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The discovery of ribozymes supports the RNA World Hypothesis. A ribozyme or ribonucleic acid enzyme, is an RNA molecule that is able to perform highly specific biochemical reactions that are similar to protein enzyme actions. This theory states that earlier life forms may have relied on RNA solely to store genetic information and catalyse chemical reactions. This hypothesis was first proposed in the 1960's by Francis Crick, Carl Woese and Leslie Orgel. This occurred many years before the discovery of ribozymes but shortly after the determination of DNA as a double helical structure. According to this hypothesis, DNA and proteins came into use later as life had evolved to use them as opposed to RNA due to RNA's poorer catalytic properties and instability relative to DNA.
Picture (1) A hairpin ribozyme.
3) RNA plays a major role in the process of creating proteins from DNA. It is known as the "Central Dogma" of molecular biology. The genetic information of an organism is stored in their cells' DNA and is encoded as a linear base sequence. During transcription, messenger RNA or mRNA, an RNA copy of DNA, is made. This strand of RNA can then form a protein after being translated by a ribosome.
The structure of RNA is very similar to that of DNA, the main difference between the two is that the RNA has a hydroxyl group on ribose sugar backbone that DNA does not (as seen in diagram 2). Another minor difference is that DNA uracil replaces the base thymine in RNA. Although they are very similar in the sense of structure, DNA and RNA have extremely different roles from each other in cells.
Diagram (2) - The structural differences between DNA and RNA in diagram form. At http://cyberbridge.mcb.harvard.edu/images/dna3_1.png
Another major difference between the two is that DNA has a double-stranded form in cells, while RNA is most often found in single-strands. The single stranded structure of RNA allows it fold into three-dimensional structures which are highly unique and highly complex. This folding is similar to that of DNA by forming base to base pairs. The only difference being that the bonds are formed within just the one strand in RNA, rather than between two in the case of DNA.
4) In basic terms support for the RNA world theory comes from RNA's ability to process genetic information by storing, transmitting and duplicating it. The strongest piece evidence for the RNA world theory is the fact that the ribosome, which is a large molecular complex that assembles proteins from mRNA strands, is a ribozyme. Although ribosomes are made up of both protein and RNA elements, an analysis of both its chemistry and structure has revealed that the mechanisms for translation are catalysed by RNA and not by proteins. This therefore suggests that the use of RNA by early life forms to carry out chemical reactions may have preceded the use of proteins. An experiment was performed by John Sutherland and his colleagues from the University of Manchester that greatly supports the RNA world hypothesis. He and his team created a ribonucleotide, which is a major part of RNA, from simple chemicals. These chemicals are those though to be present on the early earth, or primordial soup. Donna Blackmond, a chemist at Imperial College, stated that "this is extremely strong evidence for the RNA world. We don't know if these chemical steps reflect what actually happened, but before this work there were large doubts that it could happen at all."
Furthermore, RNA molecules may have survived for much longer periods of time in early earth conditions than it can today. This is because UV light can cause the polymerisation of RNA and can also cause the break down of organic molecules that could otherwise have catalyzed RNA and its breakdown. Therefore, suggesting that RNA may in fact have been very common on the early earth.
5) Some critics of this hypothesis suggest that rather than nucleic acids, other organic molecules were the first self-replicating substances capable of storing genetic information. This notion also suggests that during the course of evolution these other hereditary systems were replaced by nucleic acids. A further criticism is that it has been shown that many of the steps needed for nucleotideÂ formation do not occur correctly inÂ prebioticÂ conditions. It has been argued by that nucleotides cannot link unless there is an activationÂ of theÂ phosphate group and this, according to researchers is "totally implausible in any prebiotic scenario". A further problem of the RNA world theory is the assumption of the presence of nitrogen bases that make up RNA strands as there has been no evidence for their creation from early earth. Although the creation of these bases has been demonstrated by scientists, it required a complex series of random steps to combine these bases with the phosphate and sugar groups which are required for the formation of an RNA strand.
Many prebiotic simulations that have made nucleotides have incompatible conditions for making sugars. This may be due to them containing large quantities of substances like formaldehyde. Therefore suggesting that they may have to be synthesized in another way and then brought together.Â This problem leads to the creation of the 'perfect accident'.
6) There are many other theories that contradict the RNA world theory. Firstly, the electric spark theory which is based around the generation of amino acids and sugars from the atmosphere with the use of electricity. TheÂ Miller-Urey supports this theory. This experiment, performed in 1952, is thought to have simulated the conditions that many think were present on theÂ early Earth and tested for the origins of life and their chemical basis. Using the molecules thought to be present on early earth and an electric current it was observed that a low percentage of the carbon used had gone onto forming some amino acids that are used to make some of the proteins still present today. The Miller-Urey experiment therefore shows that amino acids and other organic substances could be made fairly easily under these conditions. Another theory is that of community clay, according to Alexander Cairns-Smith the first molecules of life might have joined together on clay. These environments may have concentrated the organic compounds together and also helped to sort them into patterns, in the same way our genes today do. A further theory is that of Panspermia, this theory suggests, controversially, that life may have formed out in space and been brought to earth through cosmic impacts or on comets in the form of microbes.
There are many other theories of the origins of life, some of which include the chilly start theory which suggests life began in icy conditions which could have protected early life from impacts and UV light from the sun. One of the more heavily favoured theories is that life may have evolved from simpler systems than that of the RNA world theory. It suggests that smaller molecules may have interacted with each other and formed life and that over time more complex molecules have evolved, which have taken over these simple systems.
7) This RNA world theory is the most favoured theory that explains the origins of life. There are vast quantities of supporting evidence that have lead many people to favour this particular theory. However, there is actually no scientific evidence to show or even suggest that RNA was created in such a spontaneous manor and then able to form the first life on earth. There has also been no scientific evidence of RNA performing this act today. While some laboratories have created artificial ribozymes there are still some quite large gaps in the reproduction of an RNA world model to support the theory. The ribozymes that have been created actually lack the ability to process genetic information and even the ability to process themselves. There is also no substantial evidence of them being able to produce sequences of nucleotides either. However the lack of production of cellular life by a model is not unique to the RNA world theory it is true for all origin of life hypotheses as none have formed cellular life when tested. Furthermore, there is an ongoing, important debate among numerous scientists about the conditions of the early earth and the substances and molecules that made it up; therefore until this has been determined no true model of this can be made. In conclusion when taking into account the extensive levels of research done to both support and contradict this hypothesis, The RNA world hypothesis does not help us to fully explain the origins of life. This is because the starting conditions can never exist on the earth today or in fact anywhere in the known universe. These conditions that are thought to simulate early life can actually only exist in test tubes with the aid of highly controlled conditions and researchers. The origins of life still remain a mystery today.