The chemical structure of DNA II - Genes - the units of inheritance (4/10)

Uploaded by OUlearn on 25.07.2008

The breakthrough was in 1953,
thanks to Cambridge scientists, Francis Crick and James Watson,
in the most celebrated research of the 20th century.
Using the x-ray pictures of DNA,
taken by Rosalind Franklin and Maurice Wilkins,
they struggled to make the data about the spaces between the atoms
fit into a structure that made sense.
When they had finished, it did make sense
but in a more profound way
than a structure of any molecule before or since.
The most important thing about that structure
was not that it was a double helix,
which is the icon of the 20th century we all are aware of,
but the fact that it was a very long linear molecule
and that these bases, the A, G, C, T symbols,
were distributed linearly along it.
They spotted and confirmed the notion,
that people have been fighting against that it was digital
because it seemed unreasonable
that something sort of soft and squidgy and lifelike like us
would be coded for in this hard, digital fashion.
But it is so and that's the breakthrough,
which now we're exploiting today to actually read out that code.
Although the double helix looks complicated,
what it does is very simple.
Crick and Watson had opened up a world
not of chemistry but of information.
Each strand of DNA carries a set of four bases
abbreviated to A, C, T and G.
The bases can occur in any order along the length of the molecule
and this is how the information is carried,
just as the letters of the alphabet make words.
But the crucial thing is the way
the bases on the opposite strand are joined.
Chemistry allows A only with T, C only with G.
And they saw, also, and this is the famous sentence,
"it has not escaped our attention
that this provides a mechanism for replication"
because they saw that if you pulled
the two halves of the double helix apart,
then used those same rules to reconstruct new strands,
you'd have two new double helices looking just like the old one.
You had replicated DNA and as Francis Crick said,
"That's the secret of life."
Crick and Watson realised
that each strand could act as a template for the other.
If you split them apart into two single strands
and then throw in a mixture of A, C, T and G,
the bases can only join up in the right places.
A T opposite every A, an A opposite every T,
a C opposite every G, a G opposite every C
and you have accomplished one of the great tricks of nature.
You've taken one copy of the code and made two identical perfect copies
that can pass on to the next generation.
Through chemistry and physics, Crick and Watson have discovered
how the instruction for life are passed on.