RNA World hypothesis

Amongst the four basic biomolecules are the proteins and the nucleic acids: DNA, RNA: that play a vital role in the biochemistry of a cell. DNA is the storage of information that encrypts for RNA which is then translated into proteins: the work horses of the cells. Yet in both transcription and regulation of gene expression processes, DNA requires the aid of various proteins ^(I). This fascinating interaction in between these three bio molecules leave abiogenists with the question ‘Which of these molecules appeared first in the primordial soup 3 billion years ago, to form the first life form or the common ancestor from which all typical cells nowadays are believed to have evolved from?’ ^(II) RNA world hypothesis is one of the possible explanations which was first suggested by Francis Crick, Carl Woose, Leslie Orgel (1968) ^(III) ; yet the term “RNA world” was introduced by Walter Gilbert in 1986^(IV).

As predicted if RNA is the first to appear, it should demonstrate properties of all these three molecules. With the discovery of catalytic RNA or the ribozymes it was proven possible that RNA can perform enzymatic activities like proteins. (Thomas R. Cech 1980)^(V) Also varieties like the hammerhead ribozyme which has the ability to perform self cleavage, would have contributed in inventing new nucleotide sequences that might have created diversity within the RNA world. ^(VI) Ribozymes and riboswitches would have contributed in gene regulation instead of proteins. ^(VII) Ribozymes act as auto catalysts of self replication which makes it possible for RNA to create more copies of its own sequence just as DNA. If we consider the building blocks of the nucleic acids they differ only in two ways; either due to the presence of the 2’OH in the sugar molecule or due to the presence the base thymine or uracil. Other than that the chemical composition is likely to be similar which would give RNA the ability to store information like DNA. Also in the Fisher-Tropsch experiment it was found that only adenine, guanine, cytosine and uracil could be produced under the conditions of the pre biotic atmosphere. Creation of thymine wasn’t expected because it required methilization of uracil which wasn’t possible at that time. ^(VIII)

How was DNA and proteins first created? rRNA, one of the major components of the ribosomes, is always involved in the assortment of amino acids in order to form peptide linkages to create proteins. In the same manner it may have begun protein synthesis. With reverse transcriptase cDNA can be produced; and still the central dogma of molecular biology is secured. (Francis Crick 1958)^(IX) Likewise during evolution DNA would have formed.

Then the question comes; why was RNA replaced by proteins in catalytic tasks while DNA became the dominant molecule of heredity? The vast chemical diversity among the amino acids due to different functional groups, allow proteins to step ahead of RNA in reactivity properties. Higher reactivity provides more advantages for a catalyst and thereby proteins may have evolved to be the biocatalysts. The RNA molecule is destabilized by base induced deprotonization of the 2’OH in the ribose sugar component of the nucleotides. As it will not occur in the deoxyribose in DNA, it’s considered prior to RNA in stability. Therefore flow of information via DNA is favoured in highly organized species. ^(X) But still the idea of RNA world remains a hypothesis as it has to compete with other suggestions such as ‘Iron- Sulfer world hypothesis’, which are also put forward to explain the origin of life.

References:

• I. Garette, R., Grisham, C. (2003). Nucleotides and nucleic acids. 3^rd ed: Biochemistry (328). USA: Brooks- Cole publications
• II. Horten, H.R., Moran, L.A., Scrimgeour, K.G., Rawn, J.D.(2008). Introduction to biochemistry. 4^th ed: Principles of Biochemistry (114). USA: Pearson publications
• III. Cox, M.M., Nelson, D.L., (2008). RNA metabolism. 5^th ed: Leninger: Principles of Biochemistry (1057). NY: Freeman and company.
• IV. Gilbert, W. (1986, Feb.). The RNA world. Nature, (618):319
• V. Cox, M.M., Nelson, D.L., (2008). RNA Metabolism. 5^th ed: Leninger: Principles of Biochemistry (1033-4). NY: Freeman and company.
• VI. Voet, D., Voet, J.G., Pratt, C.W., (2008). DNA structure and interaction with proteins. 3^rd ed: Principles of Biochemistry (870). NJ: Wiley & sons, inc.
• VII. Cox, M.M., Nelson, D.L., (2008). Regulation of gene expression. 5^th ed: Leninger: Principles of Biochemistry (1133). NY: Freeman and company.
• VIII. Murey, R.K., Granner, D.K., Mayes, P.A., Rodwell, V.W. (2003). Metabolism of purines and pirimidines. 26^th ed: Harper’s Illustrated Biochemistry (301-2). USA: McGraw Hill Companies.
• IX. Berg, G.M., Tymoczko, J.L., Stryer, L. Exploring genes. 5^th ed: Biochemistry(258)
• X. Voet, D., Voet, J.G., Pratt, C.W., (2008). DNA structure and interaction with proteins. 3^rd ed: Principles of Biochemistry (871). NJ: Wiley & sons, inc.
• Exploring Life's Origin
• The RNA World