only in certain conditions. If the conjugative plasmid is integrated into the chromosome of a bacterium, then cells are formed that are able to transfer the genetic material of the chromosome during conjugation with the recipient cell.
In conclusion, plasmids play an essential role in giving bacteria extrachromosomal genetic variation and properties, which can increase the chances of overall survival. They can act as a method of direct intercellular DNA transfer that makes allow other F bacteria to acquire additional information without dividing into daughter cells.
Stranahan, Lauren W., et al. “Rhodococcus Equi Infections in Goats: Characterization of Virulence Plasmids.” Veterinary Pathology, vol. 55, no. 2, 2018, pp. 273-276.
Sun, Jingjing, et al. “Intracellular Plasmid DNA Delivery by Self-Assembled Nanoparticles of Amphiphilic PHML-b-PLLA-b-PHML Copolymers and the Endocytosis Pathway Analysis.” Journal of Biomaterials Applications, vol. 31, no. 4, 2016, pp. 606-621.