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- Hello, I'm Lindsay Turnbull

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and I teach biology at
the University of Oxford.

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In this video,

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I want to tell you how
cells process information.

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It's a crucial concept

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from chapter one of my new book,
"Biology: The Whole Story".

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(birds chirping)

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(frogs croaking)

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All Life on Earth is made from cells.

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Indeed, most cells live alone

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and they're too small
for us to really notice.

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But what we can't help noticing

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is all the larger beings
that roam the Earth,

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and they too are made from cells.

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You and I, for example,

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are each made from
around 37 trillion cells,

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which is a pretty mind-boggling number.

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And if we are going to function properly,

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then every single one of those cells

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needs to know exactly what
it's supposed to be doing

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or we'll kind of fall apart.

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So how do cells know

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what it is they're supposed to be doing?

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Well, they have to make all
the right tools and machinery

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and spare parts that they need.

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And in order to do that,

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they have to have the right information.

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So if we're going to understand

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this process of information flow in cells,

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then we're going to have
to look inside them.

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And there are two real
problems with doing that.

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The first is that cells are really tiny,

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but the second is that cells
are fiendishly complicated.

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But the good news is that
I can show you a diagram

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that's really very simple

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because it's just going to
contain these core components

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that all cells have.

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Let's start by just putting
on the outer membrane.

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That's important for all
cells, because it means

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that they can keep all
their components together

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and they won't just drift off.

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And now let's put these
crucial things inside.

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So the very first thing that
we're going to add is a genome,

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and that is a massive
book of instructions.

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So all the information that
the cell could possibly need

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is contained within this genome.

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The second thing are messages

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that are going to fly out of the genome

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that just carry individual instructions.

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The third thing are
the cell's key workers,

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and they're called ribosomes.

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And their job is to trap
these messages and read them,

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and they use the information
contained in the message

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to build all of the tools, machinery,

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and the spare parts that the cell needs.

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So this, this set of things here

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is really central to the
functioning of any cell on Earth.

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It doesn't matter whether
that cell lives alone

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or whether it's part of a larger body.

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Let's take a closer look now

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at each of those four crucial
components inside the cell.

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So the first thing we
looked at was the genome.

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Now, the genome is made
from a molecule called DNA.

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And like many molecules in cells,

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a long strand of DNA is formed

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by sticking together smaller molecules,

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in this case the DNA letters.

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Now, the DNA letters come in four kinds,

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the letters A, C, G, and T,

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and they can be stuck
together in any order.

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And then we get this very long strand.

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So just how many letters do
we need to build an organism?

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Well, if you want to build a bacterium,

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you're going to need
around a million letters.

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This worm, probably gonna
need a hundred times more.

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And if you wanna make a
shark or a pigeon or a horse,

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then you're probably gonna need
a billion letters and more.

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So we need that molecule
to be enormously long,

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and that means it's got to be very stable.

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And that's why you get this
classic double helix shape.

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So only one of the strands
carries the information

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and the second strand is really there

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to support the first strand

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and make sure that the molecule is stable.

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The second thing we looked
at was those little messages

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that were flying out of the genome

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just carrying a single instruction.

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And they're made from
a different molecule,

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although it's very
similar, it's called RNA,

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and it's also letters

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but it's got a slightly
different alphabet.

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So it uses the letters A, C, G, and U.

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The third thing we looked
at was those ribosomes.

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Now they're very large molecular machines.

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Now we've drawn them as
sort of fat guys in braces,

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but of course that isn't
really what they look like,

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and they're actually made from
a type of RNA and protein.

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The fourth thing we looked at

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was actually all that machinery

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and all the tools that
the cell was making.

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And they're not made from DNA or RNA,

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they're made from protein.

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Now, proteins are also
made by sticking together

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lots of smaller molecules
into a long chain.

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But those smaller molecules in this case

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are called amino acids.

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And once they've been stuck
together into a chain,

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then the chain starts to fold and twist

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and bend itself into an exquisite shape.

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And the shape of the molecule

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then allows it to
perform whatever function

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or job that it needs to around the cell.

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Okay, so proteins we said
are the molecule of choice

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for making all of the
machinery and the tools

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and spare parts that the cell needs.

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So how is it possible
for just one molecule

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to make such an extraordinary
array of shapes?

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You know, cells make
some incredible things,

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not just crude tools.

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They make locks with
perfectly fitting keys.

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They can make channels that
they can put into the membrane

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which only certain molecules
will be allowed to enter,

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and they can even make an
amazing molecular turbine

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to generate all the energy that they need.

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So how can this one molecule protein

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make all of those shapes?

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Well, as we said, once
the chain of amino acids

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is put together by the ribosome,

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it can twist and turn
into an amazing shape.

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And because there are
20 different amino acids

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to choose from,

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that's why each chain can
really be quite different.

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And those different amino acids

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have very different properties.

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So some of them attract each other

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and some of them repel each other.

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And that's why the chain
can fold up and bend

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and twist in the way that it does.

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Now, what's absolutely crucial, of course,

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is that the amino acids
are joined together

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in exactly the right sequence.

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If that doesn't happen,

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then the thing isn't going
to function properly.

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So that's the job of the ribosome,

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is to make sure that it
reads the message carefully

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and it puts the amino acids together

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in exactly the right sequence.

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And the way it does that
is rather interesting.

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So when the message arrives,

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it's going to read the letters
three letters at a time,

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and each of the three letters corresponds

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to a single amino acid.

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So by reading the three letters,

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it can decide which amino
acids should I add next.

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For example, the three
letters GGA tell the ribosome

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that the next amino acid to
add to this chain is glycine.

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And what's truly incredible
is that it wouldn't matter

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what cell you went into anywhere on Earth,

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if you gave it the letters GGA,

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the ribosomes in that cell would know

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to add the amino acid called glycine.

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And that's one of the reasons
why we're very, very confident

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that all life on Earth is related

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and descended from a
single common ancestor.

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So hopefully we can see
now why this information

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is so crucial to the functioning of cells.

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They have to get the right information

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from the genome via the message

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so that the ribosomes can
actually make the right protein,

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the one that's gonna work properly.

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So let's imagine that ribosomes

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can make some kind of hammer.

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In reality, cells make much more

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sophisticated things than hammers,

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but it's something we can think about.

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So the message comes out
and they build the hammer.

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But in some cells,

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it might be that there's some
slight change to that message.

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Perhaps one of the letters in
the genome has been changed.

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And that does happen, and it's
a process called mutation.

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So sometimes the letters can
change from one into the other,

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an A might become a C, for example.

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Or sometimes a few letters
get added or they're missing.

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And that means that the
message that the ribosomes get

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is slightly different.

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And they'll still go
ahead and translate it,

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but this time they might
put the wrong amino acid in.

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And so the hammer might
look a bit different.

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It might look like this now.

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And that hammer might
not work quite as well

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as the normal hammer that
they should be making.

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And actually most of us have
got a few mutations like that.

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So there'll be a few proteins
in your body and in my body

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that are a little bit suboptimal.

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But for some people, those
changes are much more serious,

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and that can lead to something
called a genetic disorder.

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And examples of those are
things like cystic fibrosis,

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which affects a protein
that's very important

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in the functioning of lungs,

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or sickle cell disease,

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which affects the protein
called haemoglobin

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that transports oxygen around the body.

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So genetic disorders can be mild

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and they can be very serious,

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and they're all down to changes

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in those DNA letters in the genome.

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And that's a little bit of the problem

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with this information transfer process.

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It only flows one way.

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The ribosomes can't try
out different proteins.

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They can't send feedback to the genome

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and say, well, this
hammer's not quite right,

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thanks very much.

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Information flows one way
through cells from the genome

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to the RNA messages, to the
ribosomes, and that's it.

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So we might think, well,
that seems a bit terrible.

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Why do we have genetic disorders?

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Why do we have mutations?

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But of course,

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mutations are absolutely
crucial for evolution.

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Organisms look different

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because their genomes are different.

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So if an organism

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is going to change
through time and evolve,

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then its genome has to change.

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And the only way its genome can change

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is through this process of mutation.

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Now, most mutations, as we've seen,

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are going to be harmful

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because you are a very well-oiled machine,

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and some random change

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is unlikely to make you
function any better.

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But that's not to say
that it can't happen.

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Sometimes, for example,

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there might be a change in the genome

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in the instruction for building a hammer

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that makes a hammer like this

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which is much better
than the original hammer.

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And if that happens,

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then that individual is
going to be more successful

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perhaps than those around it.

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So what we'll see in the next episode

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is exactly how evolution works.

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So I hope you enjoyed that video,

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and if you did, then please
do share it with friends

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and colleagues who you
think might also enjoy it.

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There's also the book itself, of course,

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and there's a link below

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if you'd like to buy it
and have your own copy.

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There's a lot more depth
and detail in the book

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on this topic and on others.

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So for example, in chapter one,

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there's a whole load of stuff
about the origins of life

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and how understanding
information flow in cells

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can help us understand the
origins of life better.

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We're also planning to
make more in this series.

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So look out for the next videos.

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We're hoping to make one for each chapter.

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And the next one coming
up is all about evolution.

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(birds chirping)