Size Limits of Very Small Microorganisms Proceedings of a Workshop (1999) / Chapter Skim
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Pages 107-136
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From page 107... ...
A primitive organism might have relied on a single biopolymer, RNA, which might or might not have catalyzed its own reactions. Such an organism would not have required proteins, ribosomes, and other modern cellular machinery and conceivably could have been the smallest self-sustaining chemical system capable of Darwinian evolution.
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From page 108... ...
. The problem is that nucleic acids are generally not good catalysts: one must sort through 2 x 10~3 random RNA sequences to find one that modestly increases the rate of a templated ligation.
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From page 109... ...
A cell with an impermeable membrane would need to be complex enough to both evolve the barrier and encode a transport system, perhaps with nucleic acids or peptides serving transport functions.
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From page 110... ...
He proposed that life arose rapidly, perhaps many times, until it finally was sustained. He disputed the notion of tiny protocells harboring RNA molecules that exhibited both genetic and catalytic activity swimming in a sea of activated nucleotides.
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From page 111... ...
He provided a brief review of the definitions put forward over the past 100 years and then focused his discussion on recent definitions. The simplest is, "Life is a self-sustaining chemical system undergoing Darwinian evolution." He proposed a modification of this definition for "adherents of the RNA world" that life is "a population of RNA molecules (a quasi-species)
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From page 112... ...
A Summary of the Current State of Prebiotic Synthesis of RNA The basic ingredients required for proposed models of primitive life are RNA, peptides, or proteins and membrane constituents. I recognize that the first life may not have utilized the types of biomolecules present in contemporary life, but at the present time there is very little information as to the possible structures of alternative life so the focus here will be on those molecules where information on possible prebiotic syntheses exists.
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From page 113... ...
Prebiotic Syntheses of Polypeptides There have been many reports of the prebiotic synthesis of short peptides in aqueous solution (for a brief summary see Liu and Orgel, 1998a) , and it has been possible to make those that contain more than
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From page 114... ...
. Positively and negatively charged oc-amino acids and ,8-amino acids form long chain polypeptides on mineral surfaces when carboxyl activating groups are added to the reaction mixture 20 to 50 times (Figure 4; Hill, Jr., et al., 1998; Liu and Orgel, 1998b)
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From page 115... ...
The hydrolysis of esters and anhydrides of fatty acids results in their conversion to fatty acids, which associate into vesicles with diameters that range from 10 to 45 microns (Figure 5) (Walde et al., 1994~.
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From page 116... ...
For an alternative view of both the RNA world and the thesis that RNA viruses contain vestigial RNA see Benner and Ellington (1987~. Primitive life probably required the presence of preformed biomolecules that could be appropriated for its own purposes.
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From page 117... ...
bNot counting the phospholipid membrane, which is assumed to be 4 rim thick. Table 2 Volume and Radii of Spherical Primitive Life Determined on the Basis of the RNA Packing in Q,8 and L-A Virusesa Qua L-A GenesVolume (nm3)
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From page 118... ...
An exhaustive survey of potential mineral catalysts is required before undertaking such a search for these minerals in ancient rock formations. Such a search would be facilitated if there was data that suggested that primitive life provided conditions resulting in the deposition of a mineral that does not form under the usual environmental conditions.
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From page 119... ...
Cold Spring Harbor, New York: Cold Spring Harbor Laboratory Press. McCollum, T.M., G
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From page 120... ...
Introduction: Early Cellular Life It is important to distinguish between truly early cellular life and the last common ancestor of existing life. The structural and biochemical similarities of all existing branches of life point to a complex cellular structure for the last common ancestor, characterized by a DNA genome encoding at least several hundred and possibly several thousand genes, ribosome-catalyzed protein synthesis using the standard genetic code, membrane-surrounded cells with a wide range of protein transporters, and a complex metabolism supporting sugar, amino acid, nucleotide, cofactor, and lipid biosynthesis based on ATP synthesis from an electrochemical proton gradient (1~.
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From page 121... ...
Perhaps the easiest way to understand this function is to consider what would happen in the absence of compartmentation. Imagine an initial population of RNA replicase molecules in free solution with activated monomers, but without any other RNA molecules present to complicate matters.
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From page 122... ...
Size and the Cell Cycle Membrane vesicles can be made from a wide range of phospholipids and other components, in a wide range of sizes. The vesicles that bud spontaneously from the surface of dried phospholipid films suspended in buffer tend to be large (1-10 ~m)
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From page 123... ...
fission (12~. Can we conceive of an analogous spontaneous cell cycle for larger vesicles?
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From page 124... ...
Vesicles as small as 50-nm diameter can be generated, could encapsulate small replicating informational polymers, and have at least some attractive properties in terms of the potential for a spontaneous cell cycle. Although such structures may not be the most likely form of primordial life, it would not be wise to ignore this possibility.
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From page 125... ...
(1993~. Nucleic Acids Res.
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From page 126... ...
The terrestrial version of two-biopolymer life contains a well-recognized paradox, however, one relating to its origins. It is difficult enough to envision a non-biological mechanism that would allow either proteins or nucleic acids to emerge spontaneously from non-living precursors.
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From page 127... ...
Conversely, if the meteorite structures are indeed fossils, then they almost certainly are fossils of an organism that used only a single biopolymer. Does a Single Biopolymer Exist That Is Capable of Genetics and Catalysis?
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From page 129... ...
Protein is not COSMIC-LOPER, and is not expected to serve well as a genetic biopolymer, despite its acknowledged virtues as a catalytic biopolymer. Starting in the 1980s, various groups altered the structure of nucleic acids to learn what structural features enable the rule-based molecular recognition properties (for a review, see Benner et al., 1998~.
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From page 130... ...
. At the very least, a single-biopolymer attempting to support Darwinian evolution must reflect some sort of structural compromise between these goals.
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From page 131... ...
, and their coworkers introduced "in vitro selection" as a combinatorial tool to identify RNA molecules that catalyze specific reactions. If RNA was indeed as effective a catalyst as the reconstruction of the RNA world would imply, in vitro selection should rapidly generate the ultimate goal, an RNA (or DNA)
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From page 132... ...
suggested that perhaps only 107 random sequences must be searched to get a similar catalytic effectiveness as is observed in a library of 10~3 RNA molecules. This suggests that peptides are intrinsically a millionfold fitter as catalysts than RNA.
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From page 133... ...
Some of these might even be placed by parsimony in the protogenome (Benner et al., 1989~. A third way to expand the functional diversity of nucleic acids is to increase the number of nucleotides in the nucleic acid alphabet.
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From page 134... ...
Redesigning nucleic acids.
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From page 135... ...
Usher, D.A., and McHale, A.H., 1976. Hydrolytic stability of helical RNA: A selective advantage for the natural 3',5'-bond.
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