RNA world

Biologists have long been unhappy with aspects of the Oparin-Haldane model. They have pointed out, for example, that the two fundamental functions of any living thing are that it must have some form of genetic code, the ability to pass on information from one generation to the next, and it must be able to perform chemical reactions, to break down food, for example. These are, respectively, the functions of genes and enzymes. Genes are the segments of the genetic code, written in the sequence of bases in the DNA (deoxyribose nucleic acid), that specify particular functions. Enzymes are chemicals that stimulate, or catalyse, chemical reactions. The conundrum was to determine whether life originated according to a 'genes first' or 'enzymes first' model.

The solution seems to be that perhaps both functions arose at the same time. In 1968, Francis Crick (1916-2004) suggested that RNA was the first genetic molecule. He argued that RNA could have the unique property of acting both as a gene and an enzyme, so RNA on its own could be a precursor of life. RNA (ribonucleic acid) is one of the nucleic acids and it has key roles in protein synthesis within the cells. The genetic code, the basic instructions that contain all the information to construct a living organism, is encoded in the DNA strands that make up the chromosomes. Different forms of RNA act as the template for translation of genes into proteins, transfer amino acids to the ribosome (the cell organelle where protein synthesis takes place) to form proteins, and also translate the transcript into proteins.

o t When Walter Gilbert from Harvard University first used the term

| 'RNA world' in 1986, the concept was controversial. But the first

1= evidence came soon after when Sidney Altman of Yale University and Thomas Cech of the University of Colorado independently discovered a kind of RNA that could edit out unnecessary parts of the message it carried before delivering it to the ribosome. Because RNA was acting like an enzyme, Cech called his discovery a ribozyme. This was such a major finding that the two were awarded the Nobel Prize for Chemistry in 1989; Altman and Cech had confirmed part of Crick's prediction.

But how could naked RNA molecules exist, and how could they act as a foundation for life? The argument was that the simple RNA molecules may have assembled themselves by chance in rock pools, more or less following the assumptions made by Oparin and Haldane, and as shown in the Stanley Miller experiment. These simple naked RNA molecules mainly existed and then disappeared, but perhaps one or two were able to copy themselves, and they could have become dominant.

To take this forward to create a living cell, there might have been two stages, the production of a protocell by combination of two components, an RNA enzyme and a self-replicating vesicle (Fig. 4). This satisfies the minimum requirement that two RNA molecules should interact, one to act as the enzyme to bring together the components, and the other to act as the gene/ template. Together the template and the enzyme RNA combine as an RNA replicase. But these components have to be kept together inside some form of compartment or cell, or they would only occasionally come into contact to work together. This is the second pre-life structure, termed a self-replicating vesicle, a membrane-bound structure composed mainly of lipids (organic compounds that are not soluble in water, including fats) that grows and divides from time to time. The RNA replicase at some point entered a self-replicating vesicle, and this allowed the RNA replicase to function efficiently (Fig. 4).

This is a protocell, but it is not yet living. It is just a self-replicating membrane bag with an independent self-replicating molecule f inside. To make the protocell function both components have to e interact, the vesicle protecting the RNA replicase, and the RNA replicase perhaps producing lipids for the vesicle. If the interaction works, the protocell has become a living cell. The cell is alive because it has the ability to feed itself, to grow, and to replicate. Evolution can happen because the cells show differential survival ('survival of the fittest'), and the genetic information for replication is coded in the RNA.

Some aspects of the RNA world hypothesis have been tested, but much remains to be done. And in any case, the model remains hypothetical, because none of these stages would be likely to be fossilized. If the RNA world existed, it had to pre-date the oldest fossils, and the Earth had to be cool enough for the organic elements to survive being burned off. Some estimate that this might have been a time of 100-400 million years, somewhere between 4.0 and 3.5 billion years ago.

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