In Jonathan's <a href="http://ttaxus.blogspot.com/2007/08/ismb-2007-vienna-part-i-keynotes.html">recent post on ISMB/ECCB 2007</a>, he touches on the lively topic of "organism snobbery". I define this as the belief that your favourite taxonomic group (anything from species <i>Mus musculus</i> to class <i>Mammalia</i>) is the only thing of interest in biology and anything else (smaller, "simpler" - whatever that means - or most commonly, single-celled) is irrelevant and not worthy of study.
I always find this attitude perplexing because to me, biology is about universal processes and their variations. All cells need to transport molecules in, out or around, respond to stimuli via signalling pathways, divide, assemble their components, fold their proteins and so on. All that really distinguishes eukaryotic from prokaryotic cells is that the former have more compartments and chromosomes to worry about. Still, a belief persists that prokaryotes are essentially "simple" and that certain features are exclusively eukaryotic.
Let me point you to a few examples that might make you think again.
<ol>
<li><b>Multiple chromosomes</b></li>
Under the impression that all bacteria possess a single circular chromosome and perhaps a plasmid or two? Meet <a href="http://microbewiki.kenyon.edu/index.php/Borrelia"><i>Borrelia burgdorferi</i></a>, an obligate parasitic bacterium that causes Lyme disease. The genome of Strain B31 consists of a large linear chromosome, 9 circular plasmids and 11 linear plasmids - some strains have even more. The small linear plasmids terminate in a closed hairpin structure.
Links:
<li><b>DNA repair</b></li>
<a href='http://nsaunders.wordpress.com/files/2007/08/drad.png' title='drad.png'><img src='http://nsaunders.wordpress.com/files/2007/08/drad.thumbnail.png' alt='drad.png' align='right' /></a>Larger genomes means more things that can go wrong, so eukaryotes possess a multitude of enzymes that ensure fidelity when DNA is copied. But how many of them can rebuild a completely shattered genome in a few hours?
The king of genome repair is <i>Deinoccous radiodurans</i>.
<li><b>Internal membranes</b></li>
Membrane-bound organelles: that's a eukaryotic feature, right? Wrong. Here are some <a href="http://www.calpoly.edu/~rfrankel/mtbphoto.html">lovely images</a> of magnetotactic bacteria. They contain structures called magnetosomes, which are made from crystals of the mineral magnetite, wrapped in a lipid bilayer. The magnetosomes align the cell with the geomagnetic field, presumably pointing them in the direction of their optimal growth environment. Learn more about the species <i>Magnetospirillum magneticum</i> at <a href="http://microbewiki.kenyon.edu/index.php/Magnetospirillum_magneticum">MicrobeWiki</a>.
The magnetosome membrane is derived from invaginations in the cytoplasmic membrane, but other bacteria possess more extensive and impressive membrane-bound structures. <a href="http://microbewiki.kenyon.edu/index.php/Gemmata"><i>Gemmata obscuriglobus</i></a> belongs to the phylum <i>Planctomycetes</i> and is unusual in that its DNA is wrapped in a nuclear body with a double-membrane, superficially similar to the eukaryotic nucleus. <a href="http://www.anammox.com/research.html"><i>Candidatus Brocadia anammoxidans</i></a>, another planctomycete, contains a membrane-bound organelle named the anammoxosome where hydroxylamine oxidation takes place. The lipids in this membrane are described as "bizarre and unique"; the annamoxosome is their only known natural source.
<li><b>Splicing</b></li>
When eukaryotic DNA is transcribed to mRNA, the transcript usually contains features called introns. These segments do not contain protein coding information and so they are chopped out of the mRNA molecule which is then stitched back together before translation, in a process called splicing.
Splicing is far from unique to eukaryotes.
<li><b>Non-coding RNA</b></li>
Jonathan's original post highlighted a common misconception amongst eukaryote snobs: the notion that all prokaryotic cellular regulation is protein-based and only eukaryotes, with their high proportion of non-protein coding DNA, use ncRNA in regulation.
<li><b>A cytoskeleton</b></li>
There are still people around who visualise bacteria as little bags of water, or jelly, with stuff floating in it.
<li><b>Mitosis</b></li>
<li><b>Serine-threonine and tyrosine kinases</b></li>
Eukaryotes are chock-full of protein kinases: the human and mouse genomes encode over 500 and some estimates place the proportion of phosphorylated proteins in eukaryotic cells at 30-50%. Most of these kinases attach a phosphate group to their substrates at a serine, threonine or tyrosine residue. The conventional wisdom is that this type of kinase is a hallmark of eukaryotes. Bacteria and archaea, on the other hand, use two-component systems for cell signalling in which a different type of membrane-bound protein kinase phosphorylates substrates at a histidine residue.
In fact, there are many examples of "eukaryotic-type" protein kinases to be found in prokaryotes.
</ol>
