Page 78 - Physical chemistry understanding our chemical world

PHYSICAL AND MOLECULAR INTERACTIONS 45

appreciated that it is the chromosomes within a cell nucleus that
The word ‘theory’ comes
dictate hereditary traits. And such chromosomes consist of DNA
from the Greek theoreo,
and protein. In 1944, the American bacteriologist Oswald Avery
meaning ‘I look at’. A
showed how it was the DNA that carried genetic information, not
theory is something we
the protein.
look at, pending accep-
The next breakthrough came in 1952, when Francis Crick and tance or rejection.
Donald Watson applied X-ray diffraction techniques to DNA and
elucidated its structure, as shown schematically in Figure 2.5. They
showed how its now famous ‘double helix’ is held together via
a series of unusually strong dipole–dipole interactions between The rules of ‘base pair-
precisely positioned organic bases situated along the DNA poly- ing’ (or nucleotide pair-
ing) in DNA are: ade-
mer’s backbone.
nine (A) always pairs
There are four bases in DNA: guanine, thymine, cytosine and
with thymine (T); cyto-
adenine. Each has a ketone C=O group in which the oxygen is quite
sine (C) always pairs
electronegative and bears an excess negative charge δ , and an
−
with guanine (G).
amine in which the electropositive hydrogen atoms bear an excess
1 nm

3.4 nm

(a)

H
H O H N N H
C
C C C C
N
H C N H N C
N C C N DNA backbone
DNA backbone O H N
Cytosine H Guanine

H
H 3 C O H N N H
C
C C C C
H C N H N C N
N C C N DNA backbone
DNA backbone O H
Thymine Adenine
(b)

Figure 2.5 (a) The structure of the ‘double helix’ at the heart of DNA. The slender ‘rods’ represent
the hydrogen bonds that form between the organic bases situated on opposing strands of the helix.
(b) Hydrogen bonds (the dotted lines) link adenine with thymine, and guanine with cytosine