DNA is often referred to as the instruction manual for building an organism or the blueprint for an organism.
And I like that.
It's a good one sentence summary of what's going on, but obviously it leaves this huge gap in understanding.
Because, if you think about it
like, what really is DNA, like on the molecular level?
Well in simple terms DNA is a really long molecule made of smaller molecules chained together,
and there here are four possible small molecules that DNA can use they're called A, T, C, and G for short.
For example, maybe this is a adenine, this is thymine, this is cytosine, and this is guanine,
and the particular sequence of A's, T's, C's, and G's
that make up this chain of DNA is like a code and it's a code that tells a cell how to build proteins.
The way your cells build proteins based on the code that's inside DNA is really interesting.
Basically, you've got these giant molecules inside your cells called ribosomes,
and a ribosome is basically like a robot. I mean it is a robot.
It's just a molecular size robot and a ribosome will grab ahold of one of these chains that we've been talking about,
one of these A, T, C, and G chains,
and it will pull the chain through itself,
and the particular sequence of A's T's C's and G's as they pass through the ribosome will cause the ribosome
to grab different components from its environment.
And so, for example, suppose the sequence coming in is A, A, T.
Well, then the ribosome is going to grab hold of an asparagine molecule, but if the sequence is G, G, C
then it's gonna grab a glycine molecule.
There are 20 possible molecules to choose from they're all called amino acids,
and actually the ribosome doesn't grab the amino acid directly instead it grabs hold of a carrier molecule.
Those are shown in green in the animation and a little red blob on the end.
That's the amino acid that it's carrying. And actually when I say grab,
the ribosome isn't reaching out and getting these things.
It's just that they're all jumbling around in solution,
And occasionally the right carrier is going to come along to match the sequence of "letters"
that's currently inside the ribosome, and the ribosome is chaining these components together to form the protein.
So you've got this chain that's being pulled through the ribosome,
and at the same time the ribosome is spitting out a new chain,
and that new chain will fold up into some complex protein.
So, you can see how the specific sequence of A's, Ts C's, and G's defines the sequence of ingredients
that are pulled in by the ribosome to form the specific protein that it wants to build.
So that's how DNA codes for proteins.
I have actually missed out a step where a copy of the DNA is taken first.
The copy is called RNA and all the T's are switched for U's.
But here's the thing, when the ribosome is finished building the protein it just spits it out,
and the protein is then just swimming around in solution. It's moving around aimlessly
in the fluid inside your cells.
And, that seems like a really random process, really undirected, and
And, that's where the chasm of understanding is.
You've got this really stochastic looking process: the building of proteins,
and then them sloshing around in solution .
And then, on the other end, you've got the organism which seems really
directed, really deliberate, like, you know, five fingers go on the end of this thing five fingers go on into this thing
So, you can get from one to the other.
You can fill in the gap between the stochastic process going on at the molecular level and the very
deliberate process of the organism.
And, this video is going to look at the early stages.
So, how do you get from these molecules and proteins moving around in solution
to some kind of meaningful structure. And it turns out, that the way viruses reproduce is a really good example
of how meaningful structure can arise from molecules swimming around in solution.
A virus is basically just a relatively short piece of DNA
surrounded by molecular shell called the capsid. The capsid protects the DNA inside.
Actually, viruses sometimes store their genetic code in RNA molecules instead of DNA molecules,
but the point is, viruses are really small and really simple. There's no room for
molecular robots like ribosomes
So, for a virus to reproduce, it needs to hijack
the molecular machinery of a host cell
Once a virus gets inside a cell,
the protective capsid falls away leaving the DNA or the RNA exposed, just floating around.
Let's assume it's RNA in this case. Eventually it will bump into a ribosome,
and ribosomes will just blindly read whatever RNA it comes across.
So, the ribosome is feeding the RNA into itself and spitting out proteins
according to the genetic sequence that's encoded in the viral RNA. In other words,
the cell starts to produce new viral proteins. And here's the really clever part,
these viral proteins are just floating around aimlessly in solution,
but, they've been built in just such a way that when they bump into each other they snap together,
and they snap together at just the right angle.
Here's what happens when you've got a whole load of them jiggling around together
A meaningful structure arising from a random process
You might have noticed that the virus has icosahedral symmetry or
dodecahedral symmetry it has twelve identical faces, like this,
and that's really common for viruses.
And the brilliant thing is, the RNA of the virus only needs to code for this one single protein
to build its protective capsid shell. If you pump out enough of them, eventually
twelve will bump into each other and form a capsid. In reality, one of these faces
of the capsid may be made of a handful of different proteins,
but still, it's a really efficient use of genetic code.
One of my favorite examples is a family of viruses called circoviridae.
The DNA of these viruses codes for just two proteins:
one that forms the outer shell capsid of the virus, and the other that helps with DNA replication.
The DNA is so short. It's just half a kilobyte in size.
That's as much data as you could store in a 22x22 pixel grayscale image.
For comparison, the human genome is 700 megabytes in size
Not all viruses have this Icosahedral symmetry, some have spiral symmetry
but again, it's really efficient use of genetic code because it's made of just a handful of repeating protein units.
Among the most complex viruses are ones that infect bacteria, because
bacteria have a cell wall that needs to be overcome.
So, bacterial viruses have evolved a way to inject their DNA into the bacterial cell from the outside.
And yet, the Assembly of the virus inside the cell happens in much the same way, just a jumble of proteins
coming together and snapping into the right position.
This video is made possible by my patrons on patreon, but I'm also really grateful for the continued support of brilliant.org
who are sponsoring this video. Two new things? I want to say about Brilliant that You probably don't know yet.
Well, actually first of all, if you don't know what it is
it's a website full of amazing maths, science, and engineering problem solving courses.
The two new things are well, first of all, it's gotten really interactive,
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And if you ever do get stuck, there's a whole community of learners just like you discussing the solutions.
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The link is also in the description.
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courses and daily challenges. Thanks also to Hamish Todd who lent me his virus self-assembly model.
He makes really interesting interactive content. I'll leave a link in the description for his stuff,
and thanks also to Arthur Olson who actually devised that model.
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