Thursday, May 14, 2015

How eukaryotic cells arose

Examples of eukaryotes
The discovery that there are two main types of living cells (prokaryotes and eukaryotes) gave rise to a revolution in the way of classifying living things. Although (as usual) biologists do not agree on a single classification, the following one seems very reasonable:
1.      Empire prokaryote (bacteria). DNA free in the protoplasm.
a.       Kingdom eubacteria (true bacteria). They use acyl ester lipids.
b.      Kingdom archaea. They use isoprenoidal-ether lipids. They include sulphobacteria, methanobacteria and halobacteria.
2.      Empire eukaryote (cells with nuclei). DNA inside the nucleus. They have a cytoskeleton.
c.       Kingdom archaezoa (primitive eukaryotes). They have no organelles.
d.      Kingdom protozoa (advanced unicellular eukaryotes) with symbiotic organelles.
e.       Kingdom Fungi.
f.       Kingdom metaphyta (plants).
g.      Kingdom of metazoa (animals).
The last three eukaryotic kingdoms are multi-cellular beings who came from protozoa.
The archaezoa (primitive eukaryotes) are very close to the archaea, so it seems likely they came from them by acquiring a nucleus. Recently a possible candidate for the archaea closest to us has been discovered in the bottom of the Arctic Ocean and given the name lokiarchaea.
Lynn Margulis
But the most surprising eukaryotic mystery is the origin of cellular organelles (mitochondria, peroxisomes and chloroplasts). Since 1967, Lynn Margulis was the first to propose that this happened by endosymbiosis. Let's see how.
Cell organelles belong to three main types. They contain their own DNA, sometimes with striking differences in the genetic code.
         Mitochondria: they carry out cellular respiration, which takes advantage of oxygen to obtain energy.
         Peroxisomes: they contain peroxidases and aid in the digestion of fatty acids.
         Chloroplasts: they perform the chlorophyll function.
Endosymbiosis asserts that organelles were acquired successively by primitive archaezoa by means of symbiosis: first mitochondria (bacteria capable of breathing oxygen); then peroxisomes (gram-positive bacteria capable of generating peroxidases); eventually (after separation of the strains that resulted in fungi and animals) some protozoa acquired chloroplasts (they were originally autotrophic Cyanobacteria). From this group, plants arose later.
How could this be accomplished by symbiosis? It’s a mystery. Obviously some primitive archaezoa swallowed a bacterium capable of breathing oxygen, in order to digest it. What happened to prevent the prey to be digested? How could the bacterium get used to live and reproduce inside the membrane of the archaezoa and share with it the results of its respiratory process, which generate much more energy than anaerobic processes? Why was the reproduction cycle of both the host cell and the symbiotic organelles get synchronized, so that when the eukaryotic cell is divided the cellular organelles also divide and are shared between the two daughter eukaryotic cells? Why and how some genetic information originally contained in the organelles was transmitted to the nucleus of the eukaryotic cell? Nobody knows just now.

Manuel Alfonseca

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