Hi. It's Mr. Andersen and in this podcast I'm going to talk about the Meselson-Stahl
Experiment. And so after Watson and Crick came up with the shape of DNA, this characteristic
double helix, the next thing to unlock was how does it make a copy of itself? And so
there were immediately three proposals as to how it might copy itself. Number one, it
could kind of unfold in the middle and then it could copy new templates on either side.
This is kind of what Watson and Crick proposed. There was another what's called the conservative
theory that somehow the DNA was kind of wrapped around these histone proteins. And then it
would make an exact duplicate of itself, almost like a photocopier. And then there's the dispersive
model that maybe it would chunk itself into little ten, twelve, nucleotide segments. And
then each of those would be copied. And so what Meselson and Stahl did is they figured
out which of these three is right. And in this picture, down here at the bottom is,
it was later figured out that this is Martha Chase, who's also famous for that Hershey-Chase
experiment. And so what did they do? Well basically what they did was they grew e. coli,
so this is Escheriichia coli, or a bacteria. And they grew it in heavy nitrogen. What does
that mean? Well nitrogen normally has an atomic or mass number, excuse me, of 14. That means
it has 7 protons, 7 neutrons. But heavy nitrogen is going to have an extra neutron. And so
it's slightly heavier. And when you spin it in an ultra centrifuge, DNA that contains
this heavy nitrogen is going to be moved to the bottom. And those that contain lighter
nitrogen is going to stay up near the top. And so we can look at the density of the DNA
to figure out what nitrogen is actually being added. And so if you think about it this way,
thought experiment. Let's say we started with heavy nitrogen. And then we were to copy that.
So as the bacteria make copies of themselves, they're going to unzip that DNA in the middle.
And since they're just in nitrogen 15, you're going to have two strands of DNA. But each
of those are going to be all heavy nitrogen. So it would go to the bottom of a centrifuge.
If they were to copy themselves again, the e.coli and make new template strands, 100%
of them are going to be of heavy nitrogen. And so what they did is they bred them in
there until the point at which they were all nitrogen, all heavy nitrogen. They then switched
them to e.coli living in the lighter normal nitrogen. And then they observed what happened
with each generation. And so basically the lighter nitrogen in my little simulation,
we'll color in orange color. And so bacteria on generation 0, we'll say, are going to be
100% heavy nitrogen or 100% N15. As they unzip and make copies of themselves, that new template
strand that they're going to add is going to contain nitrogen 14. And so at this point,
so kind of to keep track of everything, we had 100% heavy nitrogen the first time. On
this first generation, now we're going to have 100% of the strands are a hybrid between
the two. Half of them are heavy nitrogen and half of them are not. We're then going to
go through another copying process. And so at this point we've got one hybrid strand.
Another light strand. Another light strand and then a hybrid strand. And so at this point
it would be 50%, if this semiconservative theory is right, 50% light nitrogen and 50%
of the hybrid. Now think in your head, if we were to do one more copy, which I don't
have room for over here on my simulation, well this strand would create a hybrid. But
the next six strands as they split in half are going to form light nitrogen and this
would form a hybrid. So now we'd have a 75% to 25%. And so that's what the semiconservative
would predict. And that's what they found in their experiment. So we had 100% heavy
nitrogen in the first, 100% hybrid in the second. And then it split 50 / 50 again. This
is when my simulation stopped. But then it went to 75 / 25. And if we were to keep going,
then it would eventually drop off. And so what did that tells them? It told them how
DNA makes a copy of itself. It simply splits in half in the middle. And then we're going
to add a new template strands, or we're going to read that template to create new strands
on the other side. And so that's the Meselson-Stahl experiment. It's very elegant. It's also very
important and I hope that's helpful.