PrimerWk5F11 - Week 5 THE DNA WORLD The entire story up...

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Week 5: THE DNA WORLD The entire story up till now was pretty much run by RNA. This molecule can do it all. It can be a template that directs its own replication, or it can be a catalyst that accelerates chemical reactions. We considered its catalytic activity as a replicase in previous lectures, but RNA is much more versatile than that. Its ability to fold into three- dimensional shapes according to the interactions of its internal complementary base pairs means it can facilitate all kinds of chemical reactions. Thus, one would think that cells could survive just fine with RNA as the master chemical. In fact, although RNA is a good jack-of-all-trades, it is a master of none. First, as a template , RNA is relatively unstable, and although some mutation is good in order to generate diversity, the original requirement for persistence of chemical form demands that the RNA not change too much. RNA makes about 1 mistake for every 1000 nucleotides polymerized, so it really needs to be shorter than 1000 nucleotides, or persistence will be insufficient. A more stable replicator would allow a longer, more complex nucleotide, and thus more potential functions. Second, as a catalyst , RNA molecules fold into different shapes according to the hydrogen bonding in their various nucleotide sequences. But the variety of such shapes is actually somewhat limited. A more versatile catalyst could facilitate a much broader range of reactions. Lets start with this second deficiency first, and return later to the first deficiency. So, as best we can sort out, based on a wide variety of observations and experiments, the original RNA world gradually handed over the duty of catalysis to a much more versatile class of molecule, proteins. Proteins consist of a linear string of amino acids. Such strings of amino acids have much greater versatility than do strings of ribonucleotides (RNA) for two reasons. First, there are 20 amino acids from which to choose, leading to many more combinations for a polymer of any particular length. Second, the range in structural differences between amino acids on a protein is much greater than the range in structure available in the 4 ribonucleotides. Notice in the following figure the huge diversity in the side groups of amino acids, the white parts of each of the 20 amino acids. Thus, the number of different side groups, as well as their large range in chemical structure, has the consequence that proteins are vastly more versatile than RNA at catalyzing cellular reactions. In fact the vast majority of cellular reactions in modern cells are catalyzed by protein enzymes rather than RNA ribozymes. RNA retains its catalytic function in critical processes as we will see, but protein is by far the most ubiquitous catalyst.
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Most early life researchers believe there was a transitional period when RNA was still used for replication, but its ribozyme function was handed off to coalitions of RNA and protein, and finally to pure protein. Catalysis by proteins provided even faster
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This note was uploaded on 10/23/2011 for the course BIO 101 taught by Professor Wright during the Spring '11 term at Chapman University .

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PrimerWk5F11 - Week 5 THE DNA WORLD The entire story up...

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