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THE ORIGINS OF LIFE Biochemical evolution © 2010 Paul Billiet ODWSODWS
Biochemical evolution Life appeared after a period of chemical reactions According to physical and chemical laws. © 2010 Paul Billiet ODWSODWS
Prebiotic Earth Conditions on the primitive Earth were not the same as those present today No spontaneous generation of life today (Pasteur)… because the necessary conditions no longer exist. © 2010 Paul Billiet ODWSODWS J.B.S. Haldane (credit: Bassano and Vandyk Studios) Image Credit A.I. Oparin A.I. Oparin
Conditions on early Earth Reducing atmosphere on the primitive Earth. No free oxygen (O 2 ) Free hydrogen (H 2 ) and saturated hydrides (CH 4, NH 3 and H 2 O) Energy for chemical reactions between these gases could come from electric discharge in storms or solar energy (no ozone layer) The Earth’s surface temperature probably hotter than today. © 2010 Paul Billiet ODWSODWS
The formation of monomers Miller and Urey recreate these conditions in vitro The water is heated and the mixture circulates for many days. H 2 NH 3 CH 4 H 2 O H2OH2O Heat Electric discharge Samples © 2010 Paul Billiet ODWSODWS
Results After a week 15 amino acids in the mixture Other biologically important molecules had been formed including ethanoic acid, lactic acid and urea Later similar experiments were done using CO 2 that produced nucleotides. Image Credit Stanley MillerStanley Miller © 2010 Paul Billiet ODWSODWS
Conclusion These experiments cannot reproduce the exact conditions on the primitive Earth We shall never know exactly what happened BUT it can be shown that the basic building blocks for the large macromolecules can be synthesised in vitro from inorganic compounds. © 2010 Paul Billiet ODWSODWS
Comets Comets are dirty snowballs Formed in the outer part of the solar system Contain many organic molecules. Image Credit Comet Hale-Bopp 1997 Comet Hale-Bopp 1997 © 2010 Paul Billiet ODWSODWS
Comet Shoemaker-Levy Collision with Jupiter Image Credit July 16 through July 22, 1994 July 16 through July 22, 1994 © 2010 Paul Billiet ODWSODWS
‘Comets giveth and comets taketh away’ Carl Sagan ( ) Jan 2006 comet dust sampled Wild-2 comet Organic compounds, including glycine, found. Image Credit Stardust mission 2006 Stardust mission 2006 © 2010 Paul Billiet ODWSODWS
From monomers to polymers Amino acids into polypeptides, could have occurred when dry or highly concentrated monomers are heated Condensation reactions take place forming peptide bonds between amino acids or phosphodiester bonds form between nucleotides. © 2010 Paul Billiet ODWSODWS
Early catalysts As molecules adsorb to the clay mineral particles they become concentrated (stick to the surface particles) These clay particles (coacervates) may have been essential catalysts in the formation of polymers. © 2010 Paul Billiet ODWSODWS
The first polynucleotides Once formed polynucleotides show a tendency to copy themselves using complementary base pairing This was probably catalysed by the presence of clay particles and metal ions These single stranded polynucleotides would have been the equivalent of RNA. © 2010 Paul Billiet ODWSODWS
The first hereditary information RNA was probably the first hereditary molecule having the ability to copy itself RNA shows enzymic (catalytic) properties – called ribozymes Polynucleotides are very good molecules at storing and transmitting information but they lack the versatility for all the chemical functions of a cell. © 2010 Paul Billiet ODWSODWS
A great partnership Polypeptides, which can form complex 3- dimensional structures (proteins), are much better at complex cell functions A partnership must have formed between the polynucleotides and the polypeptides The polynucleotides directed the synthesis of the polypeptides Today it is clear that information only flows from polynucleotides to polypeptides. Translation had started. © 2010 Paul Billiet ODWSODWS
The origin of DNA Later the hereditary information was probably stored in the form of DNA which is more stable than RNA The passage of information from RNA to DNA is possible in nature The reverse transcriptase enzyme of the retro viruses shows this. Image Credit: DNA DNA © 2010 Paul Billiet ODWSODWS
The first membranes, the first cells If a piece of RNA codes for a particularly good protein then there is nothing to stop that protein being used by other RNA molecules If however the RNA is enclosed in a membrane then it can keep it’s protein to itself and gains a selective advantage So membranes probably pushed evolution by natural selection forwards. © 2010 Paul Billiet ODWSODWS
Membranes defined the first cell The phospholipids form lipid bilayers when they are surrounded by water All the components of a simple prokaryotic cell were now assembled They diversified in their metabolism By 2 billion years ago free oxygen was appearing in the atmosphere due to the activity of photosynthetic bacteria. © 2010 Paul Billiet ODWSODWS
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