Re: Why are bacterial genes and plasmids in a closed loop?

First, let's make a small correction...

Both eubacteria and the archaebacteria contain a single, circular molecule of double-stranded DNA molecule that is often called the bacterial "chromosome" (some scientist do not like this term because of the difference in structure between prokaryotic and eukaryotic chromosomes) as well as circular, double-stranded extrachromosomal DNA molecules called plasmids. Plasmids are replicated independently of the main bacterial chromosome.

Why do bacteria have a single circular chromosome, rather than multiple linear chromosomes like eukaryotes? One consequence of the mechanism of replicating DNA is that since replication is discontinuous on one strand (the lagging strand) it is very difficult to replicate all the way to the very ends of a linear double-stranded DNA molecule. The ends become progresssively shorter and over time genetic information at the ends can be lost. Eukaryotes have evolved a system to deal with this (telomeres/ telomerase). Since bacterial chromosomes are circular, this is not a problem.

We don't know why bacterial chromosomes are circular, but eubacteria and archaebacteria are the oldest organisms on earth and some species of both have adapted to environments not inhabited by other organisms. The structure of bacterial chromosomes may reflect their ancient history as well as their lifestyle. Most bacteria reproduce rather rapidly and the circular nature may in some way aid in allowing this rapid reproduction.