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Bacterial are a great group of organisms. They have circular genomes and never went toward linear genomes while other organisms show the opposite strategy and don't have circular genomes (disregarding their cytoplasmic genome). Why they have followed these different strategies?

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The bacterium Borrelia burgdorferi has a linear chromosome as well as a number of linear and circular plasmids. pbil.univ-lyon1.fr/members/lobry/articles/… –  Alan Boyd Feb 8 '13 at 15:31
    
Interesting. Why it has not circular chromosome? –  MySky Feb 8 '13 at 17:59
    
I checked mentioned article to see how bacteria terminate replication and what do with its telomeres; its what I saw: >The mechanisms for termination of replication and resolution of the telomeres remain important unanswered questions –  MySky Feb 8 '13 at 18:04
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@AlanBoyd related to the discussion below regarding replication speed, it is interesting to note that B. burgdorferi has a very slow doubling time of 12-18 hours. –  Bitwise Feb 13 '13 at 16:39
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You can package linear genomes much more efficiently than circular genomes, and bacteria simply don't require the information density to be prosperous.

To be a bit more specific, it's the torque strain put on the double-helix while it's being wound that makes the difference. Linear genomes can be wound around Histones, and these Histones can be further formed into more complex and dense patterns so that the DNA of a linear genome is condensed to a mere fraction of the size it was before. The high density allows cell division to occur much, much easier than it would otherwise happen in Eukaryotic cells.

Circular genomes cannot get rid of the torque stresses like linear genomes can. To attempt a similar packaging scheme as Eukaryotic cells would result in the DNA breaking apart at some point before you reached a high level of information density.

I'll look through my BioChem books again to find a reference later on, but hopefully you should have a solid idea of "why" from the above. For now you can do a little thought experiment: Think of all the ways you can wind a single piece of string or thread around things to make it as dense as possible. Then think of all the ways you can try to achieve the same density with a rubber band without breaking it. The string should win out in both the ease at which you can manipulate it, and the level of density you can achieve.

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While I agree with your answer, the question remains why don't the bacteria have linear genomes also, since it is very easy to create linear genomes. My guess is that a circular genome might be easier to replicate faster, since eukaryotes typically have longer cell cycles. Anyone else have ideas about that? –  Bitwise Feb 8 '13 at 16:07
    
@Bitwise why easy?You need all sorts of helper proteins, helicases, histones etc. –  terdon Feb 8 '13 at 16:57
    
@Bitwise - Speed is definitely part of it. It takes time to condense all of a linear genome into Chromatin. A circular genome isn't necessarily easier to replicate (the proteins involved operate at about the same speeds if I remember correctly), but when your species depends upon a very, very fast generation time then a genome that you don't have to pack is a huge advantage. –  MCM Feb 8 '13 at 17:11
    
I don't know if it is a significant factor, but maintaining the linear DNA takes some effort in terms of making sure the telomeres stay intact for cells that divide often. A circular DNA can completely avoid that complexity. –  Mad Scientist Feb 8 '13 at 18:27
    
@MadScientist good point, I hadn't thought about that. Still, if you take a eukaryote like a yeast, it seems to manage with telomeres and with having a relatively short cell cycle time (~twice that of E coli). So I am still not sure that speed or not having telomeres are a sufficient advantage. –  Bitwise Feb 12 '13 at 4:06
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