Eukaryotes and Prokaryotes

I’ve long found it fascinating that every living thing on this planet can be cleanly split into two categories – prokaryotes and eukaryotes.  The prokaryotes consist of all the bacteria while the eukaryotes include animals, plants, fungi, and various protozoa.  The core life processes of the two cells are much the same, being built around the triad of proteins, RNA, and DNA, relying on the ribosome to build the proteins that synthesize everything else, including RNA and DNA, using ATP as the primary energy currency, and using lipid bilayer membranes to compartmentalize.  So what makes the two cell plans so different?

Below is a nice figure that helps you answer this question.

 

As you can see, there are two primary differences: size and level of compartmentalization.  Typical eukaryotic cells are much larger than bacteria and show a much more extensive level of compartmentalization given the numerous membrane-bound organelles and membranous folds.

Yet a question to ponder is why there are two cell types and only two cell types?  The non-telic perspective would explain this (away?) as simply an artifact of a contingent past.  There is no reason to ponder the question “why?”  It just happened that way.  But the telic perspective allows us to think of these two cell plans at a level that runs deeper.

I’ve long found it fascinating that these two types so nicely fit within a teleological template.  The prokaryotes are the terraformers, as they are built to disperse, infiltrate, and transform.  Many aspects of their cell biology fit these objectives perfectly, whether it be their high reproductive rates, remarkable ability to adapt to all sorts of extreme environments, or their ability to physiologically and genetically communicate with each other.  As Craig Venter says, “We live on a microbial planet.”

But why terraform in the first place?  That objective assumes the Earth needed to be transformed to adopt and facilitate later events.  Enter the eukaryotic cell.  Why does it even exist?  The biosphere successfully existed for a very long time without it and when it comes to “life as a cell,” it is needlessly complex in just about everyway.  Yet once we realize that nothing akin to an animal would have ever evolved without this cell design, suddenly its existence makes sense in the light of foresight.  It exists because it is the cell type that can be used to facilitate the emergence of animal-type and plant-type complexity.  Remember, once the eukayotic cell plan emerged, it did not need to be re-tooled any further to make metaozoan life possible.  The potential for such later evolution was embedded in the very design of the cell type.

What is most interesting is that over the last year I have highlighted three features of the eukaryotic cell that all converge on the same point.  It began with a discussion of protein-coding introns, where I responded to the faulty idea that introns do not fit into a teleological perspective.  On the contrary, we uncovered good reason to think introns facilitated the emergence of metazoan-type complexity.

Later, I discovered a putative homolog of beta-catenins in green algae.  As we looked more closely, the evidence held and, what’s more, we uncovered the plausible scenario where beta-catenins, needed for form epithelial tissue, were front-loaded to emerge because of their uncanny similarties with the alpha-importins.

Finally, I brought to your attention another team of scientists who have raised the strong argument that mitochondria were needed for metazoan life to emerge.

Yet I now want to bring to your attention that these three independent considerations all converge on the same point – the nucleus.  Protein-coding introns exist because the nucleus separates RNA-processing events from translation by ribosomes.  Beta-catenins exists because alpha-importins are needed to shuttle proteins in and out of the nucleus.  The mitochondria exist because their genes were successfully shuttled to the nucleus.  Add it all up and we can begin to appreciate that the nucleus, long recognized as the defining feature of eukaryotic cells, may very well have been the key feature that poised such cells for the eventual emergence of animals and plants in a world prepared for them by bacteria.

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5 responses to “Eukaryotes and Prokaryotes

  1. Pingback: Eukaryotes and Prokaryotes (via ) « Interstitial

  2. So was the nucleus the result of an endosymbiotic event? Or were there pre-existing cells that already had nucleui, that allowed for further endosymbiotic events? If the latter, were these pre-existing, nucleated cells part of the original design event? Or were they the result of additional intelligent intervention?

  3. I must admit that I don’t know the answer to any of these questions. It’s important to remember that I don’t have a preconceived story in mind. Instead, I am trying to piece together an account by following the clues. In this case, I did not begin with the belief that the nucleus would be key. I began by responding to something biologist Steve Matheson said about introns. Then later, I decided to do some BLASTing to see if my hypothesis of front-loading epithelial tissue could be further supported. Then, I ran across the study that argued mitochondria were essential for the evolution of metazoans. I then realized there is a common theme running through these three independent lines of inquiry and proposed my hypothesis:

    we can begin to appreciate that the nucleus, long recognized as the defining feature of eukaryotic cells, may very well have been the key feature that poised such cells for the eventual emergence of animals and plants in a world prepared for them by bacteria.

    From here, we can go in two directions. Look back and ask, “Where did the nucleus come from?” Or look forward and ask, “How else might the nucleus have facilitated the emergence of metazoan?” I will mostly likely focus on the latter simply because a) that has been my focus for years and b) there is likely to be more solid information. However, I will also start poking around about the origin of the nucleus.

    Nevertheless, don’t lose sight of the big picture here. Your questions are good and show that the rabbit can guide an investigative approach.

  4. The terraformers were willing to wait 2 billion years for part 2.

  5. Not only is the nucleus important but so is the location of the chromosomes within the nucleus. The action is in the nucleolus- “genes at the periphery of the nucleus are often inactive”*. It appears the transcription factories reside in the interior of the nucleus. So it would make sense to have the genes that require activation to be near these factories.

    This location also makes it possible to transcibe from more than one chromosome at a time.

    * Feb 2011 SciAm page 71.

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