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1 of 31 © Boardworks Ltd 2009 2 of 31 © Boardworks Ltd 2009 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). 3 of 31 © Boardworks Ltd 2009 Producing multiple copies of DNA 4 of 31 © Boardworks Ltd 2009 Locating genes 5 of 31 © Boardworks Ltd 2009 The Sanger method 6 of 31 © Boardworks Ltd 2009 PCR – the basic ingredients 7 of 31 © Boardworks Ltd 2009 Studying genes – true or false 8 of 31 © Boardworks Ltd 2009 Materials used in DNA sequencing 9 of 31 © Boardworks Ltd 2009 10 of 31 © Boardworks Ltd 2009 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. 11 of 31 © Boardworks Ltd 2009 Minisatellites 12 of 31 © Boardworks Ltd 2009 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 13 of 31 © Boardworks Ltd 2009 Genetic fingerprinting 14 of 31 © Boardworks Ltd 2009 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. 15 of 31 © Boardworks Ltd 2009 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? 16 of 31 © Boardworks Ltd 2009 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. 17 of 31 © Boardworks Ltd 2009 How is a DNA profile created? 18 of 31 © Boardworks Ltd 2009 19 of 31 © Boardworks Ltd 2009 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 20 of 31 © Boardworks Ltd 2009 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. 21 of 31 © Boardworks Ltd 2009 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. 22 of 31 © Boardworks Ltd 2009 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. 23 of 31 d D d D © Boardworks Ltd 2009 Gene therapy – types 24 of 31 © Boardworks Ltd 2009 Carrying out gene therapy 25 of 31 © Boardworks Ltd 2009 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. 26 of 31 © Boardworks Ltd 2009 Understanding gene therapy 27 of 31 © Boardworks Ltd 2009 28 of 31 © Boardworks Ltd 2009 Glossary 29 of 31 © Boardworks Ltd 2009 What’s the keyword? 30 of 31 © Boardworks Ltd 2009 Multiple-choice quiz 31 of 31 © Boardworks Ltd 2009