Biology: Watson and Crick: The Clues

Rosalind Franklin

11/30/2012

~ ycamacho

You seem to have forgotten that Rosalind Franklin played a huge role in the discovery of DNA!!! Why was she not mentioned?

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Molecular Genetics
DNA Structure Revealed
Watson and Crick: The Clues Page
X-ray diffraction, one of the most powerful tools that we can use to visualize a molecule. No one has seen molecules.
You might, when you look at say paper, you know that it’s a collection of molecules. But the bottom line is, just like in
the days of Watson, Crick, Rosalyn Franklin, and Maurice Wilkins, we find that molecules are simply things we can
visualize. And in the case of x-ray diffraction, we visualize them by literally exposing a photographic plate and taking
a photo, if you will, of a molecule.
And thus we have, once again, this photo of DNA, and the possibility of the fact that it’s a helical structure. And in
fact, there was an awful of controversy as to the helical nature of DNA. One famous scientist, you may have heard of
this guy, Linus Pauling. You’ve heard of Linus Pauling. He was in the hunt. He was in the race. Linus Pauling
proposed a three-helix molecule for DNA, three strands. Other people were proposing one. But nobody really had it
down yet. But let’s look at what was known by this time, partly because of Chargaff’s rules, partly because of the xray
diffraction of Franklin and Wilkins.
First of all, what do we know? We know that the amount of A is going to equal the amount of T. And G is going to
equal the amount of C. Now you’re going to see when Watson and Crick worked on this, at first, this wasn’t apparent
to them. And at first, they tried some combinations other than this. But that’s what you and I know. So there’s what
we know. A equals T. G equals C. Ah, if we could have only been back there.
The second thing we know, we know three magic numbers. What are those magic numbers? 0.34, 3.4 and 2.0,
repeating numbers that came back over and over again when mathematical measurements and calculations were
done of this x-rayed diffraction. And what we see here, when you look at this x-ray diffraction, as you did those
measurements, there were repetitive occurrences of these numbers. It just didn’t happen once in a while. Consistent
measurements of 2.0, consistent measurements of 0.34, and I’m sure it hasn’t escaped you, consistent
measurements of a multiple of 0.34, 3.4.
And the third thing we know from the vision of this photograph is that this DNA molecule seems to be a helix of some
kind, some kind of twisty turny kind of thing. Not a whole lot to work on, but it was a start.
Now we come to the brilliance of the work of Watson and Crick. Let’s take a look at Watson and Crick and what they
did. I call this the tinker toy experiment. Here’s Watson and Crick and their model. So here’s Watson and Crick, and
here they are. And I love to look at this picture, because number one I think it’s posed, but number two, it’s very clear
what these guys did. What they did was they used molecular models. They went to their lab and cut out sheet metal
models, not exactly like these. This is just something we can use to demonstrate some organic bonding. But what
they did was they cut out models to approximate the size of what they know. Crick, particularly, was a chemist. And
they knew a lot of very sophisticated chemistry. So they were able to cut out models and start to integrate their
knowledge into a working physical model of what DNA may be. How did they do this? Well it took a lot of painstaking
work, and a lot of manipulation.
For example, they guessed, correctly it turns out, that it was—and this was Watson, he remembered that photograph
he saw. And his memory said, “You know, I’m going to be guessing that this is a two-helix. This thing has two
strands, this helix. It’s a double helix.” So they worked on that model. So their basic premise was it’s a double helix,
not three, not one. And then they said, “Now this whole 0.34 thing, this 0.34 nanometers sounds suspiciously like the
thickness of what a nucleotide would be.” In other words, because of their knowledge of the physical nature—and
knowledge is everything when it comes to putting pieces together. Because of their knowledge of the physical nature
of the three-dimensional structure of each of these things called nucleotides, they knew that one of these, like a
penny, picture a penny, like a penny, would have a depth of 0.34. And now if that penny is 0.34, what would a stack
of ten of these pennies be, 3.4. And so we’re starting to put the pieces together. So DNA must be a double helix.
And that double helix must be somehow arranged in a sequence of ten of these things. What’s with the 2.0? Well if
it’s indeed a double helix, the 2.0 must represent the diameter of the helix, 2.0. So now let’s figure out how this can
possibly come into some kind of physical being, some kind of physical structure.
So here’s what they did. Using the parameters that the molecules must fit within a diameter of 2.0 from here to here,
they tried different combinations. So what would they do? The first thing they did was they said, “Okay, let’s put the
sugars and the phosphates towards the middle of the molecule, and the nitrogenous bases on the outside.” So they
would try. I’m just going to draw this, but they actually used their models to do this. So they would try something like
this, where they would have the phosphate groups toward the inside, and the nitrogenous bases out here. So they
put their pieces together. And remember it’s helical. They’re trying to fit ten of them, and that didn’t work.
Then they said, “Aha. Well we know that thiamin and cytosine are a group of molecules called pyrimidines. It must be
that pyrimidines go together.” So they tried putting them together. But then they found out that pyrimidines, when put
together, didn’t make the 2.0 mark. They were too narrow. And purines and purines together were too wide. Could it
be that a purine and a pyrimidine fit together? BAM, purines and pyrimidines made up the rungs of the ladder of DNA.
And now all we have to do is piece this molecule together and figure out how these things will bond. Well there it is. It
must be hydrogen bonding. There must be hydrogen bonding involved between A and T, and G and C. The world
wasn’t ready, but in 1953 they published an article that would rock the scientific community.