| | | Lazcano, A.; Fox, G.E.; Oró, J.F.Life before DNA: the origin and evolution of early archaean cells. In The Evolution of Metabolic Function (R.P. Mortlock, ed.), pp. 237-295. Boca Raton, FL: CRC Press, 1992.
Schopf, J.W.; Hayes, J.M.; Walter, M.R.Evolution of earth's earliest ecosystems: recent progress and unsolved problems. In Earth's Earliest Biosphere: Its Origin and Evolution (J.W. Schopf, ed.), pp. 361-384. Princeton, NJ: Princeton University Press, 1983. |
|
| | Bartel, D.P.; Szostak, J.W.Isolation of new ribozymes from a large pool of random sequences. Science 261:1411-1418, 1993 [PubMed]
Cech, T.R.RNA as an enzyme. Sci. Am. 255(5):64-75, 1986 [PubMed] |
|
|
|  | Specialized RNA Molecules Can Catalyze Biochemical Reactions5
Natural selection depends on the environment, and for a replicating RNA molecule a critical component of the environment is the set of other RNA molecules in the mixture. Besides acting as templates for their own replication, these can catalyze the breakage and formation of covalent bonds between nucleotides. For example, some specialized RNA molecules can catalyze a change in other RNA molecules, cutting the nucleotide sequence at a particular point; and other types of RNA molecules spontaneously cut out a portion of their own nucleotide sequence and rejoin the cut ends (a process known as self-splicing). Each RNA-catalyzed reaction depends on a specific arrangement of atoms that forms on the surface of the catalytic RNA molecule (the ribozyme), causing particular chemical groups on one or more of its nucleotides to become highly reactive.
Certain catalytic activities would have had a cardinal importance in the primordial soup. Consider in particular an RNA molecule that helps to catalyze the process of templated polymerization, taking any given RNA molecule as template. (This ribozyme activity has been directly demonstrated in vitro, albeit in a rudimentary form.) Such a molecule, by acting on copies of itself, can replicate with heightened speed and efficiency (Figure 1-7A). At the same time, it can promote the replication of any other type of RNA molecules in its neighborhood (Figure 1-7B). Some of these may have catalytic actions that help or hinder the survival or replication of RNA in other ways. If beneficial effects are reciprocated, the different types of RNA molecules, specialized for different activities, may evolve into a cooperative system that replicates with unusually great efficiency.
|
