DNA Structure and Classic experiments, excerpt 1 | MIT 7.01SC Fundamentals of Biology

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MIT OpenCourseWare at ocw.mit.edu. PROFESSOR: So here’s
what we did. We found mutants that
effect biochemistry. That’s one way to make the
connection between function and gene. But if we want to go the other
way, we now have to do the biochemistry of genetics. What’s biochemistry about? It’s purifying things
in a test tube. What’s genetics about? Heredity. So what do we have to do? We have to purify heredity
in a test tube. But that’s it. That’s what we have to do. If we’re going to make that
connection in that direction, all we need is to take a cell,
grind it up, and purify not the enzyme that digests
a sugar, but heredity. We need to get a pure
tube of heredity. You can imagine that this wasn’t
an obvious thing to do, how you get a pure
tube of heredity. The problem is you
need an assay. If you want to find an
enzyme that digests a sugar, you have an acid. You purify different fraction,
you make different fractions of a cell you see which fraction
of the cell is able to digest the sugar. But if I make different
fractions of the cell, it’s not obvious how I figure out
which fraction of the cell has heredity in it. And yet, that’s exactly
what was done, and that’s today’s lecture. So purifying heredity. The discovery of the
transforming principle. By the transforming principle,
I don’t mean an idea, like a principle. I mean a substance. This is an old medieval kind
of word, a principle. It’s kind of a Harry Potteresque
kind of word or something like that of what is
the transforming principle, the transforming substance. It’s the kind of word alchemists
would like to use, but it was actually what’s
attached to this, and it was called at the time the
transforming principle, the transforming substance. And it really is the work in
1928 of a young scientist F. Griffiths in London. Griffiths was particularly
interested in studying the bacteria pneumococcus. Why was Griffith so interested
in studying the bacteria pneumococcus? Well, not so long before in
1918, there had been the terrible Spanish influenza
epidemic that had killed millions of people around the
world, the worst flu ever. Millions of people died by
this flew, and one of the reasons they died when they
have the flu was they got pneumococcal infections. Griffiths was trying to make a
vaccine against pneumococcus, a pretty good idea trying to
make a vaccine against pneumococcus. Pneumococcus highly virulent
stuff, but particularly if you’re compromised
by the influenza. So what did he do? Well, Griffiths had a strain
of pneumococcus. He didn’t, by the way, infect
people with it. He infected mice with it, that
being considered a somewhat more ethical way to
do the experiment. So he had a strain of
pneumococcus that had a smooth, glistening coat. When you looked at it, it
was smooth and white and glistened, the colonies
that it made smooth, white, glistened. And it was virulent. If you inject it into a mouse,
the mouse got pneumococcus infection, and it died. It turns out we now know
that it has a beautiful polysaccharide coat around it
that provides resistance to the host’s immune system,
et cetera. He also had a strain
of pneumococcus that had a rough coat. It was not glistening. It was kind of dull looking,
and it was non-virulent. He injected it into a
mouse, mouse lives. It happens to be the case that
we now know that it had a mutation in a gene that produced
that coat, but that doesn’t much matter. It didn’t produce that
polysaccharide, and therefore, was more easily fought off
by the immune system. So Griffiths does the following
experiment. He takes his smooth, virulent
bacteria, he injects it into a mouse, that’s a mouse. And what happens to the mouse? AUDIENCE: Dies. PROFESSOR: Dies. Exactly. That’s a dead mouse. And one of the easier assays
in molecular biology is the feet up, feet down assay. All right. Dead mouse. Now, what does he do? He takes the rough,
non-virulent, he injects it into a mouse, and
what happens? Lives. Right? Lives. Then what he does, he takes the
smooth, virulent and he bakes it in an autoclave. He heat kills this bacteria. Now, this heat-killed, dead,
virulent bacteria when injected into a mouse, what
happens to the mouse? It’s alive. The mouse is fine. The bacteria was dead. And then he does the following
truly weird experiment. He takes the absolutely
harmless, rough, non-virulent bacteria but alive, plus the
smooth, virulent bacteria that has been killed by heat,
heat-killed. The rough stuff is harmless. The smooth heat-killed
stuff, harmless. Both are harmless. We’ve shown the mouse can
live with this injected. It can live with
this injected. He injects it into the mouse,
and what happens? Dead mouse. Very surprising. Not only that, when he takes the
blood of the dead mouse, he can culture out of it. He can culture on a Petri plate
from this dead mouse live, smooth, virulent
bacteria. How did that happen? Somehow, we didn’t have any
live, smooth bacteria. We had dead, smooth bacteria. We had live non-virulent
bacteria. Somehow, the dead stuff
transformed the live harmless stuff into virulence. It transformed it. The substance, the unknown
substance that transformed it was referred to as the
transforming principle or transforming stuff. And now we have biochemistry
because we have an assay. We could take the dead, virulent
bacteria and break it up into fractions and see
which substance, which fraction, is it a protein? Is it a carbohydrate? Is it nucleic acid? Is it something, is the
transforming substance and purify heredity because we
have seen the transfer of heredity to the harmless
bacteria. There is an assay. The minute there’s an assay,
you could do biochemistry. Now, the problem was the assay
was painfully slow. You had to grind up the dead
bacteria, you had to demonstrate by mixing it
together, putting it into a mouse, waiting months, and then
you had to get a further sub fraction. It was just painfully slow. All this mouse stuff
and all that. And Griffiths didn’t make much
progress through the 1930s, but he kept going at it. And I suspect might have gotten
there except for the fact that in 1941, his lab was
hit by a German bomb during a blitz, and he died. And so Griffiths never saw the
result of this, but he did purify fractions and all that. But thank goodness others
picked it up. He was again, Griffiths
was a great guy. He worked in World War I, worked
during World War II on important public health
problems, and really lays this foundation, but never really
purified what the final substance was because he
died in a bombing. But then you get in 1943, during
World War II, folks working in New York City at
Rockefeller University, then Rockefeller Institute, Avery,
McCarty, and MacLeod continue this work, and they do
it without the mouse. Because what they do is they
grind up the virulent stuff and they sprinkle it on the
living stuff, the living bacteria and played it out,
and just looked for the appearance of colonies
that are transformed. Skip the mouse. Skipping the mouse makes it a
lot easier, because you can do those experiments
pretty quickly. Bacteria grow quickly. Same basic idea. Purify the stuff from the smooth
dead stuff, grind it up, put it in different
fractions, apply them, and then sprinkle them on a
plate, and look for the occasional colony. And now you’re just going to
look for a fraction of the material that has the capability
to produce some smooth colonies. Well, they did that, and they
purified it, and the purified it, and they purified it, and
they purified it, and they eventually found that the
particular type of molecule that they purified appeared
to be DNA. But when they purified fractions
containing DNA, these fractions had the
ability to transform. Wow. You might immediately say,
that’s the transforming principle, DNA. That’s the transforming
substance. What do you think the
reaction was? Skepticism. First, it should be
noted it’s 1943. People were busy at
the time, right? We’re in the middle
World War II. It wasn’t exactly top on
people’s minds, but there was enormous skepticism
scientifically of those people who did follow the work. Why? Because the one thing they knew
was that DNA was truly a boring molecule. It was understood by all smart
people that DNA was an incredibly boring structural
molecule that had none of the fascinating diversity and
richness of proteins. Proteins could do zillions
of different things. DNA, you know, it’s
just scaffolding. Why? What is the structure of DNA? So let’s turn to the structure
of DNA to see why it is that people were not impressed. Of course, when people are not
impressed, you purified something and you show it
transforms, what do you say to Avery, McCarty, and MacLeod? How do you express
your skepticism? You say, it’s very good. You’ve purified this and it
contains DNA, but is it absolutely totally 100% pure or
is it possible that you’ve carried along in the fraction
that you have purified some other trace quantity of a highly
potent protein that is really causing heredity? And of course, that’s the
problem is you can never prove that there’s not a teeny smidgen
of something in there. You can only show how pure it
is, but you can never rule something out. So when people want to sort
of dis your biochemistry experiments, it’s always easy
to say, it was probably something else in there too you
just don’t know about it. And that was what the
answer was to them. But let’s look at the
structure of DNA. So DNA has three important
components which we need to learn. A, it has a sugar called
2 prime deoxyribose. So ribose is a 5-carbon sugar. A five-part carbon sugar
is a pentose. So this is a sugar, in fact a
pentose, pentose of course five, pentose meaning it’s a
5-carbon sugar, but it lacks a hydroxyl group. So it’s just slightly
different from the 5-carbon sugar. And we draw it in this
configuration where there is the 1 prime carbon here, the 2
prime carbon here, the 3 prime carbon here, the 4 prime carbon
here, the 5 prime carbon here. That’s very important to know. We’ve got an oxygen up here. Here, we have an OH and an H.
Here, we have an H and an OH. Here we have H. Here we
have our OH, H, H. But here we should have
a hydroxyl off every carbon, and we don’t. Only here are deoxy. That’s the only difference from
this being a perfectly normal ribose, deoxy at
the 2 prime position. Big deal. Now, the next component of DNA
that you need to know about are these nitrogenous bases. So hanging off our
ribose is a base. This base has carbons, oxygens, hydrogens, and nitrogens. And they come in four flavors,
adenine, A, guanine, G, thymine, T, cytosine, C. A, T,
C and G, and we’ll look at their structure in
just a moment. Then the next important part
if we look at this conceptually, is that hanging
off here, we have a triphosphate. We have a triphosphate. So this is the monomer for
making DNA, triphosphate. We have a sugar, the sugar in
exactly the same place off the 1 prime carbon there has a base,
off the 5 prime carbon, we have a triphosphate. What’s the triphosphate
good for? Energy. We’re going to do a
polymerization, and that’s going to contribute the energy
for the polymerization, and that’s pretty much it. That’s the way to
think about DNA. So when we do our polymerization
now, we polymerize and we get base,
CH2, phosphate. And then coming down this way
attached here, we have our phosphate, and that attaches
to the 5 prime carbon here, and onward that way. So notice that our polymer goes
from a 5-prime carbon here, 3-prime carbon here,
5-prime carbon here, 3-prime carbon there. And we go sugar, phosphate,
sugar, phosphate, sugar, phosphate, sugar, phosphate,
5-prime attachment, 3-prime attachment, 5-prime attachment,
3-prime attachment. That’s DNA. Pretty boring. The same sugar, same phosphates
strung together, totally boring. The only difference
are these bases. And there’s only four of them,
and they’re not very impressive. They’re pretty boring,
these bases. There are purines. The A and G are purines,
and their ring structure looks like this. This is six-membered ring and
there’s a five-membered ring. There are pyrimidines, T and
C, and they just have a six-membered ring. They’ve got carbons, and
oxygens, and nitrogens, and hydrogens, and they
don’t differ really in their charges. By compared to the amino acids,
positive charges, negative charges, hydrophobic
groups, sulfurs that are reactive. Amino acids, that’s
impressive. Those 20 different side chains
have wildly different chemical properties. These form measly bases, have
essentially, the same chemical properties. There’s nothing very different
about their chemical properties and therefore, all
smart right-thinking people recognize the DNA
could not be a particularly interesting molecule. It had to be largely
a structural molecule of some sort. So when Avery, McCarty, and
MacLeod tell us ah, the transforming principle of
DNA, nobody’s impressed. But of course, it’s
World War II. People are busy. Lot of things going on. And not that long afterwards,
not that long afterwards, another really important
experiment gets done in the early 1950s, the Hershey-Chase
experiment. Hershey is not the candy bar. It is Alfred Hershey
and Martha Chase. Martha Chase and Alfred Hershey
do a cool experiment. People were studying something
else at the time. They were studying the viruses
that infect bacteria, bacterial viruses. So it turns out just like you
may get a viral infection, E. coli gets viral infections
too. It usually dies of them or at
least often dies of viral infection, not necessarily
usually, I take that back, sometimes dies of viral
infections. So that is a virus, actually,
greatly magnified, glommed on to E. coli, virus E. coli. What happens is, if you mix
the virus with E. coli, it gloms on, and then if you wait
a little while giving it a happy medium to grow in, the E.
coli some time later, half an hour later maybe, bursts
open, dead, and spews out zillions of viral particles
which could go on to infect new cells. How does it do that? How does it instruct E. coli
to make viral particles? It must be bringing
information. It’s having progeny. It is passing on heredity too. It has some transforming
information. Where is the transforming
principle in the little virus? It gloms on to the cell somehow
gives something into the cell, and poof, 20 minutes
later, half an hour later, lots of viruses. Where’s the information
carried? Now, this was a much
simpler system. This system, you’re asking
what’s in the bacterial virus. There’s not a lot in
a bacterial virus. It’s not like a cell that
have zillions of things. The bacterial virus is a
pretty simple particle. The bacteria virus consists of
protein coat, proteins are on the outside, DNA
on the inside. That’s it. You don’t have a lot to work
with, a limited number of proteins, DNA in the middle. These things just as an aside
were thought to kind of like eat bacteria. Because they were thought to
eat bacteria in a way by at least the early things, they’re
called bacteriophage, bacteriophage. The word phage means to eat. So you may hear me talking about
bacteriophage, meaning eaters of bacteria. Indeed, actually there was some
nutty ideas in the 1920s and 1930s when bacteriophage
were first discovered that the way to cure a bacterial
infection was to drink a lot of bacteriophage. They would kill the bacteria. It’s a thought. People actually tried
these things. Anyway, it turns out not
to be such a good idea. So Hershey and Chase decided
we’re going to figure out which is it? Is it the DNA or is
it the protein? How do you find out? Yeah? AUDIENCE: [INAUDIBLE]. PROFESSOR: Put in only protein,
and see what happens. So take the bacteriophage,
purify it from protein verses DNA. I’ve got a pure component of the
protein, I sprinkle it on, nothing happens. I take the DNA, I sprinkle
it on, nothing happens. Neither works. Why is that? AUDIENCE: [INAUDIBLE]. PROFESSOR: The shape, those
little feet in the shape were critical for the
pathogenicity. So when we grind up the virus,
it doesn’t work anymore. It’s a great idea. If it worked, bingo, we’d have
it, and that should be the first experiment we do
because it’s so easy. But it turned out not to work. Yes? AUDIENCE: [INAUDIBLE]. PROFESSOR: Put a chemical marker
on the protein, put a chemical marker on the DNA,
and see which one goes into the cell. What chemical marker? How are we going to attach a
chemical marker to the protein without messing it up? We can’t mess up the
protein, right? It still got to function. How do we get a chemical
do you want me to put in? Well, how am I going to tell
whether, DNA’s got phosphorus. How am I going to follow
the phosphorus? AUDIENCE: Radioactive tag. PROFESSOR: Radioactive tag. Bingo. What if I used radioactive
tags, and I made a radioactively-labeled virus. How can I radioactively
label the DNA? AUDIENCE: [INAUDIBLE]. PROFESSOR: Sorry? AUDIENCE: [INAUDIBLE] PROFESSOR: A radioactive base. I could do that. What else could I do? Yup? AUDIENCE: Phosphorus. PROFESSOR: Phosphorus. Phosphorus has the nice property
that phosphorus is in my DNA, but it’s not
an proteins. So what do I use? Phosphorus-32, P-32. So I use P-32, and how do I
manage to chemically create a virus that has P-32 in it? AUDIENCE: [INAUDIBLE]. PROFESSOR: Just throw it in the
solution with P-32, and the virus will take care
that itself, right? So simply grow virus for a while
in the presence of P-32. Let’s do that. So grow virus in a test
tube with bacteria. Here’s my bacteria. Here’s my virus I’ve put in
there, and let me put in P-32, and what I’ll get is
P-32-labeled labeled virus. How do I label my protein? Someone said it already. What elements can we
find that’s in proteins but not in DNA? AUDIENCE: [INAUDIBLE] PROFESSOR: Sorry? AUDIENCE: Sulfur. PROFESSOR: Sulfur. Sulfur. S-35 is a radioactive isotope of
sulfur, and if I grow it, I can S-35 label the proteins
in my virus. Nice. This radioactive labeling
trick is very cool. So I take it, I take some
P-32-labeled virus where these P-32 was only in the DNA. I got some S-35-labeled
label virus where the S-35 is in the protein. I could mix them together, now
do my experiment, wait 20 minutes and, or even wait last
10, 15 minutes, and see which element has gone
into the cell. How do I do that? See I’ve got my cells here,
and I’ve got the viruses attached to them, and they’ve
injected something in here. They’ve either injected
a protein or they’ve injected DNA. What was injected? I need to carefully go in there
and remove the virus and look at just what’s
in the cell. I have to now separate the virus
glommed onto the outside of the cell from the cell. So do I use micro manipulator
tweezers to pull off the virus? AUDIENCE: [INAUDIBLE]. PROFESSOR: Well, if I
denature, I might crack open the cell. Centrifuge it. If I centrifuge it, the whole
thing will spin down. I need to kind of knock the
viruses off the cell, physically. I just got to agitate it so
I get them off the cell. With enough kind of hydrodynamic
agitation, the viruses fall off. So a device was created that
was able to just perfectly knock the viruses off. It’s referred to as the Waring
kitchen blender. It turns on your kitchen blender
is perfect for this. Take the viruses, add it to the
bacteria, sit for a little bit, put it in your kitchen
blender, press puree. And let’s say on puree setting,
the viruses fall off, and now you can spin it in a
centrifuge, the bacteria are denser, they come down. The viruses are lighter, they
stay in the supernatant, and you can take the supernatant and
the pellet at the bottom over your radioactivity counter
and see which one is in the bacteria. These were referred to as the
famous Waring blender experiments. They really are, actually. So you put this in the Waring
blender, you knock off the viruses, you spin it down, and
what happens is after you’ve done it, there’s a pellet here
of the bacteria that are spun down in the centrifuge. The virus particles
are still up here. We take this pellet over to our
counter, and which element do we find in great abundance,
S-35 or P-32? AUDIENCE: P-32. PROFESSOR: P-32. The DNA is what’s going in. Bingo. Nice experiment. Now if you were churlish,
couldn’t you say, yeah, look it’s mostly the DNA, but there’s
a little smidgen of protein maybe, that
came along too. Do you think they found
absolutely zero S-35 in there? No, because they don’t perfectly
knock the virus off. Some of it kind of sticks. There’s 1% S-35. And if you’re being really
churlish about this you would say I still don’t believe you. But now you have it from two
different directions. You have it from the
pneumococcus, this bacteria experiment from Avery, McCarty,
and MacLeod Hershey and Chase coming from two
different systems. They’re giving you
the same answer. It’s pretty clear. It’s in the air. People know DNA is the stuff. They’re believing it now. DNA is the stuff. But how does it work? How can this dumb molecule
possibly be the transforming principle? Well, smart, young people
want to know. So an erstwhile ornithologist,
that is a college kid from the University of Indiana who
particularly liked bird watching got very enamored by
this problem, actually based on some fabulous faculty at
the University of Indiana. He got really intrigued by how
could DNA possibly do this. But he recognized he didn’t
know any chemistry. He decided to go to Cambridge,
England to the Medical Research Council lab, the MRC
lab in Cambridge, England where he teamed up with someone
who did a lot of talking and very few
experiments. A physicist who had worked for
the British admiralty during World War II on classified
things and had somehow gotten interested in biology. And because he had this kid,
this recently graduated college kid, and you had this
35-year-old physicist who nobody was quite sure what to
make of, they kind of hung out with each other in
the same office. And they didn’t really do many
experiments, but boy did they do a lot of talking, and
thinking, and looking at all the data that were out there. And that’s pretty much what
James Watson and Francis Crick were doing. They knew this problem
was important. And Jim and Francis would talk
every day about this stuff, and they will talk to people
down the hall who really knew about the chemistry
of nucleic acids. And they went down to London to
Maurice Wilkins’ lab where crystals were being
made of DNA. And Rosalind Franklin, who was
a fantastic scientist and had managed to make crystals of DNA,
showed Crick and Watson her crystals of DNA. Francis Crick being a physicist
was very good at understanding crystallography
and how crystal structures and x-ray diffraction patterns
related to each other. And Francis knew immediately
this thing had to be a helix. He could tell it was a helix. And they went back, and based
on Rosalind Franklin’s x-ray diffraction patterns, went and
made a model, a model for the structure of DNA. You all know the model. You’ve seen the double
helix forever. It’s a cultural icon, but that’s
how this came about. And The Double Helix, the
Structure of DNA, April of 1953 is published. The double helix has two
strands running an anti-parallel directions, 5
prime to 3 prime, 5 prime to 3 prime anti-parallel directions,
and it has a perfect base pairing between
purines and pyrimidines. If you have a T, and
I’m just going to draw this very quickly. You can look in your book for
getting it just right. You have two hydrogen bonds. That’s T and A, and if you
have a C, you have three hydrogen bonds that perfectly
hold it to the G, et cetera. So notice C and G fit perfectly
together to make three hydrogen bonds. A and T fit perfectly together
to make two hydrogen bonds, and that was the key was to
recognize that when you stick them together in that way, you
get exactly the same width. They fit perfectly. You couldn’t match an A with a
G, an A with a C, you could only match the A with
a T. That was it. Brilliant. Beautiful. Now, you guys should read
Crick’s book The Double Helix in which he tells the stories
because it’s just a fascinating, fascinating
business. He’ll tell, or others will
tell actually, the story. So you know what this
means by the way? This means that the amount of
A should equal the amount of T. And the amount of G should
equal the amount of C. There should be a ratio, a
one-to-one ratio that the A to T ratio should be one to one. And the G to C ratio should
be one to one. Although any organism might have
more As than Ts and Gs than Cs, the ratio of these guys
should be one and these guys should be one. This actually was discovered
by a chemist at Columbia called Chargaff. These were called Chargaff’s
rules. Chargaff was a very
distinguished chemist who came up with Chargaff’s rules with
the As equals the Ts, and the Gs equals the Cs, and
he didn’t know what to make of it. By the way, Chargaff actually
visited Cambridge, England while Crick and Watson were
there before their discovery, and he had lunch with them. And he related that Crick and
Watson seemed like Bozos to him, because they couldn’t even
keep straight the exact structure of the four bases. They always had to keep
looking it up. They hadn’t memorized the
structures the four bases, and Chargaff was such a brilliant
chemist, he, of course, knew this instantly, et cetera,
et cetera. And he said, these guys are
never going to get anywhere because they really don’t even
understand the structure of the bases, haven’t
memorized it. When Crick and Watson’s
discovery turned out to be the single most important biological
discovery of the 20th century, Erwin Chargaff who
lived a very long life was sort of bitter because he’s kind
of worked it out in a way with the ratio and never figured
out what it meant. And he said one of the
bitterest, cuttingest comments I’ve have ever heard from a
scientist which is referring to Crick and Watson as still not
being impressed even after they won their Nobel
Prize for this. He said that two such pygmies
should cast such giant shadows only shows how late
in the day it is. Anyway, he was not happy to
have missed this point. Crick and Watson were
very happy to have figured this out. They, when they figured this out
in February of 1953, what do you do in England when you
make a big discovery? AUDIENCE: You have tea. PROFESSOR: No, you
don’t have tea. You go to the pub. They went to the pub. They ran down to the Eagle Pub,
they brought everybody drinks, and they told everybody
at the Eagle Pub, we’ve discovered the
secret of life. The people at the Eagle Pub
had no idea what they were talking about, but were happy to
have a round of drinks, and there you go. They immediately raced to
write this up in Nature. It appears in Nature in
April of 1953, and it’s a one-page paper. And get it on the
web and read it. It’s the single best one page
that has been written in biology in the 20th century. And of course, what
did they say? The title is kind of unassuming,
“A Structure for the Salt of Deoxyribonucleic
Acid,” nothing too exciting. But Crick and Watson
realized something. What do they realize? Crick and Watson realized
why is this a big deal? Why is this double helix
so important? Well, the implication of the
double helix is that if I have a double helix and those strands
were to separate, each would be a template for a
double helix, heredity. How do you pass information
to two daughter cells? You got a double helix. It’s redundant. If you know the A is on
one strand, you know the Ts on the other. Unzip it, copy, voila. I now have two copies
of heredity. What’s a mutation? Occasionally get it wrong. Bingo. They saw it. They knew. Now, they didn’t have
time to prove this. I mean, who has time
to prove this. This is such an exciting
discovery secret of life. Drinks for everybody, they write
it up, but in the last paragraph, they certainly don’t
want anybody to think that they missed the point. And they write the coyest
sentence in molecular biology. They write, “It has not escaped
our notice that this structure offers an
explanation for heredity and mutation. We’ll address this in another
paper.” Cute. Very cute. They put down their marker, they
knew what it meant, but they got the thing off to
Nature very quickly. It was a hot topic. They were competing
with other people. You’ll read about the
competition with Linus Pauling and other things like that. It had not escaped their notice
that this pretty much explains heredity. Now, of course, are
you convinced that it explains heredity? It’s a nice model, but don’t
we require proof? We do require proof. It’s an ex post facto model, we
need proof, It’s a pretty good ex post facto model,
but we need proof. So the last step, which I’ll
touch on very briefly, was proof of what’s called
semi-conservative replication. Meaning that each strand
is used for the other. And I might run two
minutes over. We’ll see. I’ll try to keep
it within time. Out at Caltech, two graduate
students, Frank Stahl and Matt Meselson hear of this. Matt Meselson by the
way is still working in Harvard Square. He’s at Harvard. He’s a wonderful guy. Matt is there. You could ask Matt about this,
young graduate student at Caltech in the early ’50s. Obviously, this model looks
like it must be right. How do you prove it? Well, Matt and Frank, Meselson
and Stahl came up with a cool experiment to prove it. Meselson and Stahl, they take
bacteria, here’s my DNA in there, they want to show that
each strand, each, when we make a new generation of
bacteria, the new DNA has one old strand and one new strand. That each old strand
is being used as a template for a new strand. How are we going to tell? We gotta label it somehow. We got a label the old strand
different than the new strand. How do we possibly label and
they’re the same chemical composition? What are we going to do? AUDIENCE: Radioactivity. PROFESSOR: Radioactivity or
some kind of isotope. Well, they used an isotope. What they did, super cool, they
grew this up, but not in normal nitrogen, but in N-15,
not, by the way, radioactive, but different weight. They grew it up in N-15. They added bacterias that had
grown up, and all of their DNA had lots of N-15 in it
on both strands. They then pour in a lot of
medium that has N-14. Tons, they swamp it with
N-14 so what’s this strand going to be? N-14. So what can you tell me about
the difference between this DNA and that DNA? AUDIENCE: One’s going
to be lighter. PROFESSOR: One is going
to be lighter. How do you measure how
much lighter it is? They came up, they invented the
technique for this purpose of centrifuging in
a salt gradient. They put in the right amounts of
salt, cesium salt, and they centrifuge it so hard that
there’s a gradient of densities, denser here,
lighter here. And they find that this DNA
and this DNA centrifuge to different places. There’s a difference
in their density. The new DNA is half old,
N-15, half new, N-14. It has the intermediate
density. If I grow another generation,
I’m going to see some N-15 14s, and I’m also going
to see some N-14, 14s. And that’s what they found. They invented this technique
it’s isopycnic centrifugation and Matt Meselson and Frank
Stahl provided an experimental prediction of the beautiful
Crick-Watson model that was fair to say the secret
of life. Anyway, these are
the foundations. Notice now, we’ve taken genetics
and used it to do biochemistry. We’ve taken biochemistry
and used it to understand genetics. And so finally, we are
making the bridge of molecular biology. Next time.


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