RNA world hypothesis
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The RNA world hypothesis proposes that RNA was actually the first life-form on earth, later developing a cell membrane around it and becoming the first prokaryotic cell. This hypothesis is supported by the RNA's ability to store, transmit, and duplicate genetic information, just like DNA does. RNA can also act as a ribozyme (an enzyme made of ribonucleic acid). Because it can reproduce on its own, performing the tasks of both DNA and proteins (enzymes), RNA is believed to have once been capable of independent life.
The phrase "RNA World" was first used by Walter Gilbert in 1986. However, the theory of independent RNA life is much older and can be found in Carl Woese's book The Genetic Code (New York: Harper and Row, 1967).
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The base pair
RNA and DNA are made of long stretches of specific nucleotides, often referred to as "bases", attached to a sugar-phosphate backbone. The RNA world hypothesis holds that in the primordial soup / primordial sandwich there existed free-floating nucleotides. These nucleotides would regularly form bonds with one another, but the chains would often break apart because the change in energy was so low. However, certain sequences of base pairs have catalytic properties that actually lower the energy of their chain being created, causing them to stay together for longer periods of time. As each chain grew longer it attracted more matching nucleotides faster, causing chains to now form faster than they were breaking down.
These chains are proposed as the first, primitive forms of life. In an RNA world, different forms of RNA compete with each other for free nucleotides and are subject to natural selection. The most efficient molecules of RNA, the ones able to efficiently catalyze their own reproduction, survived and evolved, forming modern RNA.
Competition between RNA may have favored the emergence of cooperation between different RNA chains, opening the way for the formation of the first proto-cell. Eventually, RNA chains randomly developed with catalytic properties that help amino acids bind together (peptide-bonding). These amino acids could then assist with RNA synthesis, giving those RNA chains that could serve as ribozymes the selective advantage. Eventually DNA, lipids, carbohydrates, and all sorts of other chemicals were recruited into life. This led to the first prokaryotic cells, and eventually to life as we know it.
Nucleic acid fragility
At first glance, the RNA world hypothesis seems implausible because, in today's world, large RNA molecules are inherently fragile and can easily be broken down into their constituent nucleotides with hydrolysis. Even without hydrolysis RNA will eventually break down from background radiation. (Pääbo 1993, Lindahl 1993).
A proposed alternative to RNA in an "RNA World" is the peptide nucleic acid, PNA. PNA is more stable than RNA and appears to be more readily synthesised in prebiotic conditions, especially where the synthesis of ribose and adding phosphate groups are problematic.
Additionally, in the past a given RNA molecule might have "lived" longer then than it can today. Ultraviolet light can cause RNA to polymerize while at the same time breaking down other types of organic molecules that could have the potential of catalyzing the break down of RNA (RNAses), suggesting that RNA may have been a relatively common substance on early earth. This aspect of the theory is still untested and is based on a constant concentration of sugar-phosphate molecules.
Implications
The RNA world hypothesis, if true, has important implications for the very definition of life. Life so far has been largely defined in terms of DNA and proteins; in today's world, DNA and proteins seem to be the dominant macromolecules in the living cell, with RNA serving only to aid in creating proteins from the DNA blueprint. But the RNA world hypothesis places RNA at center-stage when life originated, therefore requiring that we define life primarily in terms of RNA and the set of strategies that RNA has used to perpetuate itself.
In 2001, the RNA world hypothesis was given a major boost with the deciphering of the 3-dimensional structure of the ribosome, which revealed the key catalytic sites of ribosomes to be composed of RNA, with proteins playing only a structural role in holding the ribosomal RNA together. Specifically, the formation of the peptide bond, the reaction that binds amino acids together into proteins, is now known to be catalyzed by RNA. This finding suggests that RNA molecules were most likely capable of generating the first proteins.
Difficulties
Nucleotides have not been found in Miller-Urey-type experiments. They would have to have been made from their components: nucleobases, ribose, and phosphates.
The base cytosine does not have a plausible prebiotic simulation method because it easily undergoes hydrolysis.
Prebiotic simulations making nucleotides have conditions incompatible with those for making sugars (lots of formaldehyde). So they must somehow be synthesized, then brought together. However, they don't react in water. Anhydrous reactions will bind with purines, but only 8% of them are joined with the correct carbon atom on the sugar joined to the correct nitrogen atom on the base. Pyrimidines, however, will not react with ribose, even anhydrously.
Then phosphate must be introduced, but in nature phosphate in solution is extremely rare because it is so readily precipitated. After being introduced, the phosphate must combine with the nucleoside and the correct hydroxyl must be phosphorylated.
For the nucleotides to form RNA, they must be activated themselves. Activated purine nucleotides will form small chains on a pre-existing template of all-pyrimidine RNA. However, this does not happen in reverse because the pyrimidine nucleotides do not stack well.
Additionally, the ribose must all be the same enantiomer, because any nucleotides of the wrong chirality act as chain terminators. [1] (http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?holding=npg&cmd=Retrieve&db=PubMed&list_uids=6462250&dopt=Abstract)
A.G. Cairns-Smith criticized writers for exaggerating the implications of the Miller-Urey experiment. He argued that the experiment showed, not the possibility that nucleic acids preceded life, but its implausibility. According to Cairns-Smith, the process of constructing nucleic acids would require eighteen distinct conditions and events that would have to occur continually over millions of years in order to build up the required quantities.
One of the leading researchers into RNA world models, Gerald Joyce, wrote:
- The most reasonable assumption is that life did not start with RNA .... The transition to an RNA world, like the origins of life in general, is fraught with uncertainty and is plagued by a lack of experimental data. [2] (http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=2466202&dopt=Abstract)
See also
References
- Cairns-Smith, A. G. Genetic Takeover: And the Mineral Origins of Life. ISBN 0-52123-312-7
- Lindahl, T., 1993. Instability and decay of the primary structure of DNA, Nature 362(6422):709–715.
- Pääbo, S. 1993. Ancient DNA, Scientific American 269(5):60–66.
External links
- "SELF-REPLICATION: Even peptides do it" (http://www.santafe.edu/sfi/People/kauffman/sak-peptides.html) by Stuart A. Kauffman
- Nobel prize website on the RNA world (http://nobelprize.org/chemistry/articles/altman/)
- American Scientist Online article from 1995 discussing origin of life and RNA world (http://www.americanscientist.org/template/AssetDetail/assetid/21438?fulltext=true)
Creationist criticism
- The RNA World: A Critique (http://www.arn.org/docs/odesign/od171/rnaworld171.htm) (Origins & Design 17(1):9–16, 1996) (Intelligent Design publication, co-authored by Dean Kenyon, former leader in chemical evolution)
- Origin of Life: Instability of Building Blocks (http://www.trueorigin.org/originoflife.asp)(Vol. 13, No. 2 of the Creation Ex Nihilo Technical Journal) by Young Earth Creationist Jonathan Sarfati.he:השערת עולם הרנ"א