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The RNA World

The beautiful DNA/protein duality of modern life, representing information and action, respectively, is also the big unsolved mystery of the origin of life. This is the proverbial "chicken-and-egg" problem. DNA requires protein enzymes in order to function properly, but those very proteins are coded into DNA and created by the action of other proteins. (See the section on Genetics for more). How did this cycle begin? The RNA World hypothesis attempts to solve this conundrum by placing the RNA molecule (and its precursors) at the beginning, where earlier forms of life relied solely on RNA to store genetic information and catalyze the necessary chemical reactions.

What are the current arguments in favor of the RNA World hypothesis?

First, the discovery of ribozymes proved the feasibility of this hypothesis - that a polymer that stores genetic information can simultaneously have the catalytic properties necessary to replicate itself. This breaks the protein-DNA "chicken-and-egg" impasse conceptually. Moreover, it gives us hope that we can soon achieve and understand RNA self-replication.

Second, it appears we now have a working prebiotic chemistry pathway to the synthesis of RNA nucleotides, and their polymerization into RNA strands - in naturally occurring planetary environments.

Third, similar environments enable the formation of vesicles from fatty acid membranes. These vesicles are permeable enough to let the nucleotide building blocks pass into the vesicle, and then keep the newly created RNA strands inside the vesicle. Thus the stage is set for the system to exhibit Darwinian evolutionary behavior.

The fundamental properties of RNA seem to support the RNA World hypothesis in many other ways. For example, recall from the section on Metabolism that the energy-storing molecule ATP and the adenine nucleotides of RNA synthesis are essentially the same. Also, the ribosome - the large molecular complex in modern cells that assembles proteins, is essentially a ribozyme, not a protein enzyme.

According to the RNA World hypothesis, later life evolved to use DNA and protein enzymes, because RNA is relatively unstable and has less efficient catalytic properties for replication. Ribozymes were gradually phased out with the evolution of complexes like the ribosome.

Солнечная система и ее тайны