RNA world hypothesis

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RNA, or ribonucleic acid, is one of the three significant biological molecules that are crucial for all organisms, as well as proteins and DNA. The RNA world hypothesis is one possible explaination to the paradox surrounding the origin of life on Earth. It was proposed by Francis Crick and his team in the late 1960s[1]. The central dogma of molecular biology states that DNA is required to produce proteins, with RNA acting as an intermeidate[2]. The paradox arises due to the fact that DNA is required to produce proteins, but protiens are required to produce DNA[3]. This raises the question how did such an interdependant system first arise?

The RNA world hypothesis states that due to the ability of RNA to both store genetic information and catalyse chemical reactions, it may be the precursor to current life[4].   Experiments have supported this theory by showing that randomly generated RNA sequences can have useful functions such as an experiment carried out in the 1950s at the Whitehead Institute of Biomedical Research who reported on "Structurally Complex and Highly Active RNA Ligases Derived from Random RNA Sequences"[5]. Other experiments have also shown RNAs with the ability to self-replicate[6] and self-catalyse[7] as well as the ability to perform amino acid ligation[8] and peptide bond formation[9]. RNA also has the ability to self-catalyse because RNA forms a single-stranded structure which can fold itself into complex structures to form an active site, like in protein enzymes, where substrates can bind and a reaction can be catalysed. RNA can even bind metal ions at its active sites, meaning that an even wider range of catalyic activities can occur[10]. Evidence has been shown that many features of current life are descended from the RNA world. It can be argued that life was based on RNA only before modern-day cells arose. DNA later took over RNA for the function of heredity material storage with a more stable structure and evolution of new enzymes taking up most catalytic activities in the cells. The main functions of RNA today is for protein synthesis as well as a catalyst for other important reactions in the cell[11].

Ribosome Discovery

Quite possibly, the largest contributor to the supporting of the RNA world hypothesis is the discovery of the structure of the ribosome. The findings suggested that it was in actual fact the RNA that carried out the catalysis of the peptide chain from the RNA sequence and not the proteins, which caused scientists to support the claim of the RNA world, as one molecule in two different forms can carry out chemical reactions and also store the information. It was thought that only proteins could catalyse reactions, but here, the proteins were actually only seen to be playing a part in the structural support of the ribosome to hold it all together.

References

  1. Akst J. (2014) RNA World 2.0. The Scientist magazine. Available from: http://www.the-scientist.com/?articles.view/articleNo/39252/title/RNA-World-2-0/ [last accessed: 28.11.2014]
  2. Londish, H., Berk, A., Kaiser, C.A., Krieger, M., Bretscher, A., Ploegh, H., Amon, A. and Scott, M.P. (2013) Molecular Cell Biology. 7th edn. Basingstoke: Macmillan Higher Education. p. 116.
  3. Alberts, B., Bray, D., Hopkins, K., Johnson, A., Lewis, J., Raff, M., Roberts, K. and Walter, P. (2014) Essential Cell Biology. 4th edn. Abingdon: Garland Science. p. 253.
  4. Alberts, B., Johnson, A., Lewis, J., Raff, M., Roberts, K. and Walter, P. (2008) Molecular Biology of the Cell. 5th edn. Abingdon: Garland Science. pp. 400-408.
  5. Eric H. Eckland, Jack W. Szostak and David P. Bartel, "Structurally Complex and Highly Active RNA Ligases Derived from Random RNA Sequences" [abstract], doi:10.1126/science.7618102, p 364-370 v 269, Science, 21 July 1995.
  6. Johnston, W. K.; Unrau, P. J.; Lawrence, M. S.; Glasner, M. E.; Bartel, D. P. (2001). "RNA-Catalyzed RNA Polymerization: Accurate and General RNA-Templated Primer Extension". Science 292 (5520): 1319–25. doi:10.1126/science.1060786. PMID 11358999.
  7. Huang, Yang, and Yarus, RNA enzymes with two small-molecule substrates. Chemistry and Biology, Vol 5, 669-678, November 1998
  8. Erives A (2011). "A Model of Proto-Anti-Codon RNA Enzymes Requiring L-Amino Acid Homochirality". J Molecular Evolution 73 (1–2): 10–22. doi:10.1007/s00239-011-9453-4. PMC 3223571. PMID 21779963.
  9. Atkins, John F.; Gesteland, Raymond F.; Cech, Thomas (2006). The RNA world: the nature of modern RNA suggests a prebiotic RNA world. Plainview, N.Y: Cold Spring Harbor Laboratory Press. ISBN 0-87969-739-3.
  10. Alberts, Johnson, Lewis, Morgan, Raff, Roberts, Walter. Molecular Biology Of The Cell, Sixth Edition (2009), Abingdon, Garland Science Taylor and Francis Group, Page 363
  11. Alberts B, Johnson A, Lewis J, et al. (2002) Molecular Biology of the Cell. 4th edn. New York: Garland Science


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