Origin of Life – RNA Self Replicators

ResearchBlogging.org

We have previously established that organic compounds, including nucleotides, are trivially easy to produce from inorganic sources.  We have also established at least two methods from which long chain RNA molecules could develop without organic processes… including one that requires little more than warm water.

 Remember that all we have to do is show that there are no chemical or physical barriers to prevent life from developing from non-life.  No one will ever know exactly what happened.  But if there available paths from life to non-life, then there is no need to invoke something outside of observable science to explain how life got here.

I don’t think anyone would argue that a 100 nucleotide chain of RNA is ‘life’ (boy there’s another post huh?).  But we’re putting the pieces together. 

That’s all well and good we say.  But how do we get from non-life to life? 

Well, Tracey Lincoln and Gerald Joyce have started on that path by developing a pair of RNA enzymes (and several variants) that can indefinitely self replicate (provided with a supply of raw materials, which are the sub units).

 The authors converted an RNA enzyme that could replicate itself once into a version that was cross catalytic. The authors say it best:

 A plus-strand RNA enzyme (E) catalyzes the joining of two substrates (A′ and B′) to form a minus-strand enzyme (E′), which in turn catalyzes the joining of two substrates (A and B) to form a new plus-strand enzyme

 This system doubled in number about once an hour and was able to continue for 5 hours resulting in a 25-fold increase.  The growth followed an exponential curve, showing that each RNA enzyme, once assembled, could continue to assemble additional enzymes for as long as raw materials (the subunits) were available.

Now there are a couple of points here.  First, this is not how life on Earth began.  This is merely evidence that a non-biological system can begin to replicate.  I fully expect ID proponents to scream about how the RNA enzyme was ‘designed’ proving their side of the story.  Unfortunately, that’s not the point.  The point is, counter to what ID and creationists would have us believe, non-life can do some of the things that life can do.  Further, this shows that there is no intrinsic chemical or physical barrier to self replication of RNA.  And as soon as you get replication, you get… evolution by natural selection.

 This last is an important point, because the authors did more than just develop E and the process and stop.  They took it several steps beyond.

 The authors constructed 12 variants of the enzyme (variant at the active site of combination only and with approximately the same stability) and the subunits needed for assembly, then tested for replication efficiency. 

 Then they combined four of the enzymes, with a high concentration of the subunits for each and performed a serial experiment over 70 hours.  All four enzymes were still present, there were also 17 clones that were a recombination (defined as an enzyme made of two mismatched subunits.  In other words, a mistake in the chemical process resulted in the formation of new enzymes that still managed to carry on with the replication process.).

 The performed the serial experiment again except with all 12 original replicators and the necessary subunits for each.  This one lasted for over 100 hours.  In this experiment, they discovered that some recombinations were actually more efficient than their original twelve enzymes.  Evolution in action… without life being present.

 What’s also interesting is that the system of enzyme with two subunits represents a very simplified model of a genetic system with two loci and 12 alleles per locus.

 Suddenly, the concept of genetic retention of information has become very, very simple.  Instead of requiring DNA and hundreds of helper proteins and RNAs, these scientists have found that the entire concept of genetic information works with a single RNA enzyme of approximately 78 nucleotides (well within the range of be constructed by non-organic systems).

 Again, this is a model.  But this model shows many separate pieces of the possible origin of life on Earth.  We have self-replication (no other materials needed).  We have the beginning of a genetic structure with alleles.  We have the beginnings of a doubled structure.  We have catalytic activity.

 Obviously, the ultimate goal will be a self replicator that can copy itself from base nucleotides, but this model shows that everything can be done without life being involved.

Update:

I just got some e-mailed information from Dr. Joyce.  He provided a few articles and some future posting ideas for me.  He also had this to say about his work:

Our goal is not to recapitulate the origins of life on Earth, but rather to construct synthetic genetic systems of ever-increasing complexity. Currently in the lab we are working with libraries of up to 65,536 replicators, seeking to demonstrative the invention of novel function within the context of self-sustained Darwinian evolution.

While the origins of life implications are really cool, he’s working more on the evolution of novel features side of the question.

___________________________
Lincoln, T., & Joyce, G. (2009). Self-Sustained Replication of an RNA Enzyme Science, 323 (5918), 1229-1232 DOI: 10.1126/science.1167856

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One Response to Origin of Life – RNA Self Replicators

  1. Pingback: Origins of Life: General RNA Polymerases | Cassandra's Tears

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