Thursday, November 17, 2005

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.


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