[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
