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Origin of Life: What is Wrong with RNA World Hypothesis
RNA world hypothesis about origin of life was based on known properties of RNA but in formulating this hypothesis, limitations of RNA as a molecule capable of independently supporting life like activities were overlooked. These limitations are .....
The phrase "RNA World" was first used by Nobel laureate Walter Gilbert in 1986, in a commentary on recent observations of the catalytic properties of various forms of RNA.
The RNA world hypothesis is supported by RNA's ability to store, transmit, and duplicate genetic information, as DNA does. RNA can act as a ribozyme, a special type of enzyme. Because it can perform the tasks of both DNA and enzymes, RNA is believed to have once been capable of supporting independent life forms. Some viruses use RNA as their genetic material, rather than DNA.
RNA world hypothesis is further supported by computational ability of RNA. All the three i.e. DNA, RNA and Peptides have been shown to possess computational ability.
RNA computing has been used to find solutions to Knight Problem in chess. 1024 strands were used. RNA computing correctly identified 43 out of 512 possible solutions with one error. Considering very high specificity of natural biochemical reactions, error checking and correcting mechanisms must co-exist with RNA computing. This explains need for existence of antisense RNA but at the same time raises doubts about ability of RNA alone to support a living system or life like activity.
Self replicative ability of RNA is not without limitations. Only short RNA molecules have been shown to possess this ability and with a questionable fidelity. One version, 189-bases long, had fidelity of 98.9% which would mean it would make an exact copy of an RNA molecule as long as itself in one of every eight copies. This 189 base pair ribozyme could polymerize a template of at most 14 nucleotides in length, which is too short for replication. However this level of infidelity and limitation to self replication is far too high for living systems to bear. Just a single mutation affecting just one base pair in the entire genome is known to be catastrophic, therefore the kind of self replication seen under lab conditions does not support RNA world hypothesis.
RNA is much more unstable then DNA. This inherent instability of RNA is a further limitation to the ability of RNA to account for origin of life. Living organisms keep their identity and integrity over eons of time and across a vast number of generations. With no provision for RNA repair mechanisms of the kind seen for DNA in living systems, RNA is inherently incapable of performing this task.
All the properties of RNA which lend credibility to RNA world hypothesis are present in DNA but without the limitations of RNA. Moreover DNA unlike RNA is able to form much longer sequences and is much more stable then RNA (DNA has propensity to spontaneous hydrolytic cleavage @1 per million base pair per day with highly efficient DNA repair mechanisms to maintain the status quo) and thus is not only capable of keeping identity and integrity of the organism but also capable of supporting phenomenon of inheritance. Also only DNA is capable of carrying the amount of information needed to support a living system by virtue of its ability to form long sequences of nucleotide bases.
RNA world hypothesis finds some support due to existence of RNA viruses and viroids which are not dependent on DNA during their entire life cycle. But available evidence indicates that these are able to survive only within eukaryotic cells which further imply that RNA can’t account for origin of life. Since life originated about 4 billion years back whereas origin of eukaryotes has been estimated to be only 1.6 – 2.1 billion years back.
Moreover the lab conditions under which various experiments in support of RNA world hypothesis have been conducted have not been shown to naturally exist. Even the nature of chemistry demonstrated in support of RNA world hypothesis is stochastic and not definitive and deterministic. Definitive and deterministic chemistry is the hall mark of biochemistry with no room for aberrant, random, side reactions.
Therefore life could have originated only as a DNA molecule because DNA is the only molecule capable of independently supporting all life like activities.
Author: Dr Mahesh C. Jain is a practicing medical doctor and has written the book “Encounter of Science with Philosophy – A synthetic view”. The book begins with first chapter devoted to scientifically valid concept of God and then explains cosmic phenomena right from origin of nature and universe up to origin of life and evolution of man. The book includes several chapters devoted to auxiliary concepts and social sciences as corollaries to the concept of God. This is the only book which deals with origin of nature and universe from null. Twenty-ninth chapter of the book deals with the subject matter of ‘Origin of Life’.