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  • 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

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

  • I'm happy to announce that you can now alsosupport us at patreon.com/statedclearly

  • So long for now stay curious!

Stated Clearly presents

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B1 中級 美國腔

什麼是RNA世界假說? (What Is the RNA World Hypothesis?)

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    ke 發佈於 2021 年 01 月 14 日
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