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full-text.txt
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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 des-oxy-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
|
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 of 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
[diag]
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 others.
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 other 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