The replicator first scenario :
There are two approaches to life’s origin, one is the replicator or genetics -first, the other, the metabolism first proposal.
Two different conceptual models for the origin of life, “genetics first” (left) and “metabolism first” (right). In each case, one begins with small organic molecules synthesized abiotically on the Earth or even in parent bodies of meteorites.
With genetics first, an information carrying molecule arises spontaneously and eventually controls chemical reactions in primitive cells.
With metabolism first, undirected networks of chemical reactions – autocatalytic cycles – evolve to progressively higher complexity until they develop information-carrying molecules that eventually control the production of catalysts. Adapted from a figure by Trefil et al. (2009), with notional nucleotide from Englehart and Hud (2010).
In metabolism first, one argues that certain structures could form spontaneously, capable of isolating parts of the chemical soup from the environment. These vesicles, if the right chemicals were present, could have become little factories of increasing chemical complexity, eventually growing and splitting in two but still lacking a reliable (or any) genetic code for reproducing the chemical activity within them.
" Could have become little factories ". Really ??!!
The other approach focuses on the genetic code, in particular, RNA, which might have been synthesized in the environment of the chemical soup, and once synthesized, multiplied and co-opted vesicles to form cells. Neither approach yet tells a convincing story, but both have led to some tantalizing suggestions as to how life could have arisen from complex, energetic chemical systems.
Thanks for the authors admitting the obvious. No, indeed. It's not a convincing story to claim that "certain structures could form spontaneously and become little factories ".
The synthesis of amino acids is a far cry from the construction of complex, self-replicating molecules that carry enough information to construct proteins from amino acids.
The "RNA World" is essentially a hypothetical stage of life between the first replicating molecule and the highly complicated DNA-protein-based life. The chief problem facing an RNA world is that RNA cannot perform all of the functions of DNA adequately to allow for replication and transcription of proteins.
New findings challenge assumptions about origins of life
There is currently no known chemical pathway for an "RNA world" to transform into a "DNA/protein world."
But for the hypothesis to be correct, ancient RNA catalysts would have had to copy multiple sets of RNA blueprints nearly as accurately as do modern-day enzymes. That's a hard sell; scientists calculate that it would take much longer than the age of the universe for randomly generated RNA molecules to evolve sufficiently to achieve the modern level of sophistication. Given Earth's age of 4.5 billion years, living systems run entirely by RNA could not have reproduced and evolved either fast or accurately enough to give rise to the vast biological complexity on Earth today.
The RNA world hypothesis: the worst theory of the early evolution of life
(i) RNA is too complex a molecule to have arisen prebiotically;
(ii) RNA is inherently unstable;
(iii) catalysis is a relatively rare property of long RNA sequences only; and
(iv) the catalytic repertoire of RNA is too limited.
Can the Origin of the Genetic Code Be Explained by Direct RNA Templating?
In order a molecule to be a self replicator , it has to be a homopolymer, of which the backbone must have the same repetitive units; they must be identical. On the prebiotic world, the generation of a homopolymer was however impossible.
Steven A. Benner, Ph.D. Chemistry, Harvard, prominent origin-of-life researcher and creator of the Foundation for Applied Molecular Evolution, was posted on Huffington Post on December 6, 2013. In it he said,
"We have failed in any continuous way to provide a recipe that gets from the simple molecules that we know were present on early Earth to RNA."
That lead Leslie Orgel to say :
It would take a miracle if a strand of RNA ever appeared on the primitive Earth.
(Dover, 1999, p. 218).
I would have thought it relevant to point out for biologists in general that not one self-replicating RNA has emerged to date from quadrillions (1024) of artificially synthesized, random RNA sequences
One of the major assumptions of the RNA world hypothesis is that in the primordial conditions, ribonucleotides spontaneously condense into polymers to form RNA molecules. Once RNA molecules have formed, by its catalytic activity to replicate itself a population of such self-replicating molecules would arise. “It is difficult to believe,” says RNA World research scientist Steven Benner, “that larger pools of random RNA emerged spontaneously without the gentle coaxing of a graduate student desiring a completed dissertation.” In addition, researchers believe that even if RNA could have formed spontaneously, the spontaneous hydrolysis and other destructive conditions operational on the early Earth would have caused it to decompose. Joyce and Orgel recommend that “…myth of a self-replicating RNA molecule that arose de novo from a soup of random polynucleotides. Not only is such a notion unrealistic in light of our current understanding of prebiotic chemistry, but it should strain the credulity of even an optimist’s view of RNA’s catalytic potential.”
Francis Crick confirms that, “At present, the gap from the primal “soup” to the first RNA system capable of natural selection looks forbiddingly wide.” Furthermore, RNA fails to perform all of the functions of DNA sufficiently to support replication and transcription of proteins. Consequently, Leslie Orgel pointed out the inability of the RNA world: “This scenario could have occurred, we noted, if prebiotic RNA had two properties not evident today: A capacity to replicate without the help of proteins and an ability to catalyze every step of protein synthesis.” Orgel further acknowledged that, “The precise events giving rise to the RNA world remain unclear … investigators have proposed many hypotheses, but evidence in favor of each of them is fragmentary at best. The full details of how the RNA world, and life, emerged may not be revealed in the near future.” Consequently the RNA world reverie appears to be dreadfully hopeless.
IUBMB Life. 2000 Mar;49(3):173-6.
A replicator was not involved in the origin of life.
Many scientific theories of the origin of life suggest that life began with the spontaneous formation of a replicator (a self-copying organic polymer) within an unorganized chemical mixture, or "soup." A profound difficulty exists, however, with the idea of RNA, or any other replicator, at the start of life. Existing replicators can serve as templates for the synthesis of additional copies of themselves, but this device cannot be used for the preparation of the very first such molecule, which must arise spontaneously from an unorganized mixture. The formation of an information-bearing homopolymer through undirected chemical synthesis appears very improbable. The difficulties involved in such a synthesis are illustrated by considering the prospects for the assembly of a polypeptide of L-alpha-amino acids, based on the contents of the Murchison meteorite as an example of a mixture of abiotic origin. In that mixture, potential replicator components would be accompanied by a host of interfering substances, which include chain terminators (simple carboxylic acids and amines), branch-formers, D-amino acids, and many classes of substances for which incorporation would disrupt the necessary structural regularity of the replicator. Laboratory experiments dealing with the nonenzymatic synthesis of biopolymers have not addressed the specificity problem. The possibility that formation of the first replicator took place through a very improbable event cannot be excluded, but greater attention should be given to metabolism-first theories, which avoid this difficulty.
Given the extreme difficulties associated with synthesizing an RNA molecule containing 200 or more bases, it is unlikely that even one such molecule ever existed on the primitive earth, and 15 trillion are needed to just get 65 functional ribozymes. Furthermore, ribozymes are not self replicators. The knowledge required for self replication is certainly many orders of magnitude more than the 44 bits required for a marginally functional ribozyme. Finally, the 44 bits calculated above is in a test tube where all competing side reactions are eliminated. If the real primordial soup contains free amino acids, aldehydes, and undesirable isomers of ribose, then the 44 bits will increase by a factor similar to the increase seen for the protein insulin in chapter 5. Taking this last factor into account, the 44 bits is at least one order of magnitude too small.
The odds of suddenly having a self-replicating RNA pop out of a prebiotic soup are vanishingly low," says evolutionary biochemist Niles Lehman of Portland State University in Oregon.
A New Study Questions RNA World
A new study in PLoS One shows that RNA and the proteins involved in protein synthesis must have co-evolved. This flies in the face of RNA-world theories, which presume that RNA formed first and that catalytic function (usually performed by proteins) was completed by catalytic RNA, known as ribozymes.
Researchers at the University of Illinois used phylogenetic modeling methods to evaluate the evolutionary history of the ribosome by correlating RNA structure and the ribosome protein structure. Their studies reveal several things of interest.
One of the assumptions in the RNA first hypothesis is that the active site of the ribosome, the peptidyl transferase center (PTC), which is the key player in protein synthesis, evolved first. However, Harish et al.'s studies reveal that the ribosome subunits actually evolved before the PTC active site and those subunits co-evolved with RNA, or what would eventually be sections of tRNA.
The authors conclude that their study answers some of the difficult questions associated with the RNA First World, while suggesting that there may have been a ribonuceloprotien primordial world:
Our study therefore provides important clues about the chicken-or-egg dilemma associated with the central dogma of molecular biology by showing that ribosomal history is driven by the gradual structural accretion of protein and RNA structures. Most importantly, results suggest that functionally important and conserved regions of the ribosome were recruited and could be relics of an ancient ribonucleoprotein world.
The RNA world or RNA-first hypothesis is arguably one of the stronger origin-of-life scenarios to date. While the field is still rife with inexplicable gaps in the progression from non-life to life, this hypothesis at least recognizes the fundamental need to explain the origin of the nucleotide sequence and subsequent coding for protein construction.
The cell has many types of RNA (messenger RNA, transfer RNA, ribosomal RNA, etc.), indicating that RNA can perform various functions. One particular function, when it was discovered, seemed to affirm notions that RNA may have preceded DNA and, therefore, preceded proteins. This function was the catalytic abilities of RNA. Catalysts, in short, speed a reaction. Proteins that act as catalysts are called enzymes, so catalytic RNA was thus named a ribozyme. Enzymes tend to be highly complex and specific to their particular functions.
The ribozyme seemed to answer the "chicken-or-egg" problem for origin-of-life theorists. Proteins are needed to make nucleic acids (RNA or DNA) and nucleic acids are needed to make proteins. Determining how this closed loop got started would provide answers to this most difficult origin-of-life conundrum.
However, while ribozymes were appealing in theory, they have many limitations that preclude their role as the initiators of early life. For example, RNA can cleave or link other RNA molecules, but this is only under specific laboratory conditions. Furthermore, RNA is limited in its capabilities compared to proteins. Ribozymes perform few functions, but protein synthesis requires multiple proteins, each often performing multiple functions.
This poses problems for how the first protein was produced. As the authors point out:
Thus far, in vitro peptidyl transferase activity catalyzed by protein-free rRNA derived from extant rRNA or ribozymes is not demonstrated. Perhaps, the primordial cooperative property of the RNP [ribonucleoprotein] complex explains why such attempts have failed.
In other words, the authors believe that the closely tied interaction between the ribosome and RNA cannot be separated.
Top Five Problems with Current Origin-of-Life Theories
the first RNA molecules would have to arise by unguided, non-biological chemical processes. But RNA is not known to assemble without the help of a skilled laboratory chemist intelligently guiding the process. New York University chemist Robert Shapiro critiqued the efforts of those who tried to make RNA in the lab, stating: "The flaw is in the logic -- that this experimental control by researchers in a modern laboratory could have been available on the early Earth."13
Second, while RNA has been shown to perform many roles in the cell, there is no evidence that it could perform all the necessary cellular functions currently carried out by proteins.14
Third, the RNA world hypothesis can't explain the origin of genetic information.
RNA world advocates suggest that if the first self-replicating life was based upon RNA, it would have required a molecule between 200 and 300 nucleotides in length.15 However, there are no known chemical or physical laws that dictate the order of those nucleotides.16 To explain the ordering of nucleotides in the first self-replicating RNA molecule, materialists must rely on sheer chance. But the odds of specifying, say, 250 nucleotides in an RNA molecule by chance is about 1 in 10150 -- below the "universal probability bound," a term characterizing events whose occurrence is at least remotely possible within the history of the universe.17 Shapiro puts the problem this way:
The sudden appearance of a large self-copying molecule such as RNA was exceedingly improbable. ... [The probability] is so vanishingly small that its happening even once anywhere in the visible universe would count as a piece of exceptional good luck.
The RNA world hypothesis: the worst theory of the early evolution of life (except for all the others)
1. Earth Evolution of a Habitable World, Second edition, page 152
Last edited by Admin on Fri Nov 17, 2017 3:45 pm; edited 15 times in total