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INSTRUCTOR: Good morning.

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Good morning.

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So, anybody check The New
York Times yesterday?

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What did you see?

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AUDIENCE: [INAUDIBLE]

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INSTRUCTOR: People,
what people?

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AUDIENCE: I can't remember
their names.

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[INAUDIBLE]

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INSTRUCTOR: [LAUGHS]

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AUDIENCE: [INAUDIBLE]

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I can't remember their names.

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INSTRUCTOR: Yeah, the Crick
and Watson people.

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AUDIENCE: [INAUDIBLE].

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INSTRUCTOR: Exactly.

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So, perfect timing, The New York
Times had an article in

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yesterday's paper.

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Francis Crick's correspondence
with Maurice Wilkins during

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the critical year when Crick and
Watson went down and saw

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the X-ray crystallography.

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By the way, I made a mistake.

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I said that Rosalind Franklin
showed it to Crick and Watson.

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Actually it was Maurice Wilkins
who showed it to Crick

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and Watson when Rosalind
Franklin was away.

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Which, was itself a slightly
complicated thing to be doing

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because it was really Rosalind
Franklin's work.

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But in any case, there was
correspondence, some heated

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correspondence that went
back between Crick

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and Wilkins and others.

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And it was believed that the
correspondence had been lost,

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had been thrown out.

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But in fact, it turns out that
in the papers of Sydney

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Brenner, another great person
of that period, Crick's

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correspondence had got misfiled
in Sydney's files.

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Sydney had donated his files
to the Cold Spring Harbor

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Laboratory on Long Island.

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And they went through the files
in the past several

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months and found them.

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So we actually now have, as The
New York Times reported

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yesterday, the letters with
Crick and Watson and Maurice

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Wilkins about just that period
I was telling you about.

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It doesn't radically change
any of the story.

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But it really shows
you the attitude.

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And if you read the New York
Times article, you'll find all

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sorts of juicy quotes about the
attitude back and forth

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there, including the first model
Crick and Watson made

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where they totally screwed up
because they had misunderstood

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some number.

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Anyway, it's interesting
stuff.

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And the reason I bring up this
history is because science is

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done by real people.

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It's a business of passion.

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It's a business of trying
to-- you know --

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science is wonderful.

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It's objective in
a certain sense.

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And it's also about
convincing others.

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You know, a scientific result
doesn't mean anything unless

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you can convince
the community.

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So it's an inherently human
activity to bring people's

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attention to things,
make things clear.

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Anyway, that was kind of cool.

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I invite you all to go look
at The Times article.

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You don't realize how much work
we have to go to in 7.01

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to arrange these things to come
out just at the right

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time during our curriculum.

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But we pay off The New York
Times and they do our bidding.

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So last time we were
talking about--

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I want to briefly go back to
the Semiconservative Model,

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which I ended with last time,
of DNA replication, the work

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of Matt Meselson and Frank
Stahl, these graduate students

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at Caltech.

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So you remember, our DNA double
helix immediately

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suggests the secret of life,
the way that you copy

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information to daughter cells.

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Each strand is a sufficient
template for the other strand.

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If you just unzipped them, each
would be able to serve as

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a template for replication
for the other strand.

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Beautiful.

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It's called Semiconservative
because you've conserved.

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You used one strand.

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And you've made a new
strand on the other.

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It seems obvious, but
we can't take things

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for obvious in science.

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Because the alternative model,
alternative, which we know

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today is of course wrong, is
that somehow the cell comes

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along, feels this double helix
as a template, and somehow

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builds two new strands
that are the same

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as that double helix.

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They build a new double helix
with both strands being new.

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That's nuts--

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why would you do it?

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It's so obvious you could
use one strand as the

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template for the other.

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But this is what Meselson and
Stahl had to rule out.

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They had to rule out that that
double helix stayed intact,

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and maybe the cell sent some
enzymes around to feel its

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shape and somehow construct
another double helix like it.

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That would be a non-conservative
model.

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You weren't using
any old strands.

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The two old strands
stayed together.

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And you made two new strands.

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That's what they were trying to
distinguish between, this

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semiconservative model or not.

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So I just remind you that their
really cool, cool idea--

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see, they couldn't use a
radioactive tracer that was

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different on the new strand
than the old strand in the

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conventional sense.

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Because, of course, they're made
out of the same atoms.

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But what they did do, as we
talked about last time, was

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they take the DNA.

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They grow it up in
heavy nitrogen.

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They then shift the bacteria
to light nitrogen.

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And, if in fact the
Semiconservative Model is

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right, then the new DNA after
one generation of growth will

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have a lighter density.

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Very simple, except that they
had to go invent a way to

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measure density.

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They had to invent this
centrifugation process where

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they spin really, really,
really, really hard.

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And the salt gets a little
denser here, and a little less

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dense here, and a little
less dense here.

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And it's not a very
huge difference.

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But if you know the density of
DNA, and you arrange your salt

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concentration at just the right
density, you can spin it

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really hard so that there's
some gradient in

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density in the salt.

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And the heavier stuff
will band here.

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The lighter stuff from
one generation of

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growth will band there.

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And normal DNA would
band there.

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And that's a pretty
convincing proof.

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After no generations
it's all in 15-15.

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After one generation it's
all intermediate.

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After two generations some
of it is now 14-14.

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Some of it is 14-15.

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And onward like that.

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Obvious, and it's a gorgeous
experiment, just a gorgeous

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experiment.

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And this idea of density
centrifugation, which they

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invented for this purpose, has
been used for many other

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purposes since.