The infinite variety: the beginning of life
Interpretation of the Miller-Urey Experiment
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- How valid was the Miller Urey Experiment
- DNA the blueprint for life
Interpretation of the Miller-Urey Experiment The molecules produced form this experiment were relatively simple organic molecules, far from a complete living biochemical system, but the experiment established that natural processes could produce the building blocks of life without requiring life to synthesize them in the first place. With time these substances probably increased and interacted with each other to form more complex molecules. Eventually one substance essential to life as we know it appeared. This substance was called deoxyribonucleic acid or DNA. This experiment inspired many experiments in a similar vein. In 1961, Joan Oro found that amino acids could be made from hydrogen cyanide (HCN) and ammonia in a water solution. He also found that his experiment produced a large amount of the nucleotide base adenine. Experiments conducted later showed that the other RNA and DNA bases could be obtained through simulated prebiotic chemistry with a reducing atmosphere. 
Conditions similar to those of the Miller-Urey experiments are present in other regions of the solar system, often substituting ultraviolet light for lightning as the driving force for chemical reactions. On September 28, 1969, a meteorite that fell over Murchison, Victoria, Australia was found to contain over 90 different amino acids, nineteen of which are found in Earth life. Comets and other icy outer-solar-system bodies are thought to contain large amounts of complex carbon compounds (such as tholins) formed by these processes, in some cases so much so that the surfaces of these bodies are turned dark red or as black as asphalt. The early Earth was bombarded heavily by comets, possibly providing a large supply of complex organic molecules along with the water and other volatiles they contributed. (This could also imply an origin of life outside of Earth, which then migrated here. See: Panspermia) How valid was the Miller Urey Experiment? There have been a number of objections to the implications derived from these experiments. The following are extracts from Wikipedia: Originally it was thought that the primitive secondary atmosphere contained mostly NH3 and CH4. However, it is likely that most of the atmospheric carbon was CO2 with perhaps some CO and the nitrogen mostly N2. The reasons for this are (a) volcanic gas has more CO2, CO and N2 than CH4 and NH3 and (b) UV radiation destroys NH3 and CH4 so that these molecules would have been short-lived. UV light photolyses H2O to H· and ·OH radicals. These then attack methane, giving eventually CO2 and releasing H2 which would be lost into space. In practice gas mixtures containing CO, CO2, N2, etc. give much the same products as those containing CH4 and NH3 so long as there is no O2. The H atoms come mostly from water vapor. In fact, in order to generate aromatic amino acids under primitive earth conditions it is necessary to use less hydrogen-rich gaseous mixtures. Most of the natural amino acids, hydroxyacids, purines, pyrimidines, and sugars have been produced in variants of the Miller experiment. More recent results may have called this into question, however. Simulations done at the University of Waterloo and University of Colorado in 2005 indicated that the early atmosphere of Earth could have contained up to 40% hydrogen, implying a much more hospitable environment for the formation of prebiotic organic molecules. The escape of hydrogen from Earth's atmosphere into space may have occurred at only 1% of the rate previously believed based on revised estimates of the upper atmosphere's temperature. One of the authors, Prof. Owen Toon notes: "In this new scenario, organics can be produced efficiently in the early atmosphere, leading us back to the organic-rich soup-in-the-ocean concept... I think this study makes the experiments by Miller and others relevant again." Outgassing calculations using a chondritic model for the early earth, (Washington University, September 2005) complement the Waterloo/Colorado results in re-establishing the importance of the Miller-Urey experiment. Other views Although lightning storms are thought to have been very common in the primordial atmosphere, they are not thought to have been as common as the amount of electricity used by the Miller-Urey experiment may imply. These factors suggest that much lower concentrations of biochemicals would have been produced on Earth than was originally predicted (although the time scale would be 100 million years instead of a week). Similar experiments, both with different sources of energy and with different mixtures of gases, have resulted in amino and hydroxy acids being produced; it is likely that at least some organic compounds would have been generated on the early Earth. However, as soon as oxygen gas is added to the mixture, no organic molecules are formed. Recent research has been seized upon by opponents of Urey-Miller hypothesis which shows the presence of uranium in sediments dated to 3.7 Ga and indicates it was transported in solution by oxygenated water (otherwise it would have precipitated out) (Rosing & Frei 2004). It is wrongly argued by some, in an attempt to invalidate the hypothesis of abiogenesis, that this presence of oxygen precludes the formation of prebiotic molecules via a Miller-Urey-like scenario. However, the authors of the paper are arguing that the oxygen is evidence merely of the existence of photosynthetic organisms 3.7 Ga ago (a value about 200 Ma earlier than current values), a conclusion which would possibly have the effect of pushing back the time frame in which Miller-Urey reactions and abiogenesis could potentially have occurred, it would not preclude them in any way. Though there is somewhat controversial evidence for very small (less than 0.1%) amounts of oxygen in the atmosphere almost as old as Earth's oldest rocks the authors are not in any way arguing for the existence of a strongly oxygen containing atmosphere occurring any earlier than previously thought, and they state:"..In fact most evidence suggests that oxygenic photosynthesis was present during time periods from which there is evidence for a non-oxygenic atmosphere". 
http://biology.clc.uc.edu/courses/bio106/origins.htm (requires Netscape to do interactive parts) DNA the blueprint for life This molecule can act as a blueprint for the manufacture of amino acids and has the capacity to replicate itself. Such properties occur in all life as we know it including the simplest forms such as bacteria. DNA's ability to replicate itself is due to its double helix structure. During cell division, the DNA molecule splits longitudinally, and each side acts as template to which simpler molecules become attached until each half has once more become a double helix. The simple molecules from which DNA is built are of four kinds and are grouped in trios, and these can be abbreviated A, T, C, and G representing Adenine, Thymine, Cytosine and Guanine respectively. These arranged in particular and significant orders. Each base can only "pair up" with one single predetermined other base: A+T, T+A, C+G and G+C are the only possible combinations; that is, an "A" on one strand of double-stranded DNA will "mate" properly only with a "T" on the other, complementary strand. Because each strand of DNA has a directionality, the sequence order does matter: A+T is not the same as T+A, just as C+G is not the same as G+C; For each given base, there is just one possible complementary base, so naming the bases on the conventionally chosen side of the strand is enough to describe the entire double-strand sequence. These sequences of amino acids on the immensely long DNA molecule specifies how various amino acids are arranged in a protein, and how much protein is to be synthesized. A length of DNA bearing the information for an unbroken sequence of manufacture is called a gene. Occasionally, the DNA copying process goes wrong. A mistake may be made at a single point on the length of the DNA and a particular molecule may become temporarily dislocated and be re-inserted in the wrong place. The copy is then imperfect and the protein that it synthesizes will be different. Such mistakes are sources of variation from which natural selection can produce evolutionary change. We now know that photosynthesising organisms had evolved as long ago as 3700 million years. 
Schematic representation of the DNA which illustrates its double helix structure Download 2.01 Mb. Share with your friends:
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