Stated Clearly presents
What is the RNA world hypothesis? If you were to go back in time a 120
million years, you'd find yourself in a dinosaur world. 500 million years
ago was a world of trilobites and other strange sea creatures. 3.4 billion years
ago was the world of the first living cells
and if you were to go back further still
scientists suspect that chains of a chemical called RNA, or something similar
to RNA, kick-started this entire beautiful mess that we call life!
RNA is thought to have given rise to life for several reasons:
Chains of RNA are found abundantly in all living cells today, RNA is a closed
chemical cousin to DNA, and with very little help from researchers, chains of RNA
can replicate evolve and interact with their environments. While many details
have yet to be worked out, the RNA world hypothesis is the simple idea that
somewhere on early planet, perhaps in a tide pool or hot spring, the Earth's
chemistry was producing random chains of RNA. Once formed they begin
replicating, evolving, and competing with each other for survival. As these chains evolved and
diversified, some eventually began cooperating to produce the genetic code
a wide array of complex proteins, and even living cells which, from the
perspective of RNA, can actually be thought of as houses or survival
machines for RNA to live inside. To understand how RNA chains can interact
with their environments, replicate, and evolve; we first need to understand the
simple process of base pairing.
chains of RNA are made of nucleotides - small molecules that come in four
different types labeled A, C, U, and G. The backbone atoms of a nucleotide, shown
here is a yellow bar, can form strong chemical bonds with the backbone atoms
of any other RNA nucleotide. This means that different chains can have
completely different sequences from left to right. The parts we call the bases of
nucleotides - the colored section labeled A, C, U, or G - are attracted to other bases
sort of like a magnet but they're selective about who they will stick to.
G selectively pairs with C, A selectively pairs with U. When basis find their
matches and stick together, we call it "base pairing". Researchers have found that
with a little bit of assistance, base pairing allows chains of RNA to
replicate and evolve.
Here's how it works: When a long chain of RNA is suspended in cool water with high
concentrations of free nucleotides, the chain can act as a template for its own
replication. Nucleotides automatically base pair with their partners on the
existing chain. If their backbone atoms form chemical bonds with each other (and, by the way
this is the part that currently requires assistance from researchers,
we're not yet sure how this would have happened in the wild)
a complimentary RNA strand is born - one with the exact inverse sequence of the
original! If the water is then heated, paired basis lose their grip allowing
both chains to act as templates when the cycle repeats. The great thing about this
process is that every other RNA chain produced as a copy of the original, but
sometimes mutations slip in. This means that as these chains compete for survival and
reproduction, true evolution - descent with modification acted upon by selection - can
operate on chains of RNA. As amazing as replication is, base pairing also gives
RNA chains a second special ability. When placed in water cool enough for base pairing
but without enough free nucleotides for replication, chains will fold up and
base pair with themselves!
The end result is a complex shape with certain sticky basis pointing outward
because they weren't able to find partners.
These sticky, outward-facing bases can cause unique chemical reactions by
interacting with other molecules in their environment. A folded chain of RNA
capable of guiding a specific chemical reaction is what we call a ribozyme. Some
ribozymes break certain molecules apart
others joined certain molecules together. A ribozyme's specific function is
determined by its specific shape, and its shape is determined by its sequence. If a
mutation changes a ribozyme sequence the, shape can be modified and so can its
function. When ribozymes were first discovered, scientists wondered how
difficult it would be for random chains of RNA to evolve legitimate survival
functions. Imagine, for example, a ribozyme that could build nucleotides out of
molecules it finds in its environment. Across multiple generations, natural
selection could promote and refine this ribozyme because the chain would tend to
have access to more free nucleotides than its rivals, allowing it to replicate more
often. To explore this idea
researchers at Simon Fraser University produced a large group of random RNA
chains and examined them to see if any happened to be able to make nucleotides.
Surprisingly, some actually could, but they weren't very efficient. Researchers
selected out the successful chains and then use a lab technique called PCR to
quickly replicate them with slight random mutations. After just 10 rounds of
PCR followed by selection, highly efficient nucleotide building ribozymes
evolved. These are molecules with the life-like ability to actively participate
in their own survival! These ribozymes, and many others produce through similar
experiments, are beginning to blur the line between living things and simple
chemistry! So to sum things up, the RNA world hypothesis is the simple idea that
the first things to replicate and evolve on our planet, may have been chains of
RNA or something similar to them.
While the basic idea of the RNA world does seem to give us a promising pathway
to the origin of life, it's still very much a work in progress. As mentioned, one
of several unsolved problems is: how did nature get backbone binding to function
without the special enzymes or lab techniques we use today? While many
researchers continue to focus on RNA, others are investigating alternative
molecules: chemical systems that might replicate and evolve without assistance
and could have given rise to RNA. Continual breakthroughs are being found
in both avenues of research.
I'm Jon Perry, and that's the RNA world hypothesis Stated Clearly.
This video is funded by the Center for Chemical Evolution, the National Science
Foundation, and NASA! Though we do receive grants from time to time, Stated Clearly
is made possible with financial contributions from viewers like you. To
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So long for now stay curious!