this is one of the most important images
in biology because it helped us to
figure out that DNA had helical
structure and this image also tells us
some of the important details of that
helical structure it was generated by a
scientist called rosalind Franklin using
a technique called x-ray crystallography
and in this video I want to demonstrate
how the helical structure of DNA can be
deduced from an image like this by doing
the reverse I'm gonna create an image
like this starting off with an object
that has helical structure in this case
I'm not gonna used in itself I'm gonna
use the filament of a light bulb and I'm
not going to use x-rays I'm gonna use
visible light so I'm going to use a red
laser so first of all x-ray
crystallography is a diffraction
phenomena so let's first of all
demonstrate diffraction we're going to
shine our laser through two slits this
is a classic experiment Young's double
slit experiment you may have seen it
before so we'll just very quickly go
over it so you shine a laser through the
two slits and so beyond the slits you
have now two wave fronts here I'm
representing the peaks of the wave in
whites and the troughs of the wave in
green and in different regions those
waves will either constructively
interfere each other or destructively
interfere with each other for example in
this position here because there's a
slight difference in travel time from
the two slits you find that the peaks
from one wave meet the troughs from the
other wave and vice versa so they cancel
each other out you don't see any light
in this region but in this region the
difference in travel time just so
happens to make it the the peaks from
one slit meet the peaks from the other
slit they add to each other they
constructively interfere and you do get
lights in those regions so if you were
to map out all the points of maximum
constructive interference and maximum
destructive interference it would look
like this so if you were to put a screen
in front of the two slits you will see
what's called a diffraction pattern
where you have several peaks and troughs
in brightness here's the pattern that I
got with my laser you'll notice that
this pattern is very bright in the
middle and it very quickly dips away to
quite dim
spots as you move outwards if instead of
having two slits you have multiple slits
known as a diffraction grating then the
brightness falls off much more slowly
you get a nice even distribution of dots
this is an obvious thing to say but it
becomes important later look if I tilt
the diffraction grating then obviously
it tilts the pattern now look what
happens if I take a second differential
grating and I put it in front of the
first one at 90 degrees to it we get
this grid of dots and that makes sense
really because every dot from the first
diffraction grating is now split into
several dots by the second diffraction
grating but look what happens if I tilt
that second diffraction grating laser
pointers often come with interchangeable
tips and these tips just have different
types of diffraction grating in them to
produce different patterns of laser
light this tip for example I haven't
taken it part I don't know what kind of
diffraction grating or gratings are
inside but if I shine a laser through it
then this is the pattern I get and look
I can twist it to change the pattern so
although I don't know what's inside here
I can deduce it because look the pattern
is the same as the pattern I got when I
was manually moving to diffraction
gratings around so it must be the same
thing going on in here and that's what
x-ray crystallography is all about
really it's about looking at the pattern
you get and then working backwards to
figure out what kind of crystal must I
be shining my x-rays through to produce
this pattern if you didn't already know
the structure of DNA you might assume
looking at this diffraction pattern that
DNA has a structure something like two
diffraction gratings held at an angle to
each other because both of these images
have that cross of dots but DNA doesn't
have that structure so what's going on
this is where it gets interesting
so I smashed an incandescent bulb to
reveal the filament inside and that when
you shine a laser through the filament
you get this pattern and you can just
about see that same cross of dots in the
middle that we got when we were playing
with two diffraction gratings one in
front of the other and that makes sense
because if you look
clothes at the filament it's essentially
like a spring isn't it and the laser has
to pass through two sides that the first
side of the spring that it passes
through is like a diffraction grating
that's tilted in one direction and when
it passes out the other side of the
spring it's like it's passing through a
diffraction grating that's tilted in the
other direction so it's just like the
setup we've got with these two
diffraction gratings that are tilted at
an angle look what happens when I
stretch and compress the filament it
changes the angle between those two
lines of dots so you can work out from
the angle between those lines the pitch
of the spring how tightly coiled it is
and the same is true for DNA to get this
to work the wavelength of the light you
use needs to be roughly of the same
order of magnitude as the distance
between the coils of the spring so in
the case of DNA you can't use red laser
light you can't use the visible light at
all you have to use x-rays so they're of
the same order of magnitude as the
distance between the atoms in DNA which
is why it's called x-ray crystallography
two ways in which this analogy breaks
down that are really interesting the
first is it's x-ray crystallography so
you're dealing with crystals who'd have
thought you could grow DNA crystals
turns out if you've got fewer than a
hundred base pairs you can actually get
these molecules to crystallize so the
image you're seeing here is generated
when you shine x-rays through a
repeating pattern of helical molecules
whereas we've got a single helix of bulb
filament the other thing is that DNA is
a double helix whereas we've got a
single helix in our setup you can
actually see evidence of the double
helix in the image because it turns out
if you've got a double helix like the
one in DNA then you would expect some of
the dots to disappear from the image and
that's what's going on here these
missing dots suggest that you have a
double helix isn't that amazing
so two scientists called Watson and
Crick analyzed the image there was no
franking created and they were able to
figure out all this stuff you can figure
out from the
angle of the dots the the ratio between
the width of DNA and the pitch of DNA
where the pitch is like the length of a
full turn of the helix it turns out to
be about 1.7 so the wizard DNA is 2
nanometers and the length of a full turn
is 3.4 nanometers and with this
information and the fact that DNA is a
double helix Watson and Crick were able
to work out the arrangement of the
various components of DNA essentially
how do these nucleotides fit together
necessarily figured out the structure of
DNA absolutely amazing at this point a
lot of you might be thinking well hold
on how does this analogy work at all
because well in your light bulb filament
you've got nice gaps between the spirals
for the laser light to pass through but
DNA isn't like that there aren't any
gaps so if you look at the space inside
a DNA molecule it's full of these base
pairs that reach into the middle well it
turns out that these base pairs are
pretty much transparent to x-rays the
only thing that x-rays can see is the
phosphor molecules in the backbone
spirals of DNA so it still works by the
way when I was looking at this whole
crystallizing DNA thing I found some
amazing images and time-lapse footage of
DNA crystals growing isn't this
beautiful
you use the fact that DNA can turn
polarized light and you use to protoid
filters different angles to create this
kind of rainbow effect so you can really
see the details of the crystals as they
grow these were made by Linden Gledhill
he does amazing things with cameras and
microscopes and science just really good
I recommend you check out his work the
link is in the description my channel
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