What is Life?:How chemistry becomes biology by Pross Addy

Author:Pross, Addy [Pross, Addy]
Language: eng
Format: epub
Publisher: Oxford University Press
Published: 2012-08-01T20:00:00+00:00

‘Replication first’ scenario

As noted above, the ‘replication first’ scenario for the origin of life rests on the idea that life originated with the emergence of some oligomeric self-replicating entity and that replicating entity then proceeded to mutate and complexify until it became transformed into some minimal life form. Historically that idea can be traced back as far as 1914, to an American physicist, Leonard Troland, but that scenario was given a major boost through the contributions of Sol Spiegelman in the late 1960s that we described earlier. Within a short period of time those ideas were given further support through the pioneering works of Manfred Eigen and Peter Schuster in the 1970s.44 Central to ‘replication first’ thinking was the proposal of an RNA-world that preceded the interdependent world of nucleic acids and proteins which forms the basis of all modern life.45 A key attraction of the RNA-world scenario was that it appeared to solve the long-standing ‘chicken and egg’ dilemma with respect to the dual world of nucleic acid and protein. All modern life forms depend critically on this interdependence. DNA, the nucleic acid in which all heritable information is coded, cannot replicate without the elaborate involvement of protein enzymes, and those protein enzymes cannot be generated without the prior existence of the DNA molecule, which codes for those enzymes. So how could this dual world have come about? The RNA-world hypothesis appears to resolve this dilemma through its proposal that RNA originally functioned as both the carrier of genetic information and the provider of enzymatic activity. The fact that RNA can carry genetic information is not surprising. It is, after all, a nucleic acid closely related to DNA. But the discovery by two American researchers, Thomas Cech at the University of Colorado and Sidney Altman of Yale University, that RNA can also act as an enzyme and catalyse key biochemical reactions, gave the RNA-world viewpoint a major boost (as well as a Nobel prize to Cech and Altman). But the RNA-world view critically depends on the idea that a self-replicating molecule could have emerged spontaneously on the prebiotic earth, and that idea has continued to meet with opposition.

A central criticism of the ‘replication first’ scenario is based on the view that conditions on the prebiotic earth were not consistent with the spontaneous emergence of a molecule possessing a self-replicating capability. However, as discussed earlier, this view has no sound basis. The term ‘prebiotic conditions’, so frequently quoted in the origin of life literature, may convey some general information, but is totally devoid of specific information and so cannot be used to rule out any process, if that process is consistent with the basic rules of chemistry. Replicating molecules can be synthesized in the lab, so their spontaneous appearance on the prebiotic earth cannot just be dismissed ad hoc. Our ignorance regarding the prebiotic earth means that we cannot rule out the possibility that such an entity did in fact emerge on the prebiotic earth.

A more fundamental problem with the

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