Plasmids are mainly helping in antibiotic resistance and sometimes in replication. But why are they present as a separate entity in bacterial cells?
Hi!
As you know, bacteria have a chromosome located in the nucleoid, and also smaller circular DNA called plasmids. Unlike the chromosome which contain mostly essential genes, bacteria can pick up new plasmids that contain few non essential genes (at least for day to day survival) from the environment or even from another bacteria (through conjugation). Note that plasmids get copied independently from the chromosome. Plasmids can be easily lost too, for example when the condition does not require the presence of the plasmid anymore such as the absence of a specific stress.
Although I mentioned that plasmids contain only few non essential genes, plasmids can give additional functions to the cell such as surviving certain stresses like antibiotic exposure. In certain environment with specific stresses, it is then very useful for a bacteria to have a plasmid that confer resistance to the stress. This then might give you a hint on why bacteria also have plasmids in addition to the chromosome. Although retaining a plasmid inside a cell requires energy for DNA replication, usually bacteria with a plasmid can live longer as they have better chances in surviving occasional stressful situations.
Simply because it is much easier to propagate plasmids than genomic DNA. Plasmids can be transferred from one bacteria to another by several mechanisms (conjugation, transformation etc.) without killing the donor bacteria. They can also be replicated within the cells in numerous copies which helps their dissemination.
To answer your question, there is no "need" for plasmid DNA to be independant of genomic DNA. But the fact that they are hugely promotes horizontal gene transfer, which is beneficial to the bacteria in the long run.
plasmids can give additional functions to the cell such as surviving certain stresses like antibiotic exposure. In certain environment with specific stresses, it is then very useful for a bacteria to have a plasmid that confer resistance to the stress. Plasmids can be transferred from one bacteria to another by several mechanisms like conjugation, without killing the donor bacteria. They can also be replicated within the cells in numerous copies which helps their dissemination.
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