Cambridge International A Level Biology Revision Guide

Chapter 6: Nucleic acids and protein synthesis
Chemical factories of the future?
All of us – and all other living organisms – have cells
that contain DNA. DNA is constructed from a chain
of smaller molecules called nucleotides, and the
sequence of the bases in these nucleotides acts as a
genetic code, determining the proteins that are made
in the cell and hence the organism’s characteristics.
The genetic code is universal. It is the same in all
organisms.
But recently, the genetic code of a bacterium,
Escherichia coli (Figure 6.1), has been deliberately
modified. One of the three-letter ‘words’ of its genetic
code, which originally told the bacterium’s ribosomes
to stop making a protein, has been changed to code
for an amino acid that is not found in nature. The new
code word now instructs the bacterium to insert the
unnatural amino acid into a protein.
These modified bacteria can be made to take up
different unnatural amino acids. This means that these
bacteria can be used to produce new proteins with
specific, unusual properties that could be of use to
us. The possibilities are almost endless. For example,
a completely new structural protein could be made
that is able to bind to a metal, which could be used to
build new structures. A new enzyme could be produced
that is only active in the presence of another molecule,
which could be used as a therapeutic drug to treat
human diseases.
The fact that the novel amino acids are not found
in nature means that these modified bacteria can only
make the new proteins in laboratory conditions, where
they are supplied with these amino acids. There is no
chance of them surviving if they were to escape into
the environment. Nevertheless, our ability to make
such fundamental changes to an organism is thoughtprovoking.
The chemistry of proteins will look very
different in the future.
Figure 6.1 False colour scanning electron micrograph of
Escherichia coli (× 1000).
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If you were asked to design a molecule which could act as
the genetic material in living things, where would you start?
One of the features of the ‘genetic molecule’ would
have to be the ability to carry instructions – a sort of
blueprint – for the construction and behaviour of cells and
the way in which they grow together to form a complete
living organism. Another would be the ability to be copied
perfectly, over and over again, so that whenever the nucleus
of a cell divides it can pass on an exact copy of each ‘genetic
molecule’ to the nuclei of each of its daughter cells.
Until the mid 1940s, biologists assumed that such a
molecule must be a protein. Only proteins were thought
to be complex enough to be able to carry the huge number
of instructions which would be necessary to make such a
complicated structure as a living organism. But during the
1940s and 1950s, a variety of evidence came to light that
proved beyond doubt that the genetic molecule was not a
protein at all, but DNA.
The structure of DNA and RNA
DNA stands for deoxyribonucleic acid, and RNA for
ribonucleic acid. As we saw in Chapter 2, nucleic acids
such as DNA and RNA, like proteins and polysaccharides,
are macromolecules (page 29). They are also polymers,
made up of many similar, smaller molecules joined into a
long chain. The smaller molecules from which DNA and
RNA molecules are made are nucleotides. DNA and RNA
are therefore polynucleotides. They are often referred to
simply as nucleic acids.
Nucleotides
Figure 6.2 shows the structure of nucleotides. Nucleotides
are made up of three smaller components. These are:
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■■
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a nitrogen-containing base
a pentose sugar
a phosphate group.