DNA in eukaryotes

[arildno]

Why is the DNA in eukaryotes an open ended string rather than a closed loop?

[spuriousmonkey]

because it is too long to be in a loop????

[Monique]

..something with D-branes.. I don't remember :wink:

But seriously, it is a good question.. let's turn it around: why is the DNA in prokaryotes circular and when did the separation between circular and linear DNA start. I guess spuriousmonkey has a point.

[arildno]

This is how I think about it:
that by using an open ended string form, it is easier to build up
a more complex organism which might carve out new ecological niches for themselves.
But since this is wild, unsubstantiated speculation from my side, I wanted to hear an expert's view on the matter..

[iansmith]

I don't think it is a question of length. Many bacteria have more than one chromosome and if you add all the genetic material (including plasmid) some bacteria have large genome. Linear chromsome tend to be unstable if some component are not present. Circular chromosome can be done artificially in yeast (Circular chromosome formation in a fission yeast mutant defective in two ATM homologues (http://www.nature.com/cgi-taf/DynaPage.taf?file=/ng/journal/v20/n2/abs/ng1098_203.html))

I think the difference between linear and circular is probably due to their distinct evolution and early in their evolution, eukaryotes acquire genes that stabilize the linear form. As far as complexity and exploting unique niche, bacteria are ahead of eukaryotes. Linear chromsome seems better suite for sexual reproduction. Recombination between two linear chromosome seem to be easier than circular DNA.

This is paper on the evolution of linear chromosome in bacteria.
A new beginning with new ends: linearisation of circular chromosomes during bacterial evolution (http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6T2W-405SX5F-1&_user=458507&_handle=B-WA-A-A-BW-MsSAYZW-UUA-AUYZAZDVBE-AUYBDVYWBE-BEBWEBBUY-BW-U&_fmt=summary&_coverDate=05%2F05%2F2000&_rdoc=1&_orig=browse&_srch=%23toc%234929%232000%23998139997%23192052!&_cdi=4929&view=c&_acct=C000022002&_version=1&_urlVersion=0&_userid=458507&md5=66cbcaff630244073273ac3dca1eed2f)

[arildno]

iansmith, great!!
So, linear DNA is the best tool for gene-mixing when gene-mixing is to occur by means of sexual reproduction?

[Monique]

I think he has a very good point too.. on average 40 cross-overs occur during meiosis, I don't see that happening with circular DNA.. :)