Chemistry of Life
By analogy with contemporary living systems, some scientists believe that primitive life emerged as a cell-like organized system while others feel the development of the cell occurred at a subsequent step in the evolutionary process. The cell-like system required, at least, boundary molecules able to isolate the system from the aqueous environment (membrane), catalytic molecules providing the basic chemical work of the cell (enzymes) and information molecules capable to store and to transfer the information needed for reproduction (RNA).
Primitive membranes could initially have also been formed by simple terpenoids. Chemical agents, such as carbodiimides, are able to condense amino acids into peptides in water. Minerals have been used to enhance these reactions by binding the amino acids to their surfaces. Chemical reactions capable of selectively condensing the protein amino acids at the expenses of the non-protein ones have been described. When hydrophobic and hydrophilic amino acids alternate in a polypeptide chain, stereoselective and thermostable b-sheet structures are formed. Short peptides have been shown to exhibit catalytic properties.
Efficient clay-catalysed condensation of nucleotides into oligonucleotides has been achieved. Non-enzymatic replication of a preformed pyrimidine-rich oligonucleotide active as a template has been demonstrated. It is not generally agreed that ribonucleotides formed spontaneously on the primitive Earth. RNA-analogs containing much simpler structural units but capable of evolving to RNA are also being investigated.
A family of RNAs were shown to be able to act simultaneously as informative and catalytic molecules (ribozymes) and have been promoted as the first living systems on Earth (the RNA world). Strong evidence for this proposal has been obtained from the discovery that protein synthesis in the ribosome is catalyzed by RNA.
An alternative theory of the origins of life is the initiation of the formation of high concentrations of organics by autocatalytic processes which generated the informational molecules essential for life to begin. Minerals and metal ions may have had an important role in the processes.
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