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

support us, visit our website at statedclearly.com and click "contribute"

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So long for now stay curious!