The Synthesis of Organic Materials ( Life In The Universe)

We have seen that the primitive atmosphere of the Earth would have contained, at a very early stage, organic compounds of varying degrees of complexity. Indeed, the picture described so far is likely to be a fairly common one wherever planetary systems have formed; no extraordinary events have been postulated. The next step must be the construction of the proteins and nucleic acids that are the basic building blocks of terrestrial life. PROTEINS are long chains of organic compounds called AMINO ACIDS and the NUCLEIC ACIDS are long strings of compounds referred to as bases. We must first understand how the amino acids and bases them¬selves formed, and then ascertain the conditions under which they may form these long chain-like structures (POLYMERS). Two processes are needed to achieve this; there must be an enrichment of the appropriate organic material, and then it must be concentrated to the point where polymers can be formed. The enrichment process is facilitated by the absence of free oxygen and the fact that the atmosphere is exposed to a number of sources of energy that can promote the formation of molecules. Examples of such energy. sources are: solar ultraviolet radiation, volcanic activity, meteoritic bombardment of the atmosphere, cosmic rays, and lightning from atmospheric thunderstorms. .Some twenty-five years ago, Stanley Miller performed some fundamental experiments demonstrating that the action of electrical discharges on a mixture of water vapour, methane and ammonia could produce reasonable yields of simple amino acids (glycine, HCHNH2, COOH and alanine CH3 CHNH2 COOH. Subsequent experiments along the same lines by Leslie Orgel have confirmed Miller’s findings, and shown that most of the smaller building blocks of biochemistry are also produced in this way. As an aside, it is of interest to note that the Miller-Orgel experiments .simulated an environment rather similar to the lower atmospheric regions of Jupiter, where it is known that there are violent thunderstorms. Accordingly we would expect to find basic biochemical substances there: Jupiter may well be the best candidate in the Solar System for rudimentary extraterrestrial life!

Heavy molecules formed in the Earth’s atmosphere drift down¬wards to the Earth’s surface. These add to the stock of molecules put there by volcanic activity, or deposited by meteoritic and cometary impact. There are then a number of processes whereby these amino acids and bases may be concentrated and turned into biologically important polymers, the proteins and polypeptides. The evaporation of molecule-containing lakes, or even the freezing of such lakes, could increase the concentration to the point where polymerization can take place. Absorption of organic molecules onto clay or mineral surfaces could have the same effect. The basic problem, however, is more complicated than merely obtaining polymers of amino acids and bases: we need the right polymers, the proteins and nucleic acids. A considerable amount of research effort has gone into the synthesis of organic molecules from primitive conditions. We have just mentioned Miller-Orgel experiments that produced amino acids, sugars and fatty acids under conditions that one might readily accept as being primitive. On the other hand, although the nucleotides and pyrimidmes which are basic elements of DNA and RNA have been produced in some experiments, it is not generally agreed that the conditions imposed were really primitive. Moreover, no experiment has yet produced proteins or nucleic acids spontaneously.

This is a crucial point in the story. It has often been argued that once the proteins and nucleic acids assembled, then we have made the transition from chemical evolution to organic evolution. This is the origin of life since, before this point, everything that happens does so merely by chance. So let us look more closely at the question: what is so special about the proteins and nucleic acids? At this stage, let us take it for granted that amino acids and bases can be synthesized in the primordial soup. We have seen how amino acids may join up with one another to form polymers of amino acids. The simple chains of amino acids are called POLYPEPTIDES. The proteins, the fundamental constituents of living cells, are very special polypeptides. Not only does each protein have a very specific construction, but it is folded into a special three-dimensional configuration which it maintains as long as it is biologically active. The properties of the protein molecule cannot be deduced from its chemical constitution alone; the properties depend crucially on what parts of the molecule lie close to one another in virtue of the three-dimensional structure. Many proteins can act in such a way as to speed up certain chemical reactions, without themselves being affected by the reaction. Such proteins are called ENZYMES : they catalyze organic reactions. During that period of the Earth’s history when polypeptides were being made, some were perhaps easier to synthesize than others, and a few of these may in turn have acted as catalysts for other chemical reactions. We might therefore envisage a kind of natural selection operating at a pre-biological stage, favouring the formation of a small class of important proteins.

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