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Good morning. Yeah.
All right. Good.

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Something to
counteract the rainy

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days we
have here.

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All right. Today we're going to
make a very important transition.

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The transition goes
back to this picture.

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Of course, what
we want to do is

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understand biological function by
taking our two favorite approaches.

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Understanding the organism
minus an individual gene.

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Understanding an organism
minus an individual component,

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and understanding the individual
components minus the organism,

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genetics and biochemistry. And,
as we know, the geneticist went off

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on their route finding mutants,
doing mutant hunts, making crosses,

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making genetic maps, et cetera,
didn't understand really what these

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genes had to do with anything
specific in the organism,

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other than they produced
phenotypes when they were mutated.

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And the biochemist went
off purifying enzymes,

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working on biochemical
pathways, et cetera, et cetera.

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We began to see some connection when
we talked a bit about the mutants

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that affect the ability to make
arginine and the fact that they

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could encode different enzymatic
steps. And, in particular,

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I highlighted the work of
Archibald Garrett who really,

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right at the start of the century,
recognized that somehow genetic

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mutations were responsible for
somehow affecting the production of

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enzymes in important
biochemical pathways.

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So, that was sort of one connection
of genetics into protein but still a

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rather tenuous one. The real
next step to connect these

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two would be to do the
biochemistry of genes. So,

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how would a biochemist approach
heredity? A biochemist would grind

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up the organism,
fractionate it in different

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components, and attempt to
find heredity, purify heredity,

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get like a pure solution of
heredity. And that's nuts, right?

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The notion that you could purify
heredity as a biochemical entity

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because like how would you
know you had it? But, of course,

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that's exactly what happened.
That is what happened, was

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biochemistry developed sufficiently
far that folks were actually able to

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purify substances, not
just that could digest a

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particular sugar or substances
that might, you know,

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like slide by each other
like actinomycins of muscles,

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but substances that were actually
heredity. And that began the real

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unification of those, and
that's the point of today.

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And that is the field of molecular
biology. And we're going to cover

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tremendous territory in unifying
these two different fields.

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OK. So, let's dive right in.
The discovery of the transforming

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principle. It's a
wonderfully old-fashioned

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kind of word. Nobody would
use language like this today.

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The discovery of the
transforming principle. So,

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this starts, this shaggy dog story
starts in about 1928 with the work

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of Griffiths. Griffiths had no
particular interest in DNA or

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genetics or biochemistry
for that matter.

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Griffith was interested in bacteria.
He wanted to understand bacteria.

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And, in particular, he
studied pneumococcus bacteria,

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which could infect and kill mice.
And he was very interested in the

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mechanism by with these
pneumococci could kill mice.

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Now, it turns out that the
pneumococcus bacteria came in two

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different types.
One, the bacteria,

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when they grew on a Petri plate,
produced a glistening, smooth, shiny

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colony. We'll call them
smooth bacteria here.

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And these bacteria, in
addition to being smooth and

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glistening, were virulent.
That is, if you inject into the

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mouse, these bacteria would kill the
mouse. They are smooth because they

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have this encapsulating
polysaccharide coat around them.

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And it's not necessarily the case
that that's what makes them virulent,

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although it actually does have a
role in that, but it is the case

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that they're smooth and
they're virulent. So,

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you inject in a mouse, mouse
dies because the mouse is not

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resistant to these bacteria. By
contrast, there were strains of

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pneumococcus that were rough.
They did not have the same kind of

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a polysaccharide coat, and
therefore they had a very rough

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appearance. They
didn't glisten.

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And these were non-virulent. If
you inject these into the mouse,

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the mouse immune system was able
to fight these particular rough

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bacteria. OK. So,
Griffith did the obvious

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experiments. So,
take some bacteria,

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we'll take some smooth
virulent bacteria, we'll inject

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into a mouse. And
what will happen?

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The mouse will die. This is one of
the easier assays in the laboratory.

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It's the feet up, feet down
assay, you have a dead mouse.

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OK. Number two. Then
take the rough bacteria,

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inject it into a
mouse, what happens?

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Sorry? It lives. The mouse
lives because these are

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non virulent. OK. Now,
let's just do some simple

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controls. Let's take the smooth
bacteria and autoclave them,

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heat them up to very high
temperature to kill them.

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How will we know they're dead?
You try plating them out. They

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don't grow anymore,
so they're dead.

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So, take heat killed, and
you can verify in the lab that

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they're killed,
heat killed smooth,

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check that they really were heat
killed, inject them into the mouse.

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And what happens? It lives
because, I mean, they're bacteria,

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right? OK. Last of all we take the
utterly harmless rough bacteria,

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plus the utterly harmless
heat-killed smooth bacteria,

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we inject them into the
mouse, and what happens?

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It dies. That is a notable result
because the rough bacteria alone

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will not kill this mouse and the
smooth bacteria that have been

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heat-killed will not kill this mouse,
but together they killed the mouse.

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This is very puzzling. What
was even more puzzling was

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when you autopsied the mouse,
you can isolate from that mouse

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smooth, virulent, live
bacteria, but you didn't put

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any in. Very strange. So,
this actually yields live

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smooth virulent bacteria despite
not having put any in there.

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Virulent bacteria. Somehow
we were able to create

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smooth and virulent bacteria
notwithstanding not having put any

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in here. So, of course,
Griffith then attempted to say,

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well, what was it that
allowed this to happen?

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So, he could try putting in dead
rough bacteria with dead smooth

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bacteria. That doesn't do anything.
You need to have something alive.

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So, you've got to have live rough
bacteria. You can then say let me

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take the dead virulent bacteria and
start fractionating it biochemically

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and asking what fraction of that
material from the dead bacteria

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allows us to recover, to have
this property of being able

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to now produce virulent
bacteria that killed the mice?

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And do you realize how utterly
tedious and painful that

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experiment is? You
take the dead bacteria,

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you fractionate it into lots of
different biochemical fractions.

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For each fraction, how do you
test whether it has the property?

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You've got to shoot up a bunch
of mice. This is a very tedious

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procedure. I mean it is, you
know, you can't underestimate

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how important the assay is, how
important it is to come up with

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easy ways to do things in order
to be able to accelerate progress.

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Griffith tried hard and roughly
began to purify fractions and get

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information about what the
fractions were. But, in fact,

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this work really never did
lead to a clear conclusion.

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But it did tell people that
there was some material,

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which got named The Transforming
Principle. This is almost like an

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old alchemical kind of word,
a principle being a particular

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chemical composition of matter,
which if you don't know what it is

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you call it The Living
Principle or something like that.

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So, what was this
transforming principle?

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Well, it really took work
about 15 years later by Avery,

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McCarty and MacLeod to sort this out.
What Avery, McCarty and MacLeod did

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was the same experiment
basically, except minus the mice.

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What they found was you
could take the dead bacteria,

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combine it, the dead virulent
bacteria, the dead smooth bacteria,

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combine it with the live rough
bacteria, and by combining it in the

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right way in a test tube you would
be able to plate it out on a Petri

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plate and see smooth
bacteria come out.

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Sans mouse. So, they
didn't need the mouse.

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This dramatically accelerated work
because if you're able to just take

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fractions of the dead bacteria,
add it to the live bacteria and look

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for the presence of some smooth
bacteria coming out of it,

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you would be able to work much
more quickly. And they did.

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And they began purifying. And
they began purifying and they

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tried to isolate the fraction that
contained this new ability to make

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these bacteria acquire a new
property. And they knew that they

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were transforming the
heredity of this bacterium.

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They were transforming the
traits of this bacterium.

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They were, in fact,
transmitting heredity.

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And they purified and purified
and purified. And eventually,

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testing many, many fractions and
making them purer and purer and

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purer and purer, they
found that consistently the

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fraction that contained heredity
was the fraction that contained DNA.

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Now, it was a lot more work than
that because no fraction is pure.

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DNA is in multiple
fractions. But, you know,

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they kept trying to purify
it. And it sure looked like the

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transforming principle. The
property of being able to

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transform was co-purifying
along with the DNA fraction.

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And you know what the
reaction to that was? Well,

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mostly it was that they must have
goofed because all smart right

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thinking people knew that DNA
was an absolutely boring molecule.

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Because the interesting molecule
at this time was proteins.

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Everybody knew there were zillions,
there were 20 amino acids, they came

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in zillions of combinations,
they had all sorts of different

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shapes and properties, and
hydrophobic ones, hydrophilic

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ones that are enzymes. And,
clearly, anything as important

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as heredity was not going to be
encoded in some utterly boring

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structural molecule that was just
a long polymer of four virtually

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identical units. And so,
the sort of reaction was

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this is interesting but
there must be some trick,

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something must be wrong in
this experiment, give or take.

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Now, why did people think
that DNA was so boring?

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Now, DNA had been known for
a long time, since the 1860s.

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Lots of molecules were known, but
why was DNA boring and why were

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proteins kind of exciting? So,
for that, we really do have to

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look some more closely
at the structure of DNA.

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I want to review the structure of
DNA here because we're going to use

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it a lot. So, DNA
has three components,

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as you undoubtedly know.
It has first a sugar,

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or almost sugar, two prime
deoxyribose, two prime deoxyribose,

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so it's a pentose, or almost the
deoxypentose. And its structure,

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and this is an important structure.
In order to be a true sugar,

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to be ribose you would have a
hydroxyl. Deoxyribose just has a

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hydrogen there. And the
way we number these carbons

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around this five carbon sugar are
very important and we'll always talk

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about them, the one prime, two
prime, three prime, four prime

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and five prime carbons
of this deoxyribose.

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And you'll notice that it's the
two prime carbon that is dioxi.

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So, that's the sugar. The next
important component as we build up

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DNA is the base, OK?
The base is put here.

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Now, I'm going to start
simplifying our sugar.

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Base. So, there are four kinds
of bases that can go here.

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And they are adenine,
guanine, thymine, cytosine.

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So, that's the second important
part of building up DNA.

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The third important part in
building up DNA is to make the

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monomers that are used to
produce DNA, we need to put on

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a triphosphate. And he we
go. We'll take our sugar

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here, our base over here, and
then off this carbon we have our

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phosphate. And we
have a triphosphate.

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There we go. So, this is the
monomer that is used to build up DNA.

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This guy here is called
a nucleoside, note the S.

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This guy here with the triphosphate
on it is called a nucleotide.

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It's not usually written with
such a big capital letter,

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but nonetheless I point this
out. And, obviously, what is this

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triphosphate going to do for us?
It's going to provide the energy to

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allow us to make DNA polymer chains.
We're going to do a dehydration

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synthesis where we break two of
those phosphates off and use it for

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the energy to be able to
catalyze DNA chains to be made.

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OK. Now, when you combine
nucleotides into a DNA strand you do

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so to create a sugar phosphate
backbone. And you'll see for many

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molecules, I don't care that
you know their structures

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terribly well. But for
the basic structure of DNA,

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including its sugar phosphate
backbone, it's going to be important

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for all that we talk about. So,
what happens is we have a chain

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of DNA growing like this,
and we have our OH here.

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We have our base here.
And which carbon is this?

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Five prime, that's right. OK.
Which carbon is this? This one.

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Great. That's the three
prime, two prime, one prime.

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Great. OK. To this three prime
carbon we add this triphosphate

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breaking off two phosphates there.
The diphosphate gets broken off,

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the pyrophosphate. And we get a
single phosphate linkage to the next

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subunit of the chain.
So, here we go phosphate,

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sugar, phosphate, sugar.
And if we ignore these bases,

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which, you know, who cares about the
bases anyway, what we have is just

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phosphate, sugar,
phosphate, sugar, phosphate,

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sugar, phosphate, sugar. OK?
So, it's a very simple structure.

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There's nothing hard to remember
about this. And the phosphate is

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00:19:54,000 --> 00:19:58,000
always attached to the three prime
carbon of the preceding sugar and to

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the five prime carbon
of the next sugar.

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OK? So, we often speak of chains of
DNA growing from the five prime end

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to the three prime end. And
that confuses non-molecular

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biologists to no end.
What are we talking about,

231
00:20:13,000 --> 00:20:17,000
five prime ends and three prime ends?
This is what we're talking about.

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But the additions are catalyzed
onto the three prime carbon of that

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00:20:20,000 --> 00:20:24,000
sugar. It grows at it three
prime end. So, you have sugar,

234
00:20:24,000 --> 00:20:28,000
phosphate, sugar, phosphate,
sugar, phosphate. So, that's it.

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00:20:28,000 --> 00:20:33,000
We're all done. Well,
there are the bases,

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00:20:33,000 --> 00:20:41,000
I guess, too, right? So,
we'll mention these bases.

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The bases are, they come in
two types. There are purines.

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Adenine and guanine are purines.
And there's a six member rings with

239
00:20:56,000 --> 00:21:02,000
a five member ring. And
there are two bases that are

240
00:21:02,000 --> 00:21:07,000
called pyrimidines, they
are smaller, the thymine and

241
00:21:07,000 --> 00:21:12,000
the cytosine. And there are six
member rings. And they have some

242
00:21:12,000 --> 00:21:17,000
carbon, some nitrogen, some
oxygen and some hydrogen.

243
00:21:17,000 --> 00:21:22,000
But, you've got to admit
that compared to proteins,

244
00:21:22,000 --> 00:21:27,000
this is pretty boring. It's
just one long sugar phosphate

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00:21:27,000 --> 00:21:31,000
chain. And
two purines,

246
00:21:31,000 --> 00:21:34,000
slightly bigger things, two
pyrimidines, slightly smaller

247
00:21:34,000 --> 00:21:38,000
things. Very similar structures for
these two. I haven't even bothered

248
00:21:38,000 --> 00:21:41,000
to focus on the difference. And
as compared to the richness of

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00:21:41,000 --> 00:21:44,000
proteins, there's just no way
anything interesting could happen

250
00:21:44,000 --> 00:21:48,000
with this. That was certainly
the thinking at the time.

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00:21:48,000 --> 00:21:51,000
You have to understand
how important prior ideas,

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00:21:51,000 --> 00:21:54,000
prior prejudice is to science.
People look at it and say this must

253
00:21:54,000 --> 00:21:58,000
be some structural
molecule. It is scaffolding.

254
00:21:58,000 --> 00:22:02,000
It's like the studs in the wall
of the house you're building or

255
00:22:02,000 --> 00:22:07,000
something like that.
Not too interesting.

256
00:22:07,000 --> 00:22:12,000
So, what happens? Well, you
know, it takes time to sort

257
00:22:12,000 --> 00:22:17,000
things out. People come back
to this problem. Any thoughts?

258
00:22:17,000 --> 00:22:22,000
I mean I've given you one reason
why this did not make a huge impact,

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00:22:22,000 --> 00:22:27,000
because it was, you know, DNA was
kind of a boring molecule and people

260
00:22:27,000 --> 00:22:32,000
weren't really sure this was
right. It could be an artifact,

261
00:22:32,000 --> 00:22:38,000
right? Maybe some important protein
had come along for the ride with the

262
00:22:38,000 --> 00:22:43,000
DNA faction, right? What's
another reason why people

263
00:22:43,000 --> 00:22:49,000
might not have paid tremendous
attention to this result?

264
00:22:49,000 --> 00:22:54,000
Sorry? It was just bacteria.
Anything else? Couldn't imagine,

265
00:22:54,000 --> 00:23:00,000
right, how this DNA could encode
the enzyme. Anything else? Date.

266
00:23:00,000 --> 00:23:04,000
It's in the middle of the Second
World War. Maybe people had more

267
00:23:04,000 --> 00:23:08,000
important things to do,
right? So, this is right in the

268
00:23:08,000 --> 00:23:12,000
middle of the Second World War,
too. It's just worth noting that

269
00:23:12,000 --> 00:23:16,000
these guys are working in the middle
of New York City at the Rockefeller

270
00:23:16,000 --> 00:23:20,000
Institute and it's in the
middle of the Second World War.

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00:23:20,000 --> 00:23:24,000
Anyway, war is over and
some work continues on this.

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00:23:24,000 --> 00:23:28,000
And the work takes a
somewhat different attack.

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00:23:28,000 --> 00:23:32,000
Instead of working bacteria it now
is, there's work here on certain

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00:23:32,000 --> 00:23:36,000
bacterial viruses.
So, instead, see,

275
00:23:36,000 --> 00:23:42,000
bacteria get their own here.
Instead of using bacteria to infect

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00:23:42,000 --> 00:23:47,000
mice, Hershey and Chase, and
others at the time, used viruses

277
00:23:47,000 --> 00:23:53,000
to infect bacteria. So,
here the bacterium is the

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00:23:53,000 --> 00:23:58,000
victim. And people had found
and had studied these amazingly

279
00:23:58,000 --> 00:24:04,000
interesting really tiny things
that could affect a bacterium

280
00:24:04,000 --> 00:24:09,000
and kill it. These particles
that had these funny

281
00:24:09,000 --> 00:24:13,000
shapes were called bacteriophage.
What does phage mean? To eat.

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00:24:13,000 --> 00:24:18,000
Bacteria eaters.
Bacteriophages were these little

283
00:24:18,000 --> 00:24:22,000
viruses. They were incredibly tiny.
You could filter them through very

284
00:24:22,000 --> 00:24:27,000
small filters. And, yet,
when you added them to

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00:24:27,000 --> 00:24:32,000
bacteria they would
kill the bacteria.

286
00:24:32,000 --> 00:24:35,000
These were very simple things.
I'm reluctant to call them

287
00:24:35,000 --> 00:24:38,000
creatures. Are they alive? This
is a favorite question people

288
00:24:38,000 --> 00:24:42,000
would like to debate.
They say are viruses alive?

289
00:24:42,000 --> 00:24:45,000
And the answer is who cares? I
mean it depends on what you want

290
00:24:45,000 --> 00:24:49,000
to define alive to mean. To
me it's not alive in that it

291
00:24:49,000 --> 00:24:52,000
cannot replicate on its own without
a host, so I won't call it alive.

292
00:24:52,000 --> 00:24:56,000
But, anyway, I'll refer to them
loosely as these creatures that eat

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00:24:56,000 --> 00:24:59,000
bacteria. They were very simple.
And all they really had in them was

294
00:24:59,000 --> 00:25:03,000
some DNA in their capsid,
this capsid up here, and some

295
00:25:03,000 --> 00:25:08,000
protein. But they could
attach to a bacterium

296
00:25:08,000 --> 00:25:15,000
and after a certain amount of time
cause the bacteria to burst open and

297
00:25:15,000 --> 00:25:22,000
produce lots of daughter-phage,
lots of daughter bacteriophage. It

298
00:25:22,000 --> 00:25:30,000
could replicate
within this bacteria.

299
00:25:30,000 --> 00:25:33,000
So, somehow this, while I
might not want to call it

300
00:25:33,000 --> 00:25:37,000
alive, certainly can reproduce
itself, or at least with the help of

301
00:25:37,000 --> 00:25:41,000
a bacterium can reproduce itself.
When people first discovered this

302
00:25:41,000 --> 00:25:45,000
bacteriophage, what do
you think they wanted to do

303
00:25:45,000 --> 00:25:48,000
with them? Sorry?
Yeah. Where? In humans.

304
00:25:48,000 --> 00:25:52,000
The first thought about what to
do with bacteriophage were a whole

305
00:25:52,000 --> 00:25:56,000
bunch of interesting Russians who
wanted to make up large quantities

306
00:25:56,000 --> 00:26:00,000
of bacteriophage and
have people drink them.

307
00:26:00,000 --> 00:26:02,000
So, they would kill all their
bacteria. And this was the early

308
00:26:02,000 --> 00:26:05,000
ideas for antibiotics. It
didn't quite pan out that way.

309
00:26:05,000 --> 00:26:08,000
But, you know, people have all
these very exciting ideas of,

310
00:26:08,000 --> 00:26:11,000
wow, I've got something to kill
bacteria, let's pour it down a

311
00:26:11,000 --> 00:26:14,000
patient and see if that
does something good for them.

312
00:26:14,000 --> 00:26:17,000
You know, that's why there are
institutional review boards,

313
00:26:17,000 --> 00:26:20,000
too, to make sure that you can't
just do that right off the bat.

314
00:26:20,000 --> 00:26:23,000
Somebody else has got
to think about it also.

315
00:26:23,000 --> 00:26:26,000
It turns out not to be a great
way to kill patients, to kill

316
00:26:26,000 --> 00:26:30,000
bacteria, sorry. No,
it doesn't actually kill

317
00:26:30,000 --> 00:26:34,000
patients, but it doesn't also kill
the bacteria so well in human beings.

318
00:26:34,000 --> 00:26:38,000
So, anyway. So, the
question was how is it that

319
00:26:38,000 --> 00:26:42,000
these viruses kill the bacteria?
Somehow they inject something into

320
00:26:42,000 --> 00:26:47,000
the bacteria, something
causes something to happen,

321
00:26:47,000 --> 00:26:51,000
which causes virus particles to be
made. I don't put too fine a point

322
00:26:51,000 --> 00:26:55,000
on it because that's all you
could really say at that point,

323
00:26:55,000 --> 00:27:00,000
something goes in and
something comes out.

324
00:27:00,000 --> 00:27:05,000
So, what goes in? How
could we tell what goes in?

325
00:27:05,000 --> 00:27:10,000
Yup. By seeing what's left out.
How could we see what's left out?

326
00:27:10,000 --> 00:27:15,000
Just being really practical,
how are we going to tell?

327
00:27:15,000 --> 00:27:20,000
Visually look, but that turns out
to be terribly hard thing to do.

328
00:27:20,000 --> 00:27:25,000
You've got to have really good eyes
to be able to say the protein is

329
00:27:25,000 --> 00:27:30,000
still there but not
the DNA, or the DNA.

330
00:27:30,000 --> 00:27:33,000
Because the thought was if this
thing is injecting its DNA then the

331
00:27:33,000 --> 00:27:37,000
DNA must be carrying the
instructions to make phage,

332
00:27:37,000 --> 00:27:40,000
and this would be
hereditary material. So,

333
00:27:40,000 --> 00:27:44,000
what we want to show is that the
protein stays out and the DNA goes

334
00:27:44,000 --> 00:27:47,000
in. But how is that going to,
how do you do that practically?

335
00:27:47,000 --> 00:27:51,000
Radioactive labeling turns out
to be the best way to do that.

336
00:27:51,000 --> 00:27:54,000
If we could label radioactively the
DNA with one label and the protein

337
00:27:54,000 --> 00:27:58,000
with a different label, we
could see which radioactive

338
00:27:58,000 --> 00:28:02,000
isotope goes into the bacteria.
Any candidates for an element that

339
00:28:02,000 --> 00:28:06,000
we could use to label DNA
that won't be in protein?

340
00:28:06,000 --> 00:28:10,000
Sorry? I'm sorry? Oh,
who had one? Uranium.

341
00:28:10,000 --> 00:28:14,000
Somebody's thinking World War II
here, right, there would be some

342
00:28:14,000 --> 00:28:19,000
spare uranium around. The
problem with that is that DNA

343
00:28:19,000 --> 00:28:23,000
does not actually have uranium in
it, and so when you put uranium in it

344
00:28:23,000 --> 00:28:27,000
wouldn't still be DNA. We
would like to label it with an

345
00:28:27,000 --> 00:28:31,000
element that's actually in DNA.
So the only difference is that it's

346
00:28:31,000 --> 00:28:35,000
a radioisotope.
Phosphorus. Phosphorus.

347
00:28:35,000 --> 00:28:39,000
Well, there's obviously phosphorus
in that sugar phosphate backbone.

348
00:28:39,000 --> 00:28:43,000
Is there phosphorus in a typical
amino acid? Any of the 20 amino

349
00:28:43,000 --> 00:28:46,000
acids? No phosphorus.
Great. So, we could use a

350
00:28:46,000 --> 00:28:50,000
phosphorus isotope. We
could P32 label the DNA.

351
00:28:50,000 --> 00:28:54,000
But how do make live bacteriophages
that are labeled with radioactive

352
00:28:54,000 --> 00:28:58,000
phosphorus? I mean what kind
of fancy chemistry do you

353
00:28:58,000 --> 00:29:02,000
need to do that? Yes?
Perfect if you grow the

354
00:29:02,000 --> 00:29:06,000
bacteria in radioactively, in
medium. If you grow the virus

355
00:29:06,000 --> 00:29:10,000
and the bacteria in medium
that has radioactive phosphate,

356
00:29:10,000 --> 00:29:14,000
the bacteria and the virus
take care of it for you.

357
00:29:14,000 --> 00:29:18,000
The phosphate is
automatically incorporated. So,

358
00:29:18,000 --> 00:29:22,000
you don't have to do any
chemistry. You just feed phosphate,

359
00:29:22,000 --> 00:29:26,000
radioactive phosphate into the
medium. And the phage that are

360
00:29:26,000 --> 00:29:30,000
produced will be
radioactively labeled.

361
00:29:30,000 --> 00:29:34,000
Purify them and use them in
your experiment. Similarly,

362
00:29:34,000 --> 00:29:39,000
what are we going to
label our proteins with?

363
00:29:39,000 --> 00:29:43,000
Carbon? No. Hydrogen?
No. Oxygen? Nitrogen? No,

364
00:29:43,000 --> 00:29:48,000
because the bases have nitrogen.
Sulfa. We've only got sulfa.

365
00:29:48,000 --> 00:29:53,000
Where is sulfa going to be?
So, for example, cysteines with

366
00:29:53,000 --> 00:29:58,000
thiamines, right, we've
got sulfa. Here's S35.

367
00:29:58,000 --> 00:30:03,000
So, we can take bacteria and
we could, we can take phage,

368
00:30:03,000 --> 00:30:08,000
and by growing them in the
presence of radioactive DNA,

369
00:30:08,000 --> 00:30:13,000
no, radioactive phosphorus, P32
and growing them in the presence

370
00:30:13,000 --> 00:30:19,000
of radioactive sulfa, S35,
we are about to produce

371
00:30:19,000 --> 00:30:24,000
bacteriophage that are labeled.
OK? So, P32, S35. Now we infect

372
00:30:24,000 --> 00:30:30,000
bacteria with them. Let
me take a big tube here.

373
00:30:30,000 --> 00:30:33,000
I'm going to add bacteria. I've
got the phage here. The phage

374
00:30:33,000 --> 00:30:37,000
particles are attached to the
bacteria and they are going to

375
00:30:37,000 --> 00:30:41,000
inject whatever they inject.
Now what do we have to do? We've

376
00:30:41,000 --> 00:30:45,000
got to knock off the bacteriophage
particles from the bacteria.

377
00:30:45,000 --> 00:30:48,000
I want to knock them off and see
what is staying with the viral

378
00:30:48,000 --> 00:30:52,000
particles and what goes
into the bacteria. So,

379
00:30:52,000 --> 00:30:56,000
how do I get in there with
tweezers and separate off,

380
00:30:56,000 --> 00:31:00,000
peel off each virus
from the bacteria?

381
00:31:00,000 --> 00:31:04,000
Washing turns out not to be strong
enough. You've got to previal to

382
00:31:04,000 --> 00:31:09,000
get these things off, so you
really need some incredibly

383
00:31:09,000 --> 00:31:13,000
strong agitation. So,
specialized devices were used

384
00:31:13,000 --> 00:31:18,000
to create intense agitation.
What specialized devices you're

385
00:31:18,000 --> 00:31:23,000
aware of that do that?
Blenders. Kitchen blenders.

386
00:31:23,000 --> 00:31:27,000
The Waring blender turns out to be
the perfect laboratory device for

387
00:31:27,000 --> 00:31:32,000
this experiment. And this
is actually known as the

388
00:31:32,000 --> 00:31:36,000
Waring Blender Experiment.
You take the bacteria with the

389
00:31:36,000 --> 00:31:40,000
phage attached to it, you let
them attach and do whatever

390
00:31:40,000 --> 00:31:44,000
they're going to do, inject
their DNA, as we know turns

391
00:31:44,000 --> 00:31:48,000
out is the right answer. And
then you press puree and then

392
00:31:48,000 --> 00:31:52,000
vrrrr, and the viral particles fall
off. So, it's important to know how

393
00:31:52,000 --> 00:31:56,000
things really happen. So
then what happens is the

394
00:31:56,000 --> 00:32:00,000
bacteria are separated
from these particles.

395
00:32:00,000 --> 00:32:04,000
And it turns out these particles are,
the viral particles are much lighter,

396
00:32:04,000 --> 00:32:08,000
much less dense than the bacteria.
So, how do we separate them?

397
00:32:08,000 --> 00:32:13,000
Centrifuge them.
We centrifuge them.

398
00:32:13,000 --> 00:32:17,000
The bacterial particles are there
up in the supernatant turn out to be

399
00:32:17,000 --> 00:32:21,000
our phage capsids.
And now what do we do?

400
00:32:21,000 --> 00:32:26,000
We take this stuff, we measure the
radioactivity in the supernatant,

401
00:32:26,000 --> 00:32:30,000
that is the material that
stays above, and we measure the

402
00:32:30,000 --> 00:32:36,000
radioactivity in the pellet.
And what do we end up seeing?

403
00:32:36,000 --> 00:32:43,000
Where does most of the P32,
what shows up in the pellet?

404
00:32:43,000 --> 00:32:49,000
Mostly P32 shows up in the pellet.
Is there no S35 in the pellet? You

405
00:32:49,000 --> 00:32:56,000
know, in the textbook story, of
course, there's no S35 because

406
00:32:56,000 --> 00:33:02,000
they want it to be nice and clean.
But in reality there's going to be

407
00:33:02,000 --> 00:33:06,000
some S35. But it was, you
know, less than 1% of the S35

408
00:33:06,000 --> 00:33:11,000
ends up in the pellet. Most
of the S35 stays up here in

409
00:33:11,000 --> 00:33:16,000
the supernatant. Does all
of the phosphorus go in?

410
00:33:16,000 --> 00:33:20,000
No, of course not. Some of the
viruses didn't even attach and not

411
00:33:20,000 --> 00:33:25,000
everything goes in. So,
there's still radioactive

412
00:33:25,000 --> 00:33:31,000
phosphate up in the supernatant.
But the striking this is that the

413
00:33:31,000 --> 00:33:38,000
pellet primarily has gotten
the radioactive phosphorus,

414
00:33:38,000 --> 00:33:45,000
not the radioactive sulfa, and
therefore we can conclude that

415
00:33:45,000 --> 00:33:52,000
what? Well, more DNA went in than
protein. Are we therefore entitled

416
00:33:52,000 --> 00:34:00,000
to include that DNA is the
hereditary material? Why?

417
00:34:00,000 --> 00:34:03,000
Well, I mean suppose that 1% sulfate
is tracking one minor protein that

418
00:34:03,000 --> 00:34:07,000
is the secret. You can't,
it's very hard to rule

419
00:34:07,000 --> 00:34:11,000
out that there's no contaminants
traveling along with the DNA.

420
00:34:11,000 --> 00:34:15,000
And if you really truly disbelieve
DNA, you could be churlish and say,

421
00:34:15,000 --> 00:34:18,000
well, I just don't believe that
you've so purified it that you can

422
00:34:18,000 --> 00:34:22,000
completely rule out that some
minor protein component is really

423
00:34:22,000 --> 00:34:26,000
conferring heredity. In
fact, when you really look

424
00:34:26,000 --> 00:34:30,000
closely, Avery, McCarty
and McLeod's biochemistry,

425
00:34:30,000 --> 00:34:34,000
I believe, was purer than
the purity of this experiment.

426
00:34:34,000 --> 00:34:38,000
But by this point thinking had
begun to shift toward DNA being a

427
00:34:38,000 --> 00:34:42,000
reasonable hereditary molecule.
In addition, it was the second line

428
00:34:42,000 --> 00:34:46,000
of proof, different from the
pneumococcus, using a different

429
00:34:46,000 --> 00:34:50,000
system, both pointing to the same
answer. And the intellectual tide

430
00:34:50,000 --> 00:34:54,000
shifted to recognizing that
this probably was right,

431
00:34:54,000 --> 00:34:58,000
and the reason these experiments
were pointing to DNA was DNA had to

432
00:34:58,000 --> 00:35:02,000
be the right answer.
But, of course,

433
00:35:02,000 --> 00:35:08,000
how was it the right answer?
What was it about DNA that could

434
00:35:08,000 --> 00:35:13,000
confer these properties? This
was still unclear in 1953,

435
00:35:13,000 --> 00:35:19,000
but not for that long. It became
clarified relatively soon thereafter.

436
00:35:19,000 --> 00:35:24,000
And, of course, it
became clarified with the

437
00:35:24,000 --> 00:35:30,000
understanding of DNA
structure, the double helix.

438
00:35:30,000 --> 00:35:34,000
Nobody here has not heard of the
double helix. Probably there's

439
00:35:34,000 --> 00:35:38,000
nobody, no grownup who doesn't know
about the double helix and all that,

440
00:35:38,000 --> 00:35:42,000
but nonetheless I want to stop
and think a little bit also,

441
00:35:42,000 --> 00:35:47,000
I'll say on a personal note,
this is the first year I've taught

442
00:35:47,000 --> 00:35:51,000
this class after, the
first time I've taught this

443
00:35:51,000 --> 00:35:55,000
class when Crick and Watson
have not both been alive.

444
00:35:55,000 --> 00:36:00,000
Some of you may know that Frances
Crick dies just this past summer.

445
00:36:00,000 --> 00:36:03,000
Which was very sad. He was
an incredible person and,

446
00:36:03,000 --> 00:36:06,000
you know, as I've said,
Mendel was one of my heroes.

447
00:36:06,000 --> 00:36:09,000
Francis Crick was also one
of my heroes. He was just an

448
00:36:09,000 --> 00:36:12,000
extraordinary person. But
Jim Watson is still alive and

449
00:36:12,000 --> 00:36:15,000
kicking and still quite active.
And so, in any case, you're not far

450
00:36:15,000 --> 00:36:18,000
removed. So, I tell you a little
bit about this stuff as history,

451
00:36:18,000 --> 00:36:21,000
but this history I'm telling
you about, these people are,

452
00:36:21,000 --> 00:36:24,000
for the most part, Francis'
passing notwithstanding,

453
00:36:24,000 --> 00:36:27,000
alive and kicking. Jim
Watson is still quite alive.

454
00:36:27,000 --> 00:36:30,000
Actually, McCarty is still
alive. It's really, anyway.

455
00:36:30,000 --> 00:36:36,000
So, 1953, just a year later,
Jim Watson and Francis Crick are

456
00:36:36,000 --> 00:36:42,000
working in England. Watson
is a student for Indiana,

457
00:36:42,000 --> 00:36:48,000
a former ornithologist, had his
interest in ornithology originally,

458
00:36:48,000 --> 00:36:54,000
and then studies more biology and
came to England because he wanted to

459
00:36:54,000 --> 00:37:00,000
study the gene.
Francis Crick,

460
00:37:00,000 --> 00:37:05,000
a physicist who worked in the
admiralty during World War II.

461
00:37:05,000 --> 00:37:11,000
And, of course, what they did was
on the basis of an awful lot of

462
00:37:11,000 --> 00:37:16,000
modeling and getting to see
experimental x-ray diffraction

463
00:37:16,000 --> 00:37:21,000
pictures of Roselyn Franklin
from London who made a model.

464
00:37:21,000 --> 00:37:26,000
And the model is this beautiful,
and I haven't drawn it to its proper

465
00:37:26,000 --> 00:37:32,000
proportions, but this beautiful
double helical structure.

466
00:37:32,000 --> 00:37:37,000
Five prime, one chain of
DNA running in one direction,

467
00:37:37,000 --> 00:37:42,000
five prime to three prime. An
anti-parallel chain of DNA going

468
00:37:42,000 --> 00:37:47,000
in this direction, five
prime to three prime.

469
00:37:47,000 --> 00:37:52,000
It was a beautiful structure. Jim
Watson has written a whole book

470
00:37:52,000 --> 00:37:57,000
about the discovery of
the double helix structure,

471
00:37:57,000 --> 00:38:02,000
and we are only 51 years past that.
It was, anybody who hasn't read The

472
00:38:02,000 --> 00:38:06,000
Double Helix, this book,
really should. It's one of the

473
00:38:06,000 --> 00:38:10,000
treat books of science literature,
and actually is on many people's

474
00:38:10,000 --> 00:38:15,000
lists of some of the great
books of the 20th century.

475
00:38:15,000 --> 00:38:19,000
It's a wonderful competitive story
of Crick and Watson racing against

476
00:38:19,000 --> 00:38:23,000
Linus Pauling.
It's, you know,

477
00:38:23,000 --> 00:38:28,000
someone came along and had lunch
in Cambridge with Crick and Watson.

478
00:38:28,000 --> 00:38:31,000
And they came away and said, this
was before they discovered the

479
00:38:31,000 --> 00:38:34,000
structure, about a year or so before,
and said these guys are idiots.

480
00:38:34,000 --> 00:38:38,000
They can't even memorize the
structure of A and T and C and G,

481
00:38:38,000 --> 00:38:41,000
and they're trying to find,
you know, the structure of DNA.

482
00:38:41,000 --> 00:38:44,000
These guys are never
going to get anywhere. So,

483
00:38:44,000 --> 00:38:48,000
this person, who we'll come back
to in a moment, was wrong about this

484
00:38:48,000 --> 00:38:51,000
particular point. Because
what Crick and Watson did

485
00:38:51,000 --> 00:38:54,000
was they played around with the
models, and what they ended up

486
00:38:54,000 --> 00:38:58,000
noticing was a couple of things.
First off, from Rosalyn Franklin's

487
00:38:58,000 --> 00:39:01,000
pictures, that this was helical.
The x-ray to fraction pictures could

488
00:39:01,000 --> 00:39:05,000
tell you at a glance that
the structure was helical.

489
00:39:05,000 --> 00:39:09,000
They saw that. They then
tried to make helices. Now,

490
00:39:09,000 --> 00:39:12,000
other people, Linus Pauling knew
something that DNA probably had to

491
00:39:12,000 --> 00:39:16,000
be helical, and somehow he
just got it totally wrong.

492
00:39:16,000 --> 00:39:19,000
He made just a nutty model of DNA.
Linus Pauling, the smartest chemist

493
00:39:19,000 --> 00:39:23,000
of the century made a crazy model
of DNA where he took the sugar

494
00:39:23,000 --> 00:39:27,000
phosphate backbones and put all
the sugar phosphate backbones in the

495
00:39:27,000 --> 00:39:30,000
middle, and had three of them.
He had a triple helical model with

496
00:39:30,000 --> 00:39:34,000
sugar phosphates in the middle.
And what can you tell me about the

497
00:39:34,000 --> 00:39:38,000
charge on these sugar phosphate
backbones? Very negative.

498
00:39:38,000 --> 00:39:42,000
You're going to stick a whole bunch
of negative charges near each other

499
00:39:42,000 --> 00:39:46,000
in the middle? No way.
Anybody could have known.

500
00:39:46,000 --> 00:39:50,000
This was a bush-league mistake,
so Crick and Watson said phew,

501
00:39:50,000 --> 00:39:54,000
Pauling has got it wrong.
They put together this model.

502
00:39:54,000 --> 00:39:58,000
And the key to the model was
the recognition of base pairing,

503
00:39:58,000 --> 00:40:15,000
the recognition of base paring.
That if I take a thiamine here and I

504
00:40:15,000 --> 00:40:36,000
take an adenine here. That
these two groups would be

505
00:40:36,000 --> 00:40:48,000
pointing at each other in such a way
as to make two hydrogen bonds with a

506
00:40:48,000 --> 00:41:01,000
certain characteristic distance.
And, not just that, but cytosine

507
00:41:01,000 --> 00:41:14,000
and guanine could also be
fit into that same distance.

508
00:41:14,000 --> 00:41:30,000
And they would have
three hydrogen bonds.

509
00:41:30,000 --> 00:41:40,000
And here NH, H,
doo, doo, doo, doo,

510
00:41:40,000 --> 00:41:50,000
doo, doo, doo, three hydrogen bonds.
And they would fit the, what do I

511
00:41:50,000 --> 00:42:00,000
got? Oops. Thank
you. Good point.

512
00:42:00,000 --> 00:42:06,000
That's the problem. Yup.
Well, it's a little messy but

513
00:42:06,000 --> 00:42:13,000
anyway. The business end here
is three hydrogen bonds and two

514
00:42:13,000 --> 00:42:19,000
hydrogen bonds, and they
both fit into the same

515
00:42:19,000 --> 00:42:26,000
distance perfectly. So,
this double helix here could

516
00:42:26,000 --> 00:42:33,000
have either As and Ts or Gs and
Cs or Cs and Gs or Ts and As.

517
00:42:33,000 --> 00:42:37,000
And they would all fit
perfectly with each other.

518
00:42:37,000 --> 00:42:42,000
Now, there was an old observation,
not that old, there was an

519
00:42:42,000 --> 00:42:46,000
observation floating around at the
time that said when you analyze the

520
00:42:46,000 --> 00:42:51,000
amount of As and Ts in DNA, you
always found out that the amount

521
00:42:51,000 --> 00:42:55,000
of A tended to be very close to the
amount of T. The amount of C tended

522
00:42:55,000 --> 00:43:00,000
to be very close
to the amount of G.

523
00:43:00,000 --> 00:43:06,000
Although, these amounts could
be different. This was due to a

524
00:43:06,000 --> 00:43:12,000
biochemist called Chargaff. And
they're called, this was called

525
00:43:12,000 --> 00:43:18,000
Chargaff's Rule or Chargaff's
Law or Chargaff's Observation.

526
00:43:18,000 --> 00:43:24,000
Chargaff noted that the percentage
of these amounts tended to be equal

527
00:43:24,000 --> 00:43:30,000
but didn't know what to make of
it. This perfectly explained that.

528
00:43:30,000 --> 00:43:33,000
It was very good. Remember
I said somebody came

529
00:43:33,000 --> 00:43:36,000
through Cambridge and said
these guys were turkeys,

530
00:43:36,000 --> 00:43:39,000
Crick and Watson were turkeys
because they couldn't even remember

531
00:43:39,000 --> 00:43:43,000
the structure and all that?
This was a very distinguished

532
00:43:43,000 --> 00:43:46,000
chemist who said this about
Crick and Watson. It was Chargaff.

533
00:43:46,000 --> 00:43:49,000
Chargaff came through and
said these guys are turkeys.

534
00:43:49,000 --> 00:43:53,000
But it was Chargaff's Rule that
Chargaff had missed the importance

535
00:43:53,000 --> 00:43:56,000
of. He was quite bitter about
this through much of his life.

536
00:43:56,000 --> 00:43:59,000
And there's a very wonderful biting
quote that Chargaff says when Crick

537
00:43:59,000 --> 00:44:03,000
and Watson become famous
for the DNA double helix.

538
00:44:03,000 --> 00:44:09,000
Let's see if I can get it right.
He says that such pigmies should

539
00:44:09,000 --> 00:44:15,000
cast such giant shadows,
referring to Crick and Watson,

540
00:44:15,000 --> 00:44:21,000
that such pigmies should cast such
giant shadows only shows how late in

541
00:44:21,000 --> 00:44:27,000
the day it is. Anyway,
he was not happy.

542
00:44:27,000 --> 00:44:31,000
So, all right. Now, this
was a big deal thing.

543
00:44:31,000 --> 00:44:34,000
Crick and Watson knew this was very
important. They raced to publish a

544
00:44:34,000 --> 00:44:37,000
paper about this. They
sent it off to Nature.

545
00:44:37,000 --> 00:44:41,000
It's a gem of a paper.
It's a page roughly in text.

546
00:44:41,000 --> 00:44:44,000
It's very short, very
clear, has this beautiful

547
00:44:44,000 --> 00:44:47,000
picture drawn from Francis Crick's
wife, Odile. And it is just a

548
00:44:47,000 --> 00:44:50,000
charming paper. They know
that they've cracked the

549
00:44:50,000 --> 00:44:53,000
secret of life. Why do
they know they've cracked

550
00:44:53,000 --> 00:44:56,000
the secret of life? Because
the most important thing

551
00:44:56,000 --> 00:45:00,000
about this model here is
not its structure per se.

552
00:45:00,000 --> 00:45:04,000
But that it explains how it is that
a DNA molecule can be replicated,

553
00:45:04,000 --> 00:45:09,000
that somehow all it takes is for
those two strands to come apart,

554
00:45:09,000 --> 00:45:13,000
who knows how, and that when they
come apart each can serve as a

555
00:45:13,000 --> 00:45:18,000
template for the other because since
As always match Ts and Cs always

556
00:45:18,000 --> 00:45:22,000
match Gs, each strand has
enough information for the other.

557
00:45:22,000 --> 00:45:27,000
That's how replication happens.
You have two strands, each of which

558
00:45:27,000 --> 00:45:32,000
has sufficient information
to encode the other.

559
00:45:32,000 --> 00:45:35,000
They somehow come apart. They
each serve as a template for

560
00:45:35,000 --> 00:45:38,000
the other. And that's that.
That is the secret of life, how

561
00:45:38,000 --> 00:45:41,000
life replicates itself. Not
just that. We've explained

562
00:45:41,000 --> 00:45:45,000
replication. What about mutation.
What's mutation? It sometimes gets

563
00:45:45,000 --> 00:45:48,000
it wrong. It sometimes screws
up. So, for one little biochemical

564
00:45:48,000 --> 00:45:51,000
model we've explained replication
and mutation. That's pretty good.

565
00:45:51,000 --> 00:45:54,000
Now, the thing is, in writing this
paper and getting this off to the

566
00:45:54,000 --> 00:45:58,000
Journal, this was not an easy
thing to get done quickly.

567
00:45:58,000 --> 00:46:01,000
You couldn't, you just didn't have
time to explain all these details.

568
00:46:01,000 --> 00:46:05,000
They wanted to stake their claim
about this, so they wrote up the

569
00:46:05,000 --> 00:46:09,000
structure. And instead of going
through a long about how this

570
00:46:09,000 --> 00:46:13,000
explains replication and dah,
dah, dah, dah, dah, dah, dah, dah,

571
00:46:13,000 --> 00:46:16,000
there's one sentence in the paper,
the last sentence of the paper in

572
00:46:16,000 --> 00:46:20,000
which they just say it has not
escaped our notice that this model

573
00:46:20,000 --> 00:46:24,000
explains replication and everything
else. Basically the last sentence

574
00:46:24,000 --> 00:46:28,000
says, oh, and by the way, it
has not escaped our notice that

575
00:46:28,000 --> 00:46:31,000
this explains the secret of life.
Although, it didn't say it like that.

576
00:46:31,000 --> 00:46:35,000
It's the coyest sentence in
the scientific literature.

577
00:46:35,000 --> 00:46:38,000
It's really just an
amazing sentence there.

578
00:46:38,000 --> 00:46:42,000
And then they come back a couple
minutes later and write a paper

579
00:46:42,000 --> 00:46:45,000
explaining what they mean and all
that, but it's just a great sentence.

580
00:46:45,000 --> 00:46:49,000
So, you will hear molecular
biologists make reference,

581
00:46:49,000 --> 00:46:52,000
use in their speech the phrase,
it has not escaped our notice that.

582
00:46:52,000 --> 00:46:56,000
And it's always a homage back to
this particular sentence in this

583
00:46:56,000 --> 00:46:59,000
paper of Crick and Watson.
OK, now, last thing. Yes?

584
00:46:59,000 --> 00:47:03,000
Jim Watson was 25 and Francis
was 35 when he did this.

585
00:47:03,000 --> 00:47:09,000
Yes. He was a 25-year-old kid
when he did this. That's right.

586
00:47:09,000 --> 00:47:16,000
Pretty amazing stuff. So,
last point I want to touch on,

587
00:47:16,000 --> 00:47:23,000
and I'm not sure I'll get to,
get all the way there, but this

588
00:47:23,000 --> 00:47:29,000
model, this model here of DNA coming
apart and each strand serving as a

589
00:47:29,000 --> 00:47:36,000
template for the other strand
is called Semi-Conservative

590
00:47:36,000 --> 00:47:43,000
Replication. That is
one strand is used as the

591
00:47:43,000 --> 00:47:50,000
template for the other strand,
so there's an old strand and a new

592
00:47:50,000 --> 00:47:57,000
strand that's made. In
theory, you could imagine that

593
00:47:57,000 --> 00:48:05,000
DNA replication
occurred not like this.

594
00:48:05,000 --> 00:48:09,000
But instead somehow, I can't
imagine how, but you could

595
00:48:09,000 --> 00:48:14,000
imagine, and people were willing
to imagine it, that the old strands

596
00:48:14,000 --> 00:48:18,000
stayed together but somehow became
a template for making a new double

597
00:48:18,000 --> 00:48:23,000
helix without actually using them.
This model of the strands actually

598
00:48:23,000 --> 00:48:27,000
serving as a template would predict
that each new DNA double helix was,

599
00:48:27,000 --> 00:48:32,000
in fact, composed of one
old and one new strand.

600
00:48:32,000 --> 00:48:36,000
If you could prove that then you'd
have real confirmation of this Crick

601
00:48:36,000 --> 00:48:40,000
Watson model, semi-conservative
replication. And so,

602
00:48:40,000 --> 00:48:44,000
a young student, two young students,
Matt Meselson, who's still working

603
00:48:44,000 --> 00:48:48,000
and is down the road at
Harvard and a wonderful person,

604
00:48:48,000 --> 00:48:52,000
and Frank Stahl, who's still working
in Oregon, proved that the new DNAs

605
00:48:52,000 --> 00:48:56,000
that were made after each
generation were, in fact,

606
00:48:56,000 --> 00:49:00,000
composed of one old
strand and one new strand.

607
00:49:00,000 --> 00:49:06,000
How could you possibly do that?
Sorry? Radioactive labeling. But

608
00:49:06,000 --> 00:49:12,000
how do you radioactively label
it so that you can see that it's,

609
00:49:12,000 --> 00:49:18,000
that you've got a double helix
that's half one and half the other?

610
00:49:18,000 --> 00:49:24,000
Old ones labeled with, well, with
one isotope. It actually turns out

611
00:49:24,000 --> 00:49:30,000
to be nitrogen. The new
one with a new isotope.

612
00:49:30,000 --> 00:49:34,000
Say N14. You can do heavy
nitrogen and ordinary nitrogen.

613
00:49:34,000 --> 00:49:38,000
And if what you could do is grow up
your DNA, when you first grow it in

614
00:49:38,000 --> 00:49:42,000
normal nitrogen,
then you shift to N15,

615
00:49:42,000 --> 00:49:46,000
heavy nitrogen, you could make
DNA molecules that were half old,

616
00:49:46,000 --> 00:49:50,000
half new, and therefore half
labeled with normal nitrogen,

617
00:49:50,000 --> 00:49:54,000
half labeled with heavy nitrogen.
And how would I prove that these

618
00:49:54,000 --> 00:49:58,000
DNA molecules were a 50/50 hybrid?
What would be the property that I

619
00:49:58,000 --> 00:50:02,000
would be able to test? Well,
radioactivity turns out to be

620
00:50:02,000 --> 00:50:06,000
really hard to weight density.
It turns out density, if I could

621
00:50:06,000 --> 00:50:10,000
just measure the density of the
DNA, I would show that if the

622
00:50:10,000 --> 00:50:14,000
semi-conservative model is
true, the molecules will now have

623
00:50:14,000 --> 00:50:19,000
intermediate density between
all heavy and all light nitrogen.

624
00:50:19,000 --> 00:50:23,000
They had to work out a
centrifugation technique so

625
00:50:23,000 --> 00:50:27,000
sensitive, a salt gradient
centrifugation where you could put

626
00:50:27,000 --> 00:50:32,000
DNA on it. You had a really fine
salt gradient spun in a centrifuge.

627
00:50:32,000 --> 00:50:35,000
And depending on where the DNA
migrated, you could measure the

628
00:50:35,000 --> 00:50:38,000
density of the DNA. And
they were able to show that,

629
00:50:38,000 --> 00:50:41,000
in fact, newly replicated DNA
strands had this intermediate

630
00:50:41,000 --> 00:50:45,000
density that would be expected
from the semi-conservative model.

631
00:50:45,000 --> 00:50:48,000
And so, in fact, by that point
the semi-conservative model,

632
00:50:48,000 --> 00:50:51,000
I think, is well established.
In some sense you would say the

633
00:50:51,000 --> 00:50:55,000
beauty of the double helix was
almost one of these very rare

634
00:50:55,000 --> 00:50:58,000
scientific results where when
you look at it you say it can't

635
00:50:58,000 --> 00:51:02,000
possibly be wrong.
It explains too much.

636
00:51:02,000 --> 00:51:06,000
It's too beautiful. But, as
we've discussed before, that's

637
00:51:06,000 --> 00:51:11,000
not enough. You need some
proofs that it's real.

638
00:51:11,000 --> 00:51:15,000
And this Meselson-Stahl experiment
provided a real confirmation of that.

639
00:51:15,000 --> 00:51:20,000
Onto
next time.