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In my previous
post I discussed the rationale for the RNA world theory of ancient life. One question that often comes up is whether the RNA molecule itself could have been naturally produced in early earth. Let us look at the unit of RNA.
The picture above shows a DNA and a RNA unit. They are very similar and consists of three parts
1) A
nucleobase (the one with lots of N i.e. Nitrogen groups). In the above picture the nucleobase is Guanine, the (G) of the ATCG letter system. There are 4 others with somewhat different structures.
2) The central cyclic pentagonal structure with the O atom at the crown. That is the
ribose sugar.
3) Finally the
Phosphate group (with phosphorus P in the middle) that acts as a bridge linking the different units in a long chain.
Now the question is how do you make this reasonably complicated molecule naturally?
For
40 years nobody could do it, and it is instructive to see why.
1)We have 3 distinct groups and every chemist who looks at it would think that the best way to
design such a thing is to make the 3 groups separately and join them together.
2)Unfortunately for chemists (like me),
Nature is not a designer and does not have foresight. Nature does not go about thinking "
I must synthesize X-Y-Z at the end; so let me make X then Y and then Z and then paste them together like the proper intelligent designer I am. " Nature, always goes through the
path of least resistance, i.e. the path that requires the least amount of energy. And that path is often not how humans would think is logical.
3)Thus all efforts by chemists for 40 years to make phosphate, ribose sugar and nucleobase separately and then join them together resulted in useless mess of all sorts of undesirable compounds.
This went on till 2009 (with creationists referring to this failure in glee) when a group in UK (Sutherland group, Cambridge) decided to
mix up nitrogen and oxygen chemistry and try to create a sugar-base hybrid in presence of phosphates from the very beginning.
And lo and behold, the entire reaction happened quickly and efficiently and created the RNA cytosine and RNA uracil units in just 5 steps and extremely efficiently!
What starting compounds do we need? They are very simple if completely bonkers:-
1)
Hydrogen Cyanide (HCN) that most poisonous of all gases turns out to be the prime ingredient. HCN is found to form quite a bit in the oxygen poor CO2-N2 rich early atmosphere of earth when high impact meteors and comets were striking early earth.
2)
Formaldehyde (HCHO) and
acetylene (C2H2) the simplest of hydrocarbons also created in the atmosphere of early earth by photolysis of CO2 and H2O by the UV radiation of the sun (no ozone layer then) .
3)
Hydrogen Sulphide (H2S), the most ubiquitous gas coming out of volcanoes with the smell of rotten eggs. And ancient earth was much more geologically active.
4) Water soluble
phosphate that was leaching out of the iron-nickle meteors and comets that were falling to earth every month in those ancient times.
5) Copper and Zinc minerals, widely present in earth, as catalysts.
6) Lots of
UV rays (no ozone layer then)
7) Periodic wet and dry conditions with temperatures ranging from 40 C - 5 C, very normal even for early earth.
Under these simple conditions Hydrogen Cynaide reacts with formaldehyde in the presence of H2S and and Copper salts to produce two simple sugars:-
Glycolaldehyde (CHO-CH2-OH) and
Glyceraldehyde (CHO-CH(OH)-CH2-OH) along with ammonia (NH3) .
Next Hydrogen Cyanide reacts sequentially with the two sugars above in presence of phosphate as catalyst at 40 C(like sodium phosphate) to create Nitogen-Oxygen hybrid ring structures called
oxazoles. These oxazoles crystallize out of the water mixture on slight cooling.
(carbon atoms are at the vertices of the pentagon).
These oxazoles then react with cyanoacetylene (a compound formed by HCN reacting with C2H2) in the presence of phosphate and UV rays to directly create the RNA cytosine and uracil base.
So very briefly
2HCN + simple sugars ⇒ Oxazoles (in presence of phosphates)
Oxazoles + Cyanoacetylene + Phosphate ⇒ RNA units
The full pathway is shown below for reference.
I think the example teaches several things:-
1) God of the gaps
always fails. Even in 2007, the leaders of the prebiotic chemistry field were saying RNA synthesis was
impossible . There should be a permanent ban of scientists saying that unless they can formulate a law of physics that supports his case. Just because you can't do it does not make it impossible.
2) Nature has no foresight.
Economy of means ,not neat categorical planning, is the hallmark of natural phenomena. Who would have thought lethal hydrogen cyanide and sugar-base hybrid would be the means by which nature would go about creating the building blocks of life?
3) In nature, one does not have sterilized labs where only one or two compounds are present at one time. So the way forward is
systems chemistry, where most of the reagents are concurrently present and influence the reaction steps through catalytic action.
I am ending with a brief talk by the lead author of the paper (which should be intelligible now
) and the small fact that in the last 6 years the group has gone on to use those above simple reagents to construct in the lab a chemical reaction cascade that very simply and effectively produces 3 of the 5 RNA/DNA units, 12 of the 20 amino acids and the building blocks of all lipid cell membranes.
Thus now its an established fact that 70% of all the basic building blocks that life needs for its RNA-DNA-proteins and cell walls can be easily formed at one go in the prebiotic conditions of the early earth even before any evolution. They can now be created in a space of two days in a beaker with a little heating and cooling and a UV lamp once the way of thinking about how to go about the process changed.
Here is the full cascade. Its beautiful but would take me a month to explain. The snippet I tried explaining here is the one with the blue arrows (2-3-5-7-9-10).
Here is a brief and easy to understand talk on this research.
The paper
Prebiotic chemistry: a new modus operandi | Philosophical Transactions of the Royal Society B: Biological Sciences