Origin of life and start on Earth: Hypothesis of the primitive RNA and panspermia. 2o Part Disclosure 33
"In the first part of this blog we noted 11 facts and time of occurrence, since the formation of our protosolar nebula (-5000 million years ago) until the appearance of the first multicellular organisms on Earth (-1.7 billion years) .The explanation on as they occurred and when they happened, no major difficulties except your own dating, but there are two facts that are difficult to explain much about how, when and why they happened.
The two main objectives of this blog are to analyze the facts:
1. Firstly the development of complex organic compounds on Earth to be formed primitive RNA and membranes
2. Secondly as organized these elements to give rise to the formation of the first forms of life
To start our discussion we must stop at the definition of ribonucleic acid, or RNA. Treating a nucleic acid formed by a chain of ribonucleotides and which in turn is formed by a monosaccharide molecule of five carbons (pentose) called ribose, a phosphate group, and one of four possible nitrogen compounds called bases: adenine, guanine, uracil and cytosine.
Consequently to find the source of the RNA we should focus on the origin of its three main components: The pentose, nitrogenous bases and phosphate groups.
The pentoses are monosaccharides formed by a chain of five carbon atoms (C5H10O5). The composition of RNA known as a pentose find ribose while the composition is furanose DNA (deoxyribose is the form of ribose derivative of the loss by the second oxygen atom in the hydroxyl)
The nitrogenous bases are cyclic organic compounds which include two or more nitrogen atoms. They are classified into three groups: purine bases, pyrimidine bases and bases isoaloxazinicas. In RNA include the purine bases adenine (A) and guanine (G) the pyrimidine bases cytosine (C) and uracil (U) while the composition of the uracil DNA has been replaced by thymine (T). Isoaloxazinicas bases represented by the flavin is not part of the composition of RNA but have special importance in the mitochondria, cells oxidative endosymbiotic as flavin mononucleotide or phosphate group bonded to an adenine nucleotide (flavin adenine dinucleotide).
Of the union of the pentose ribose with purine bases (A and G) and pyrimidine (C and U) some macromolecules are produced nucleosides: adenosine, guanosine, cytidine and uridine.
Each one of these macromolecules may be combined with a phosphate group, a compound ion central phosphorus atom is surrounded by four oxygen atoms in tetrahedral arrangement, producing other macromolecules called nucleotides. This may not have been directly because it requires the presence of proteins called kinases that transfer phosphorus through exchange with magnesium or manganese.
When the macromolecules are nucleotide chains of themselves, by phosphodiester bonds, leading to a polymer called nucleic acid (if it contains ribose and uracil was called RNA or DNA if it contains furanose and thiamine).
In conclusion, the emergence of primitive RNA on Earth required the prior formation of the pentose ribose, the purine bases adenine and guanine, the pyrimidine bases cytosine and uracil, possibly protein kinase and phosphate groups. All these molecules combined give rise to intermediates nucleosides: adenosine, guanosine, cytidine and uridine, nucleotides and finally to the nucleic acids that are responsible for the storing of genetic information of living organisms and inheritance.
These considerations about the chemical composition and the members of the RNA us wonder when, how and why it was formed and we must get into three scenarios: the RNA world hypothesis, the hypothesis pre-RNA world and the panspermia hypothesis
Each of them looking for explanations for the existence of RNA but until now none has been able to overcome all the issues raised experimentally and therefore having no demonstration can not become a solid theory to explain the phenomenon that.
The RNA world hypothesis defends the possibility that RNA could be formed on the early Earth from locally produced organic compounds, but collides with some difficulties due to the chemical characteristics of the ribose and cytosine. The hypothesis of the pre-RNA world looking for an alternative explanation for these difficulties and considers the primitive RNA, also of local production, was formed by a tetrose, unlike the pentose ribose, called threose and other nucleotide derived from glycerol. This hypothesis is also without demonstration. Faced with the difficulty of synthesizing RNA arose early panspermia hypothesis which argues that the primitive RNA could not form on Earth and it came in the form of free RNA carried by meteorites or in primitive cellular structures that developed on Earth to find a suitable environment. Nevertheless this hypothesis does not explain or propose specific biochemical mechanism.
The RNA world hypothesis is based on the notion of abiogenesis, Greek term, which accepts the possibility of the origin of life from non-existence. In this sense, the consensus is that the life began its existence from lifeless matter sometime between 4400 million years, when the conditions of constant existence of liquid water, and 2700 million years. According to this hypothesis the complex organic material could be more likely in two environments:
a) At sea or in freshwater the aldehyde phosphate with hydroxides can produce phosphorylated sugar that could be the precursor RNA
b) hot water boilers CO and methyl groups, in the presence of pyrite, can cause acetic acid.
However there are serious difficulties since, first, that there are no chemical pathways for abiogenic synthesis of nucleotides from the pyrimidine bases cytosine and uracil under prebiotic conditions, second, the nucleoside cytosine has a half life of 19 days at 100 ° C and 17,000 years in freezing water, which is very short geological time scale, third, the ester bond between the ribose and phosphoric acid in RNA is prone to hydrolysis and, fourth, ribose has to be all enantiomer thereof, for any nucleotide with a different chirality would act as chain terminator.
A consequence of the lack of progress in the synthesis of organic substances needed in the conditions of the primitive earth scientific community has presented an alternative hypothesis called the hypothesis of the pre-RNA world that while accepting the concept abiogenesis is inclined to think that the first RNA was chemically simplest based upon binding of a tetrose with a nucleoside derivative of glycerol produced from the acetic acid.
This assumption simplifies some of the problems but does not explain how they finally could get to the composition of primitive RNA.
Consequence of the 4 difficulties of the RNA world hypothesis and the hypothesis of the pre-RNA world is returned to the idea that organic matter could not form on Earth and that this organic material arrived on the planet, as well as other planets and planetary systems, from a foreign origin. The panspermia hypothesis therefore rejects the concept abiogenesis and believes that the organic material isolated or organized came to Earth at the same time of the formation together with other materials or subsequently during the time of meteorites between -4500 and -3.7 billion years. This hypothesis has as precursors to Herman Richter, Fred Hoyle and Svante Arrhenius so that between 1865 and 1908 was formed the first scheme, the concept of the origin of life from outer space, which was from the beginning based criticism, first , cells die in space because of the radiation, second, the cells can not remain viable during the millions of years needed for interstellar travel, third entry through the layer of the atmosphere, or the impact with the land, would sterilize any materials, fourth, there is insufficient evidence that complex organisms are composable in space and, fifth, there is very low water on other planets and moons.
However, all these issues have been refuted, and are becoming more numerous evidences on the possibility of panspermia possible. We briefly mention some of them
1) In the comets are water, ammonia, formaldehyde and hydrogen cyanide that can produce amino acids. It is believed that comets could provide, for 800 million years, these materials to Earth
2) A known as Murchison meteorite containing uracil and xanthine, two predecessors of the molecules that form RNA and DNA
3) meteorite ALH84001, generally considered to be originated on Mars suggests that has structures that could have been caused by microscopic life forms. Mars was evolved 16 million years ago and landed in Antarctica 13,000 years ago.
4) Streptococcus mitis that were brought to the moon by accident in the Surveyor 3 in 1967, could be revived without difficulty when they came back to Earth three years later
5) Experiments in which recreates conditions when comets bombarded Earth, amino acids not only not destroyed, but begin to form peptides.
6) In the rings of several planets in our solar system have been detected organic compounds including methane, ethane, diacetylene, methylacetylene, cianoacetileno, acetylene, propane, along with carbon dioxide, carbon monoxide, hydrogen cyanide and cyanogen.
7) The atmosphere of some moons in our solar system have some differences of hydrogen and acetylene concentrations between the upper layers and the surface can only be explained by the existence of biochemical processes related to early life that have also been shown in some organisms land.
In conclusion we note that this hypothesis did not explain, so far as they formed or where they were formed in the organic compounds that came to Earth but in recent years there have been two vital discoveries in this direction:
1) has been shown interstellar dust, formed by particles of less than one micrometer, contains large amounts of hydrogen, carbon, silicates, water and organic compounds such as aliphatic hydrocarbons containing aromatic benzene rings
2) The study of infrared emissions detected in stars, interstellar space and galaxies, known as spectral signatures have concluded the Spitzer Infrared Space Observatory that other substances, more complex aliphatic aromatic hydrocarbons, are the generators of these emissions and that in these spaces are produced in large quantities of organic compounds in very short time scales.
It is therefore considered found that simple and complex organic compounds related to the origin of life, can be synthesized in space even in the absence of life forms. This source of organic material has been able to reach the entire universe from the first moment of the Big Bang 13,700 million years ago but has needed to develop appropriate conditions according to the laws of the natural selection on a universal scale. "