What is the advantage of circular DNA in bacteria? ?

Question

From what I understand, bacteria have circular DNA. What advantages does it have over linear strands like for eukaryotes? Do there exist bacteria with more than one ring of DNA?

Answer 1

Most bacteria have circular plasmids because their genome is much smaller than many eukaryotes, and it’s more feasible to manage. Large genomes are generally linear because it’s easier for enzymes to access it for transcription; if it were circular, trying to uncoil itself to allow access could cause strain on the DNA, possibly preventing enzymatic access to it.

Circular genome advantages are that it consistently replicates the whole genome, as opposed to linear genomes. Linear genomes actually lose a little bit of their DNA at the ends each time it replicates; these ends are called telomeres. Telomere DNA isn’t needed, but running out of telomere DNA to lose is one of the reasons people can’t live forever; eventually needed genes start getting cut. With circular DNA there is no start or end point necessarily, it just replicates the whole thing.

So the pro of having circular DNA is full genome replication, but the con is it ends up being harder to uncoil and replicate the bigger the genome gets.

There actually are some bacteria that have multiple plasmids (circular DNA). Vibrio cholerae is one (the cause of the cholera disease), and it has two plasmids that make up its genome. It’s also possible for bacteria to uptake foreign DNA in the form of plasmids. A common example of this is E. coli (which normally lives fairly peacefully in the human gut) uptaking the O157 plasmid, making it harmful to the host. There are also bacteria with linear DNA, and it’s not impossible to make adjustments to a species to switch from one type to the other.