10.
Like-with-like structure
Having
seen the X-ray photograph of the B form of DNA, Watson
worked out that it was a helix which repeated its pattern
along its axis every 3.4 nanometres. He presented this
evidence to Professor Bragg and asked permission to
get on with building models of possible structures.
Bragg agreed, and the machinist in the Cavendish began
making models of the molecules involved.
Data
obtained by X-ray diffraction and with electron microscopes
showed that the bases were stacked on top of each other,
0.3 nanometres apart and at right angles to the axis
of the helix. The helix was about 2 nanometres in diameter.
In
their first model Watson and Crick had tried putting
the sugar-phosphate backbone of DNA in the centre of
the molecule, so they wouldn't need to worry about fitting
the bases together. Watson now tried the opposite, building
a two chain molecule with the backbones on the outside.
Watson thought that hydrogen bonds might hold the bases
together between the backbones.
To
his surprise, it seemed that each of the four bases
could hydrogen bond with another base of the same type.
Thymine could bond with thymine, adenine with adenine,
cytosine with cytosine and guanine with guanine. This
would explain how DNA carries genetic information, as
each chain would be exactly the same!
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