Chemistry Reference
In-Depth Information
FIGURE 3.19
The classic Watson
e
Crick base pairing between A and T, and between G and C in DNA.
clearly distinguish (
Figure 3.18
)
a much broader side of the DNA double helix, the so-called major groove, from
the less accessible minor groove.
The DNA double helix is stabilised both by hydrophobic interactions between the bases (base stacking) and by
hydrogen bonds between the A-T and G-C base pairs, and it can be reversibly dissociated into the individual
strands by heating. This process is termed melting, and the melting temperature (T
m
) is defined as the temperature
at which half the helical structure is lost. The importance of hydrogen bonding in stabilising the double helical
structure is underlined by the observation that G-C-rich DNA has a much higher T
m
than A-T-rich DNA (G-C base
pairs have three hydrogen bonds whereas A-T pairs have two).
While RNA molecules do not have the double-stranded structure usually found in DNA, many RNA molecules
have stem-loop structures in which the antiparallel strands are connected by a 5
e
7 residue loop. Rather like the
-turn in proteins, this allows the polynucleotide chain to change direction by 180
. However, in addition to the
classic base pairs (with U replacing T), a number of non-Watson
b
Crick base pairs are also found. This is
particularly well illustrated by the structure of the first RNA molecule to have its three-dimensional structure
determined
e
of this structure is the optimalisation of hydrogen-bonding interactions, many of them non-Watson
e
e
Crick, which
ensures that the molecule attains the maximum degree of hydrogen bonding between bases. It is interesting to note
that the anticodon(G
m
AA) which interacts with the mRNA to correctly position Phe for incorporation into the
appropriate protein is situated in the three-dimensional structure 8nm away from the Phe residue, bound to the
3
0
-OH of the tRNA (highlighted in red in the figure).
Whereas DNA is mostly located in the nucleus of cells in higher organisms (with some also in mitochondria
and in plant chloroplasts), RNA has a much broader cellular distribution. RNA comes in three major and distinct
forms, each of which plays a crucial role in protein biosynthesis in the ribosome, the intracellular organelle which
is the site of protein biosynthesis. Ribosomal RNA (rRNA) represents two-thirds of the mass of the ribosome,
messenger RNA (mRNA) encodes the information for the amino acid sequence of proteins, while transfer RNAs
(tRNAs) serve as adaptor molecules, allowing the four-letter code of nucleic acids to be translated into the
20-letter code of proteins. The tRNA molecules contain a substantial number of modified bases, which are
introduced by specific enzymes.
