Download Studying Genomes

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DNA sequencing
DNA sequencing is the process of determining the order of
the nucleotide bases (A, T, G and C) in a molecule of DNA.
Before sequencing a piece of
DNA, it is important to know its
location in the chromosome or
plasmid it originated from.
The process used to determine
this is known as restriction
mapping.
DNA double helix
When analysing DNA, many copies are needed of the
relevant sections. These are created in a process called a
polymerase chain reaction (PCR).
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Producing multiple copies of DNA
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Locating genes
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The Sanger method
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PCR – the basic ingredients
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Studying genes – true or false
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Materials used in DNA sequencing
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Full genome sequencing
Full genome sequencing involves sequencing not only
nuclear DNA, but also the DNA contained within mitochondria
and chloroplasts.
With this vast quantity of
information, comparisons can
be made between individuals
of the same species and
between different species.
This gives us insights into
evolutionary relationships,
and differing responses to
medical treatments.
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Minisatellites
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Genetic fingerprinting
A genetic fingerprint is created by looking at an individual’s
minisatellites. The choice of minisatellite is very important:
the profiler needs to choose those minisatellites that show
the most variation, to reduce the chance that two individuals
could have matching fingerprints.
The main stages involved in genetic fingerprinting are:
1. Extraction
2. Digestion
3. Separation
4. Hybridization
5. Development
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Genetic fingerprinting
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Analysing genetic fingerprints
A profiler can inspect genetic fingerprints by eye to make quick
comparisons. This can be a useful tool in forensic science.
The process can also be automated with a computer using
the marker bands to calculate the size and distance travelled
by the bands in the profile.
It is sometimes necessary to
consider the odds that
somebody else in the population
has the same DNA fingerprint as
the one being studied. For
instance, to assess the risk of a
false criminal conviction.
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The Human Genome Project
The Human Genome Project (HGP) was an international,
publicly funded venture to sequence the three billion bases
in the 20,000–25,000 genes of the human genome.
The project ran from 1990 to 2003, when a first full
sequence was published. Corrections and further analyses
have been added to the results over the following years,
and are still being added today.
One of the subsidiary aims of the
HGP was to study the ethical and
social implications of the project,
and to predict and discuss the
legal issues that would arise.
Can you think of any examples?
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Designer drugs
One of the outcomes of the HGP is the development of new
drugs. Drugs can be designed using the knowledge of protein
structure, gained from the gene sequence information.
This reduces the need for trial
and error and allows doctors to
tailor specific treatments to their
patients, depending on the exact
nature of their condition.
Designer drugs can be used to
treat the fundamental causes
of a disease rather than just
the symptoms.
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How is a DNA profile created?
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Locating a specific gene
One approach to locating genes is to use a DNA probe.
This is a short section of DNA that has been labelled, for
example with radioactive phosphorous or a fluorescent marker.
The DNA is of a known sequence corresponding to the
gene being looked for, for example the cystic fibrosis
gene during clinical screening.
fluorescent
marker
DNA probe
target gene
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Using a gene probe
Before using a gene probe the DNA needs to be heated to
separate the two strands.
The temperature is then
reduced so that the probe
can ‘anneal’ or hybridize
with the sample DNA as a
result of complementary
base pairing.
hybridized probe
The location of the gene is then identified. The method
used depends on the method of labelling. Radioactive tags
are located by exposing the DNA to a photographic plate;
fluorescent tags are located by using UV light in a
fluorescent microscope.
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Cystic fibrosis
Cystic fibrosis is a heritable genetic disease that affects
the secretory glands. Its main effect is to increase mucus
production in the lungs. This mucus builds up, causing
inflammation, narrowing the airways and leading to
shortness of breath and an increased risk of infection.
The symptoms of cystic
fibrosis could potentially be
reduced by replacing the
defective gene that causes
it with a healthy version, in
the cells of the lungs where
it is expressed.
This approach to treatment is called gene therapy.
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Gene therapy
Gene therapy is a potential treatment for genetic diseases,
involving altering the genotype. The cause of the disease is
targeted, rather than just the symptoms.
There are two ways of altering
the genotype:
1. Replacing the defective gene
with a normal allele
2. Supplementing the gene by
adding copies of the normal allele,
which must be dominant, to mask
the effects of the defective
(recessive) allele.
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Gene therapy – types
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Carrying out gene therapy
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Genetic counselling
People seeking genetic
counselling may be
newly diagnosed patients,
new parents or couples
planning a pregnancy, or
relatives of a patient
concerned that they may
carry a disorder.
Genetic counsellors help people and their families faced
with the diagnosis of genetic disease. They can help them
to understand both the factual information about the
disease and the effect it will have on their lives, so that
they can reach their own decisions about the future.
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Understanding gene therapy
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Glossary
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What’s the keyword?
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Multiple-choice quiz
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