Understanding the RNA World Hypothesis

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All living creatures today reproduce and evolve using a complex gene-enzyme cycle. If we look at a cell, for example, information encoded in its genes is used to produce functional proteins called enzymes. Some of those enzymes then turn around to make copies of the cell's genes, allowing the cell to reproduce.

Because the gene-enzyme system forms a closed loop, it presents us with a classic chicken or egg conundrum: Which came first, genes or the protein enzymes they code for?

While the details are still not fully worked out, discoveries over the past few decades have lead researchers to a surprising possible solution: What really came first? Genes that act as enzymes!

The RNA World Hypothesis is the idea that before living cells, the genetic code, and the gene/protein cycle ever existed, chains of a chemical called RNA were forming naturally. Once formed, some of these chains were able to function as enzymes, and were even able to evolve by making copies of themselves with slight, accidental modifications.

While there is little doubt that RNA played a crucial role in the early development of life, the complexity of RNA is casting doubt on the idea that RNA was the first truly replicating and evolving chemical system. For this reason, alternatives to the “RNA first” view are also being investigated. Most notable is a proto-RNA hypothesis being studied by the lab of Nicholas Hud, and several metabolism first hypotheses which got their start from the work of Robert Shapiro.

At this time, all serious investigations into the origin of life are being done under the overarching idea that life emerged from chemistry.

SOURCES AND FURTHER READING

RIBOZYMES
The first RNA enzymes were discovered in 1982, now known as ribozymes. Here’s is a wonderful review of what we now know about ribozymes (well... as of 2002):

Scientific Paper: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3770912/

Diverse and highly functional ribozymes can be evolved in the lab. Selection experiments have shown there are many pathways that evolution can take to produce a ribozyme with a given function. This means the evolution of function is much simpler than previously thought!

Scientific Paper: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3902939/

Nucleotide building ribozymes have been evolved in the lab from random starting chains (this is the experiment discussed in our animation):

Scientific Paper: http://www.sfu.ca/~punrau/pdfs/Lau_JACS_2004.pdf

INITIAL RNA NUCLEOTIDE SYNTHESIS
RNA nucleotides have been shown to form without enzymes under conditions that might have been plausible on the early Earth. That said, the reactions we’ve discovered so far do not appear robust enough to claim that the problem is fully solved:

Scientific Paper: http://www.nature.com/nature/journal/v459/n7244/abs/nature08013.html

Nature News overview of the discovery with objections discussed: http://www.nature.com/news/2009/090513/full/news.2009.471.html

RNA REPLICATION BEFORE EVOLUTION OF ENZYMES
8 problems with RNA replication and likely solutions to them (note: number 8 now has two independent solutions demonstrated in the lab):
https://jsystchem.springeropen.com/articles/10.1186/1759-2208-3-2

What experiments in non-enzymatic template directed replication currently look like (experiments that don't use babysitting technologies like PCR): https://onlinelibrary.wiley.com/doi/full/10.1002/anie.202203067

ALTERNATIVES TO THE RNA WORLD HYPOTHESIS
Here is an article reviewing the metabolism first hypothesis in contrast to the RNA first view:

Blog post by professor Laurance Moran: http://sandwalk.blogspot.com/2009/05/metabolism-first-and-origin-of-life.html

Here is an article reviewing research into a possible proto RNA World Hypothesis:

Media article by quanta magazine
https://www.quantamagazine.org/20140205-chemists-pursue-possible-precursor-to-rna/

Learn more about evolution and the origin of life at http://statedclearly.com and at http://centerforchemicalevolution.com

#originoflife #chemistry #Evolution