Original article :
https://scitechdaily.com/discovery-supports-a-surprising-new-view-of-how-life-o
n-earth-originated/
Discovery Supports a Surprising New View of How Life on Earth Originated
TOPICS:DNAGeneticsPopularRNAScripps Research Institute
By SCRIPPS RESEARCH INSTITUTE DECEMBER 28, 2020
Newly described chemical reaction could have assembled DNA building blocks
before life forms and their enzymes existed.
Discovery boosts theory that life on our planet arose from RNA-DNA mix. Chemists
at Scripps Research have made a discovery that supports a surprising new view of
how life originated on our planet.
In a study published in the chemistry journal Angewandte Chemie, they
demonstrated that a simple compound called diamidophosphate (DAP), which was
plausibly present on Earth before life arose, could have chemically knitted
together tiny DNA building blocks called deoxynucleosides into strands of
primordial DNA.
The finding is the latest in a series of discoveries, over the past several
years, pointing to the possibility that DNA and its close chemical cousin RNA
arose together as products of similar chemical reactions, and that the first
self-replicating molecules ΓÇö the first life forms on Earth ΓÇö were mixes
of the two.
The discovery may also lead to new practical applications in chemistry and
biology, but its main significance is that it addresses the age-old question
of how life on Earth first arose. In particular, it paves the way for more
extensive studies of how self-replicating DNA-RNA mixes could have evolved
and spread on the primordial Earth and ultimately seeded the more mature biology
of modern organisms.
ΓÇ£This finding is an important step toward the development of a detailed
chemical model of how the first life forms originated on Earth,ΓÇ¥ says study
senior author Ramanarayanan Krishnamurthy, PhD, associate professor of chemistry
at Scripps Research.
The finding also nudges the field of origin-of-life chemistry away from the
hypothesis that has dominated it in recent decades: The ΓÇ£RNA WorldΓÇ¥
hypothesis posits that the first replicators were RNA-based, and that DNA
arose only later as a product of RNA life forms.
Is RNA too sticky?
Krishnamurthy and others have doubted the RNA World hypothesis in part
because RNA molecules may simply have been too ΓÇ£stickyΓÇ¥ to serve as the
first self-replicators.
A strand of RNA can attract other individual RNA building blocks, which stick to
it to form a sort of mirror-image strand ΓÇö each building block in the
new strand binding to its complementary building block on the original,
ΓÇ£templateΓÇ¥ strand. If the new strand can detach from the template strand,
and, by the same process, start templating other new strands, then it has
achieved the feat of self-replication that underlies life.
But while RNA strands may be good at templating complementary strands, they
are not so good at separating from these strands. Modern organisms make
enzymes that can force twinned strands of RNA ΓÇö or DNA ΓÇö to go their
separate ways, thus enabling replication, but it is unclear how this could
have been done in a world where enzymes didnΓÇÖt yet exist.
A chimeric workaround
Krishnamurthy and colleagues have shown in recent studies that ΓÇ£chimericΓÇ¥
molecular strands that are part DNA and part RNA may have been able to get
around this problem, because they can template complementary strands in a
less-sticky way that permits them to separate relatively easily.
The chemists also have shown in widely cited papers in the past few years
that the simple ribonucleoside and deoxynucleoside building blocks, of RNA
and DNA respectively, could have arisen under very similar chemical
conditions on the early Earth.
Moreover, in 2017 they reported that the organic compound DAP could have
played the crucial role of modifying ribonucleosides and stringing them together
into the first RNA strands. The new study shows that DAP under
similar conditions could have done the same for DNA.
ΓÇ£We found, to our surprise, that using DAP to react with deoxynucleosides
works better when the deoxynucleosides are not all the same but are instead
mixes of different DNA ΓÇÿlettersΓÇÖ such as A and T, or G and C, like real
DNA,” says first author Eddy Jiménez, PhD, a postdoctoral research
associate in the Krishnamurthy lab.
ΓÇ£Now that we understand better how a primordial chemistry could have made
the first RNAs and DNAs, we can start using it on mixes of ribonucleoside and
deoxynucleoside building blocks to see what chimeric molecules are formed ΓÇö
and whether they can self-replicate and evolve,ΓÇ¥ Krishnamurthy says.
He notes that the work may also have broad practical applications. The
artificial synthesis of DNA and RNA ΓÇö for example in the ΓÇ£PCRΓÇ¥
technique that underlies COVID-19 tests ΓÇö amounts to a vast global
business, but depends on enzymes that are relatively fragile and thus have
many limitations. Robust, enzyme-free chemical methods for making DNA and RNA
may end up being more attractive in many contexts, Krishnamurthy says.
Reference: ΓÇ£Prebiotic Phosphorylation and Concomitant Oligomerization of
Deoxynucleosides to form DNA” by Eddy Jiménez, Clémentine Gibard and
Ramanarayanan Krishnamurthy, 15 December 2020, Angewandte Chemie.
DOI: 10.1002/anie.202015910
Funding was provided by the Simons Foundation.
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