11.
Hydrogen Bonding
In
Watson's like-with-like model of DNA, the base pairs
are held together by hydrogen bonds.
Hydrogen
forms covalent bonds with other molecules to produce
molecules which are more stable than the free atoms.
The electron from the hydrogen is effectively 'shared'
between the hydrogen and the atom to which it has bonded.
Another electron, from the bonded atom, is also shared
between the two.
However,
if the hydrogen is bonded to an oxygen or nitrogen atom
the sharing of the electrons is not quite equal. The
bond becomes slightly polarised. This means that the
hydrogen atom has a slight positive charge while the
oxygen or nitrogen atoms are slightly negative.
If
two molecules with this polarisation are close to one
another they can 'hydrogen bond'. The positive hydrogen
atom of the first molecule is attracted to the negative
oxygen or nitrogen atom of the second molecule. This
bond is weaker than the covalent bond, but is the attraction
which gives DNA its shape.
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