[PBS Intro]
I hate to tell you this, but if you’ve ever looked at a biology textbook, chances are
it was lying to you.
Or at least not telling you the whole story– about DNA.
Today, we’re going to fix that.
[OPEN]
Real quick, what image comes to mind when I say “DNA”.
Have you got it?
You’re probably imagining something like this, or this, or maybe this?
That’s not what DNA looks like.
Of course you can’t exactly take a photograph of DNA, or see it through your typical microscope.
It’s simply too small.
A double helix of DNA is just 2 nanometers wide.
A DNA strand next to a piece of hair is like a person standing next to The State of Rhode
Island.
Even the best light microscopes can’t see anything much smaller than about 200 nm, because,
well, light can’t really interact with something smaller than its wavelength.
This is why scientists look at super-small things with electron microscopes, because
the wavelength of an electron can be a LOT smaller than visible light.
But even *that* doesn’t give us a very good picture of something as small as DNA.
Rosalind Franklin’s famous image, that solved the double helix structure, was made by shooting
DNA with X-rays, which are also smaller than visible light.
But it isn’t really a picture of DNA, it’s more like DNA’s shadow.
*The* best we’ve done, is by basically dragging a ridiculously small needle across the DNA
and feeling the bumps, sort of like a nanometer scale record player.
Now, all these methods and others have given us an accurate model of DNA’s double helix.
But still… this isn’t really the whole story, because that’s not how DNA looks
inside our cells.
Each of our cells holds 2 meters of DNA inside a nucleus just ten millionths of a meter across…
which is mind-boggling.
To put that in perspective, if a double helix were the width of a pencil line, one cell’s
DNA would stretch a thousand kilometers, then wrapped in a ball less than 5 meters wide.
To fit in our cells, DNA is wrapped around beads of protein, which are coiled again,
and then again, and again… and again, until all 2 meters of DNA in our 46 chromosomes
measure less than a tenth of a millimeter end to end.
That… is efficiency.
These squishy little shapes are how textbooks usually draw chromosomes… which is also
a problem, because that’s not what chromosomes look like most of the time.
DNA looks like this during a very short window when a cell is dividing into two different
cells.
But when DNA is packed that tight, the cell can’t do anything with it, like make stuff.
It’s like a book that’s locked shut.
Most of the time, our chromosomes are partly unwound, in one of those medium-sized coily
shapes.
Now I can’t even put headphones in my pocket without tying five knots, so you’re probably
wondering how our DNA keeps from getting hopelessly tangled.
To answer that, scientists have finally figured out how to look at a cell’s DNA in three
dimensions.
This is a genome in 3-D.
We are so used to looking at DNA on paper, or on a screen, we forget this stuff is floating
around in three dimensions.
But the nucleus isn’t just a bag full of ramen noodles.
Turns out there’s a lot more organization than we thought.
The nucleus is coated with a mesh of fibers that give it structure, and chromosomes get
anchored to this mesh.
Can’t have them just floating around all willy-nilly.
Turns out each chromosome hangs out in its own “territory” inside that web.
The part of the chromosome that’s being read and making stuff is near the center,
while DNA that’s not being read is usually closer to the edge, wound up tighter.
The two copies of each of your chromosomes aren’t even next to each other.
Genes are turned on and off not just by little flags on a string, but by how that DNA is
organized in three-dimensional space.
Even two bits of DNA on totally separate chromosomes can interact in this 3-D web.
How these loops and twists are arranged is what lets cells take billions of letters of
code and turn it into life.
This organization is important to how cells function normally, but it’s also a part
how diseases like cancer arise, even how different cells behave inside the brain.
Simplified ways of looking at DNA are useful.
They help us learn, they help us tell stories about how these complicated machines work…
but it’s important to remember that’s not the whole story.
Kind of like how the blueprints for a Saturn V can tell you how rockets work, but they
won’t tell you how to get to the moon.
Now that we’ve got a better picture, we’re able to see questions we didn’t even know
to ask.
Stay curious.
This is for eukaryotes, things with nuclei.
Bacteria pack their DNA totally differently.
If you see bad DNA, tell me on twitter, and use the hashtag #badDNA