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Part 3

Course video 21 of 68

When DNA was found to be the genetic material, it was not known how this molecule could carry information. The structure of DNA thus became of critical importance. The available X-ray images obtained by M. Wilkins and R. Franklin only yielded a rough picture, and even R. Franklin, who had the clearest diffraction data, could not decide whether the molecules contained two or three strands. Both Pauling and Watson and Crick used molecular models with known inter-nuclear distances (bond length) and bond angles to predict a structure. While the model of Pauling was hardly realistic, since it used the protonated form of the phosphate, the model proposed by Watson and Crick proposed that DNA consists of a pair of DNA strands. Furthermore, it indicated that any nucleotide sequence could be accommodated in the structure. The only central biological issue that was addressed in the first paper was replication, and the famous sentence was really nothing more than a priority claim. Much more biology was discussed in the second paper. It was assumed that base pairing is sufficient to account for the fidelity of replication. The importance of DNA polymerase in replication fidelity was first demonstrated by Speyer.

You have all heard about the DNA double helix and genes. Many of you know that mutations occur randomly, that the DNA sequence is read by successive groups of three bases (the codons), that many genes encode enzymes, and that gene expression can be regulated. These concepts were proposed on the basis of astute genetic experiments, as well as often on biochemical results. The original articles were these concepts appeared are however not frequently part of the normal curriculum of biologists, biochemists and medical students. This course proposes to read study and discuss a small selection of these classical papers, and to put these landmarks in their historical context. Most of the authors displayed interesting personal histories and many of their contributions go beyond not only the papers we will read but probably all their scientific papers. Our understanding of the scientific process, of the philosophy underlying the process of scientific discovery, and on the integration of new concepts is not only important for the history of science but also for the mental development of creative science.

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Ministrado por

Dominique Belin
Professor

Transcrição

[MUSIC]

Okay, so now we are going to read A Structure for

Deoxyribose Nucleic Acid by Watson and Crick.

Now, you may wonder why first Watson, second Crick.

Well, they just tossed a coin.

They couldn't decide between themselves, so they tossed a coin and

Watson was first.

There is no more senior or less senior.

They are essentially equal on the paper.

This suggests a structure for DNA,

and actually, they say for the salt of the acid.

The salt of the acid is already stating that Pauling was wrong with his structure.

They immediately talk about the Pauling and Corey model.

And then they say in our opinion this structure is unsatisfactory,

they don't say wrong, for two reasons.

We believe that the material which gives the x-ray diagram is a salt and

not the free acid.

That's the main reason.

That's the real reason.

And then they also say some of the Van der Waal's

distances between the atoms appear to be too small.

This is a very technical specific kind of criticism,

which is amusing if you consider that it's irrelevant.

These distances are irrelevant because the model was wrong.

So, they discuss other structure which they call for

instant the Fraser structure which they call ill-defined.

And then they put show their double helix, which is this.

Now, is this a well-defined structure?

Not really, this is a rather ill-defined structure.

It's a diagram as they write it.

But the major item here is the fact that the two helices,

move in opposites directions.

That is critical.

Then they discussed some technical aspects with the structure about the sugar and

the position of the sugar and so on.

And they give units of measurements,

the units are from here to here 34 angstroms,

and from here to here 20 angstroms.

That's the distance that they get from their models.

They also notice that if you look from the sides down to the DNA

there is something called a wide and a less wide,

a major groove and a minor groove which are seen by Rosalind Franklin.

Then they say in this paper, something which is totally correct,

but has been misinterpreted very often.

Another feature of this structure is the manner in

which the two chains are held together by the basis.

That is certainly

true, but it's the interpretation of this held together

by the bases which has to be very careful.

We will come back to that later.

The bases are perpendicular so they are here, and they are joined in pairs.

A single base from one chain being hydrogen bonded

to the single base from the other chain.

The two lie side by side.

Then they discussed the.

That's the Donahue contribution.

And so they say that if one base is an A, the opposite base is a T.

If one base is a G, the opposite base is a C.

So if I go to the documents, and

here you have the two base pairs of Watson and Crick.

Here the A with the T, the G with a C.

They quote here a biochemists named Chargaff who had

found that the ratio of A to T is always one of G to C is always one.

But some DNA have a lot more GC than others.

This is called the Chargaff rule.

And the Chargaff rule was actually discussed in the paper.

It's quoted in the references, and

it's considered to be a nice addition to the model.

Now, Chargaff was always upset because he thought they should have used his rule

to deduce the structure.

In fact, they deduced the structure and it fits with Chargaff's rule.

Then they also say it is probably possible to build this structure with an RNA.

Now, it didn't take very long to find out that RNA can be double stranded.

And it's true that a double-stranded RNA is not exactly the same as

a double-stranded DNA in terms of shape of the helix.

So in a sense, it is probably impossible to build this

structure, this structure, this you cannot make with RNA.

The structure is slightly different.

It's also a double helix and it's also anti-parallel.

So, the previous x-ray

were not sufficient to test their model, but, and they say our

model is roughly, our structure is roughly compatible, but

must be regarded as improved until it has been checked against more exact results.

Some of these are given in the two following paper, the Wilkins and

the Franklin paper.

We were not aware of the details of the result presented there

when we devised our structure.

Which rests mainly though not entirely on published

experimental data and sterile chemical argument.

Now comes a sentence that is very one of the few sentence

that is really famous in biology.

It has not escaped our notice that the specific pairing we have postulated

immediately suggest a possible copying mechanism for

the genetic material replication.

Now that sentence was the big debate.

Crick wanted to include the sentence because he said if we don't say it,

everybody's going to believe that we have not seen it, and we will look like idiots.

And Watson said but if our structure is wrong,

which it could be, or we look like fool.

And so, they argued and argued and argued.

And finally, Watson accepted to leave the sentence in, although he didn't

feel very comfortable about it because he didn't want to look like an idiot.

He was extremely self-conscious.

So that's

That's the paper, now we should look at it in terms of,

we should compare it with the structures of Franklin and Wilkins.

Now, the two following papers the Wilkins and Franklin paper are here.

I won't read you sentences from this paper,

because they are extremely x-ray jargon kind of thing.

And so they are not very easy to read.

What you can immediately see is the paper by Wilkins, the figure by Wilkins,

and the figure by Franklin, the two pictures of DNA.

I don't think it is difficult to realize that this picture, the Franklin picture,

is much better in terms of resolution than the Wilkins picture.

So in terms of quality of the image, it's absolutely clear that Franklin had

better data, which mean more precise way to measure distances and

angles than Wilkins.

Absolutely no question.

Now, Franklin, she shows one of the two DNA form,

the two DNA form, the same form as used for the model by Watson and Crick.

She has ten residues per turn, 34 angstrom.

20 angstrom of diameter.

The phosphate on the outside.

What she doesn't have is the anti-parallel helices that she missed.

And she doesn't decide whether it's two-chain or three-chain.

She cannot choose between two chains and three chains.

Wilkins on the other hand, also has helical,

also has the 20 angstrom diameter, also has the 3.4 and 34 angstrom.

He can now determine between two and three helices.

And he certainly doesn't think about anti-parallel,

but, Wilkins contribution in my mind is major because of this paragraph.

This paragraph which talks about invivo structure.

Why is this important?

The biological significance of a two-chain nucleic acid has been noted,

that was in a Greek paper.

The evidence that the helical structure discussed above does, in fact, exist in

intact biological systems is briefly as follows, they use sperm heads

where DNA is certainly biologically active or is partially active.

They use phage, bacteriophage.

And they use the transforming

principle of Avery.

Now, this means that in 1953 people

not only knew about Avery but remember about Avery.

And remember sufficiently to ask, and then it forced Taylor to give them some DNA,

transforming DNA, which if you remember is a pure DNA.

The purist that was available at the time.

Much more pure than sperm heads or phage, which are about 50% protein.

And that has the same structure.

So.

I didn't read this paper by Wilkins for,

I never read it basically for many, many, many years,

and when I discovered that, I got convinced that Wilkins

really deserved the Nobel Prize together with Crick and Watson.

Now, the question about Rosalind Franklin is completely irrelevant, because she died

in 1958 from a leukemia before the prize was discussed and given in 1962.

So she couldn't have gotten it.

She wasn't mistreated because of anti-feminist issues.

She was also not interested in DNA.

After that she moved in work on phage with Bernal, not on phage,

on the virus structure.

And she was basically a crystallographer,

whereas Wilkins had biology code thinking,

which made certainly his contribution very important.

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