Naked Science Forum
Life Sciences => Cells, Microbes & Viruses => Topic started by: granpa on 04/07/2016 02:36:54
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A pre-RNA world Genetic_code with only a single (but reversible) nucleotide (or rather nucleobase) which also happens to be an amino acid!
Specifically 3-Aminobenzoic_acid (image below is the Zwitterion form)
In the zwitterion form the acid (COOH) has donated a proton to the base (NH2)
http://en.wikipedia.org/wiki/PAH_world_hypothesis#Polycyclic_aromatic_hydrocarbons are the most common and abundant of the known polyatomic molecules in the visible universe, and are considered a likely constituent of the primordial sea. #PAHs , along with fullerenes (or "buckyballs"), have been recently detected in nebulae.
PAHs are not normally very soluble in sea water, but when subject to ionizing radiation such as solar UV light, the outer hydrogen atoms can be stripped off and replaced with a hydroxyl group, rendering the PAHs far more soluble in water.
These modified PAHs are amphiphilic, which means that they have parts that are both hydrophilic and hydrophobic. When in solution, they, like lipids, tend to organize with their hydrophobic parts protected. Unlike lipids though, they assemble into a 1 dimensional liquid crystal stack
Today there are four different types of #nucleobases which stack on top of one another like plates to form the RNA molecule
Perhaps in the original #PAH world there was only one type of #nucleobase but it could be put on the liquid crystal stack in one of two different orientations by simply flipping it over like a pancake.
I suspect that the original base looked something like this:
https://en.wikipedia.org/wiki/3-Aminobenzoic_acid (see image below of the Zwitterion form)
Note that this would have been both a nucleobase and an amino acid and is technically not a PAH.
It is soluble in boiling water but not in water at room temperature.
It was probably produced by intense lightning in a very thick atmosphere of hydrogen, methane, ammonia, and carbon dioxide.
There was so much lightning that most of the carbon was coverted to reactive compounds that ended up as carbonate rocks.
Attaching a carbohydrate monomer (perhaps originally another 3-Aminobenzoic_acid molecule) to its hydrophobic backside would have greatly increased its solubility in water as the atmosphere thinned and the earth cooled.
In todays cooler and thinner atmosphere lightning is relatively rare.
Eventually liquid crystal #PAH 's evolved sugar-phosphate backbones.
https://en.wikipedia.org/wiki/Genetic_code
Today #codons consist of 3 nucleotides and therefore have 64 possibilities
These code for 20 #amino_acids plus a start and stop #codon .
If there was originally only one base, as suggested above, then there were at that time only 4 distinct triplets
001
010
100
111
Which could simply be flipped over to make
011
101
110
000
You only need to know the first 2 bases (and the orientation) to select the right triplet
All of the codons that consist entirely of #Cytosine and #guanine only use the first two bases and completely ignore the third
http://study.com/cimages/multimages/16/Cytosine_Guanine_base_pair.png (note the similarity to 3-Aminobenzoic_acid)
These code for:
https://en.wikipedia.org/wiki/Glycine (the simplest possible amino acid)
https://en.wikipedia.org/wiki/Alanine
https://en.wikipedia.org/wiki/Arginine
https://en.wikipedia.org/wiki/Proline
phosphorus is part of the backbone of RNA. Liquid crystal PAHs lack a backbone. Maybe this is why they haven't found phosphorus in nanobes
https://en.wikipedia.org/wiki/Miller%E2%80%93Urey_experiment
A typical tRNA has 75 bases.
2^75 = 3.77 * 10^22
1 mole of tRNA would weigh 25 kg and contain 6 * 10^23 molecules
After a primordial strand replicated itself the two strands would have a strong tendency to remain together.
It is quite possible that the first useful molecule to evolve was an unzipper like the protein helicases
https://en.wikipedia.org/wiki/Coenzyme_A
http://www.chem.ucla.edu/harding/IGOC/N/nucleotide04.jpg
https://en.wikipedia.org/wiki/Citric_acid_cycle
(https://upload.wikimedia.org/wikipedia/commons/thumb/c/c2/3-Aminobenzoic-acid-zwitterion-nucleobase.png/277px-3-Aminobenzoic-acid-zwitterion-nucleobase.png)