Question

Which came first, nucleic acid or protein?

Within the boundaries of life as we know it, it takes many mature, fully-functional proteins, using RNA as an intermediate, to create any new protein from the DNA that codes for it; hence the central dogma of molecular biology: DNA to RNA to PROTEIN. Having said that, no one knows which came first, DNA/RNA or protein. A current hypothesis deals with an RNA world as the starting point, since RNA has a structure that demonstrates the complementary bonding strategy of nucleic acids (for transmitting genetic information), but can also have the functionality of a protein. Such RNA’s are called ribozymes for their enzymatic activity. Maybe an RNA was the first 'enzyme'. Maybe an RNA-protein hybrid was a transitional intermediate. There are many such functional hybrids existing in cells today such as the small nuclear ribonucleoproteins, or, snRNP’s, which help process messenger RNA, and the ribosome, a cellular organelle which carries out protein synthesis. The alternative idea that protein may be able to beget protein without the use of nucleic acids, comes from the provocative work of NAI chemist Reza Ghadiri of Scripps Oceanographic Institution. He has manufactured small, enantiomerically pure peptides which, when presented with a racemic mixture of amino acids, can aid in the polymerization of other enantiomerically pure peptides. Also interesting is the discovery of virus-like particles called prions that appear to contain only protein. But at this point in their evolution they are parasites, like viruses, and only arguably 'alive'. While the question of heredity in such a creature remains unaddressed, the 1952 Hershey-Chase experiment secured the idea that DNA, not protein, is in fact the molecule of heredity; that it’s an organisms DNA that is transferred to the next generation. How could life have arisen a lethal chemical environment such as a hydrothermal vent? Much of the current research focuses on determining how the chemistry of the available components might have operated under the conditions existing at the time. Super-heated vent fluid containing a multitude of dissolved minerals and molecules would have been flowing out of the chimney and mixing with the surrounding cold seawater. Further, the pressure of the vent fluid would have changed as it was forced through the chimney. The temperature and pressure differential of this system offers unique opportunities for uncommon chemistry to take place. The problem, however, is how the various pre-biotic compounds such as nucleic acids and amino acids were able to congregate locally rather than being swept away in the vast 'primordial soup.’ The idea of lipid molecules (fats) forming primitive membranes which served to separate the important molecules from the environment and provide them with a space to interact, is an attractive theory to resolve that problem. Also under consideration is the idea that crevasses and pores in the rock of the chimney may have served as secluded chambers in which different combinations of molecules and minerals could congregate and have a chance to react. Indeed, the contents of each such "natural test-tube" may have experienced vastly different temperature and pressure conditions depending upon where it was in relationship to the chimney, thus providing a myriad of tiny experiments in the pathway toward life.
May 1, 2002