Cambridge International A Level Biology Revision Guide

Chapter 19: Genetic technology
You can see that theoretically this could go on forever,
making more and more copies of what might originally
have been just a tiny number of DNA molecules. A single
DNA molecule can be used to produce literally billions
of copies of itself in just a few hours. PCR has made it
possible to get enough DNA from a tiny sample – for
example, a microscopic portion of a drop of blood left at a
crime scene.
Taq polymerase was the first heat-stable DNA
polymerase to be used in PCR. It was isolated from the
thermophilic bacterium, Thermus aquaticus, which is
found in hot springs in Yellowstone Park in the USA
(Figure 3.10, page 60). It is valuable for PCR for two
reasons. First, is not destroyed by the denaturation step, so
it does not have to be replaced during each cycle. Second,
its high optimum temperature means that the temperature
for the elongation step does not have to be dropped below
that of the annealing process, so efficiency is maximised.
PCR is now routinely used in forensic science to
amplify DNA from the smallest tissue samples left at the
scene of a crime. Many crimes have been solved with
the help of PCR together with analysis of DNA using gel
electrophoresis.
QUESTION
19.7 a How many molecules of DNA are produced
from one double-stranded starting molecule,
after eight cycles of PCR?
b Explain why it is not possible to use PCR to
increase the number of RNA molecules in the
same way as it is used to increase the number
of DNA molecules.
Microarrays
Microarrays have proved a valuable tool to identify the
genes present in an organism’s genome and to find out
which genes are expressed within cells. They have allowed
researchers to study very large numbers of genes in a short
period of time, increasing the information available.
A microarray is based on a small piece of glass or
plastic usually 2 cm 2 (Figure 19.14). Short lengths of singlestranded
DNA are attached to this support in a regular
two-dimensional pattern, with 10 000 or more different
positions per cm 2 . Each individual position has multiple
copies of the same DNA probe. It is possible to search
databases to find DNA probes for a huge range of genes.
Having selected the gene probes required, an automated
process applies those probes to the positions on
the microarray.
Figure 19.14 A microarray, also known as a DNA chip.
When microarrays are used to analyse genomic
DNA, the probes are from known locations across the
chromosomes of the organism involved and are 500 or
more base pairs in length. A single microarray can even
hold probes from the entire human genome.
Microarrays can be used to compare the genes present
in two different species. DNA is collected from each
species and cut up into fragments and denatured to give
lengths of single-stranded DNA. The DNA is labelled with
fluorescent tags so that – for example – DNA from one
species may be labelled with green tags and DNA from
the other species labelled with red tags. The labelled DNA
samples are mixed together and allowed to hybridise
with the probes on the microarray. Any DNA that does
not bind to probes on the microarray is washed off. The
microarray is then inspected using ultraviolet light,
which causes the tags to fluoresce. Where this happens,
we know that hybridisation has taken place because the
DNA fragments are complementary to the probes. Green
and red fluorescent spots indicate where DNA from one
species only has hybridised with the probes. Where DNA
from both species hybridise with a probe, a yellow colour
is seen. Yellow spots indicate that the two species have
DNA with exactly the same base sequence. This suggests
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