Thursday, November 17, 2005

On pluricellular organisms.

Why would one species evolve from one cell into a multicellular being?

This question can only be answered if we consider that in multicellular organisms only a small group of cells is allowed to duplicate endelessly (the germ line cells as well as the stem cells). All other cells in an organism are there just to perform a task and when they are no longer healthy they are replaced by newly specialized stem cells that promptly loose their high reproductible capability.

Two things are necessary for a cell to be useful to the organism: it must not be able to reproduce independently and it shall be specialized in a way that a specific trait (or group of traits) will be pushed to the maximum so this cell will perofrm tasks for the sake of the organism and not of itself.

Imagine for instance a single-cellular organism that creates a copy of itself but this copy due to some sort of mutation cannot reproduce. This copy will tend to grow and accumulate energy but since it cannot reproduce this process should proceed until it either dies or the nutrients flow out of its cytoplasm and be ingested by the normal reproducing cells. This is an example of "cooperation" between a germ line cell and a somatic cell. The term cooperation is quoted because in fact the copy is being enslaved by the other cell that is using it to acquire more energy and reproduce. If this cell manages to keep the capability of creating both normal daughters and "slave" copies it will have an advantage against the other cells that only create normal copies of itself.

The lack of reproducibility being the first specialization necessary for the somatic cell, let's pass to the next step: imagine that the slave cell now suffered some sort of mutation that causes it to produce an enormous amount of nutrients ( aminoacids, ATP, or any other metabolite necessary for survival) and after overproducing this payload it dies, suffers necrosis and the "master" cell captures all this energetic package. Once again we have an advantage for the master cell. This kind of specialization can be extended from energy metabolism to oxygen transport, mechanic protection, termal protection or any other kind of use; It's life using life to survive.

This makes us wonder who's really in control of our lives: our brain or our gonads?

On reduction of complexity of Central Dogma

Central Dogma states that DNA is a template for synthesis of RNA (mRNA) that in turn is a template for synthesis of proteins. The point to be addressed here is that this system cannot be reduced, i.e. if part of it is removed, say the RNA Polymerase or the Ribosomes we will not have a system that performs less well it's job, we'll have a system that does nothing at all.

Irreducible systems are generally used as a proof of design however this system is not irreducible if we consider the molecular complementarity principle in which evolution of most molecules does not happen only based on selection but also on complementation: two selected entities combine to give birth to a third one that performs another task.

Let's think now about the question: Why doesn't the Rybosome encode the proteins directly from the DNA?

One might state that there might be some sort of subcellular localization issue since proteins are supposed to be used both in nucleus and cytoplasm and it's safer that they be produced out of nucleus and only imported if necessary. However in bacteria there's no separation between DNA and cytoplasm and still the Ribosomes do not read the DNA directly (even though they are quite close since the poli-Ribosomes translate proteins while the mRNA is still being transcribed).

I'd suggest three hypothesis for the birth of Central Dogma (from now on referenced as CD):

a)Direct translation of proteins from DNA was a step that preceded modern CD. This step was replaced and the mRNA where used as intermediates.The Ribosome would'nt have to be that more different in order to handle DNA than it is today (considering that there was some sort of mechanism for openning the double strand such as the helicases) but this evolutionary step should happen a lot before the divergence of the species known today and this should be such an evolutionary leap that the other species with different DNA->Protein strategies would be extinct;

b)RNA was synthesized from DNA to do the job that is done by proteins nowadays (catalysis, structure, etc.) and later RNA was replaced by proteins. Again this must have been a leap in the early days of life. However a detail not to be forgotten is that the "genes" previously used for the synthesis of tri-dimensional catalytic RNA structures would be replaced by new "genes" dedicated to the synthesis of mRNAs with no tri-dimensional meaning but with a specific linear code that was very little error-robust (the functionality migrated from the form to the content). One might say that such a transition would explain why we have som much "junk" DNA around the real encoding regions but this hypothesis would be valid only if all species had similiar amount of non-coding DNA and this is not true;

c)In early times there were two different types of life: the first was based on DNA (double strand or not) and its mebaolism was catalyzed by RNA enzymes. The second type of life would have its "genes" encoded in RNA and it would synthesize proteins for its metabolism. So in the frist case we have RNA Polymerase acting as a enzyme generator while in the second case this enzyme fuctions as a genetic information replicator. By the fusion of two of these entities a hybird was born capable of both keeping large amounts of genetic information stored and also with the flexibility of producing proteic enzymes instead of RNA catalysts.

The last hypothesis reminds me the theory of the mitochondria swallowed by an early cell and used in complementarity with the cell's metabolism.