Friday, 10 July 2015

A Most Important Discovery

On March 19th of 1953, weeks before it was announced to the public, scientist Francis Crick excitedly wrote a letter to his son and told him of one of the most important scientific developments of modern times: his co-discovery of the “beautiful” structure of DNA, the molecule responsible for carrying the genetic instructions of living organisms; or, as Crick explained it to 12-year-old Michael, “the basic copying mechanism by which life comes from life.” Although DNA was isolated back in the 1860s by Friedrich Miescher, its now-famous double-helix structure wasn’t correctly modelled until the early 1950s by Crick and his colleague, James Watson, thanks in no small part to work already done by Maurice Wilkins, Rosalind Franklin, and Raymond Gosling. In 1962, Crick, Watson and Wilkins were awarded the Nobel Prize for their efforts.

In April of 2013, this letter became the most expensive in history after being sold at auction for $5.3million.

(Source: The Letters of Note book - reprinted by permission of the family of Francis H. C. Crick.)









Transcript

19 Portugal Place
Cambridge

19 March ‘53

My Dear Michael,

    Jim Watson and I have probably made a most important discovery. We have built a model for the structure of dex-oxi-ribose-nucleic-acid (read it carefully) called D.N.A. for short. You may remember that the genes of the chromosomes — which carry the hereditary factors — are made up of protein and D.N.A.
    Our structure is very beautiful. D.N.A. can be thought of roughly as a very long chain with flat bits sticking out. The flat bits are called the “bases”. The formula is rather like this.

|
               sugar —— base
|
       phosphorus
|
              sugar —— base
|
      phosphorus
|
             sugar —— base
|
     phosphorus
:
   and so on.

Now we have two of these chains winding round each other — each one is a helix — and the chain, made up sugar and phosphorus, is on the outside, and the bases are all on the inside. I can’t draw it very well, but it looks like this.

[diagram of the double helix]

The model looks much nicer than this.
    Now the exciting thing is that while there are 4 different bases, we find we can only put certain pairs of them together. The bases have names. They are Adenine, Guanine, Thymine & Cytosine. I will call them A, G, T and C. Now we find that the pairs we can make — which have one base from one chain joined to one base from another — are only

       A with T
and  G with C.

Now on one chain, as far as we can see, one can have the bases in any order, but if their order is fixed, then the order on the other chain is also fixed. For example, suppose the
first chain goes ↓ then the second must go

A - - - - - - - - - - - - - - - - - T
T - - - - - - - - - - - - - - - - - A
C - - - - - - - - - - - - - - - - -G
A - - - - - - - - - - - - - - - - - T
G - - - - - - - - - - - - - - - - -C
T - - - - - - - - - - - - - - - - - A
T - - - - - - - - - - - - - - - - - A

It is like a code. If you are given one set of letters you can write down the orders.
Now we believe that the D.N.A. is a code. That is, the order of the bases (the letters) makes one gene different from another gene (just as one page of print is different from another). You can now see how Nature makes copies of the genes. Because if the two chains unwind into two separate chains, and if each chain then makes another chain come together on it, then because A always goes with T, and G with C, we shall get two copies where we had one before.
For example

A — T
T — A
C — G
A — T
G — C
T — A
T — A

chains
↙ separate ↘
A T
T A
C G
A T
G C
T A
T A

new chains form

A — T                             T — A
T — A                             A — T
C — G                             G — C
A — T                              T — A
G — C                              C — G
T — A                              A — T
T — A                              A — T

In others words we think we have found the basic copying mechanism by which life comes from life. The beauty of our model is that the shape of it is such that only these pairs can go together, though they could pair up in other ways if they were floating about freely. You can understand that we are very excited. We have to have a letter off to Nature in a day or so.
    Read this carefully so that you understand it. When you come home we will show you the model.

Lots of love,
              Daddy