The control of gene expression (AQA A2 Biology) PART 7 of 8 TOPICS
Gene technologies allow the study and alteration of gene function allowing a better understanding of organism function and the design of new industrial and medical processes – differences in DNA between individuals of the same species can be exploited for identification and diagnosis of heritable conditions:
Gel electrophoresis is where DNA fragments of desired genes are separated by size. This is how it’s done:
- A gel with wells along one edge is placed in a tank full of buffer where each of the wells has the DNA fragments of the desired gene. These fragments have been cut out by restriction enzymes and have got a marker on them either carbon-13 or fluorescence e.t.c.
- The buffer is used to dissolve the DNA fragments. NB: You do not need to know the buffer that is involved in this process.
- The edge that has the wells full of the DNA fragments is towards the negative terminal making the other towards the positive terminal. This because DNA fragments are negatively charged because of the phosphate group.
- The circuit is switched on.
- The DNA fragments then move to the positive terminal at different speeds due to their size where the largest DNA fragments being more towards the negative terminal and the smallest ones being more towards the positive terminal.
- UV-light is shone over the gel to allocate where the DNA fragments are on the gel if a marker for fluorescence has been used. Carbon-13 will go black in the presence of light.
- The DNA fragments are then transferred on to a nylon membrane using a weight so that it can be stored forever as the gel can dry out. NB: The name of this process is called Southern transfer, which is named after Edwin Southern, does not need to be known for the exam.
DNA probes are used to identify which of the DNA fragments has the desired gene as they have complementary bases:
- DNA hybridisation:
- Gel electrophoresis takes place.
- As DNA probes are single stranded, the DNA molecules have to be separated by breaking the hydrogen bonds. This is done by heating the DNA fragments to 95 degrees to break the hydrogen bonds. Other techniques can be used to break hydrogen bonds e.g. using DNA helicase or soaking the gel in alkaline solutions.
- The DNA probes with a marker, e.g. fluorescence, are added and are washed away to get rid of any unbound DNA probes.
- Any DNA probes that have bounded will glow under UV-light shows that the DNA fragments that are complementary to the DNA probes have the gene that you are looking for. This is a good technique to use for when you are looking for a mutation in a gene that causes disease.
- DNA microarray:
- This involves using a glass slide with different DNA probes on it.
- A DNA sample is added to the slide where only one type of DNA probe is complementary to the desired gene on the fragment. The fragments have a marker on them e.g. fluorescence.
- The slide is then washed to remove any unbound DNA fragments.
- The parts of the slide that fluoresce under the presence of UV-light shows that the desired gene is present. This technique can screen for more than one type of gene at the same time.
Genetic counselling is the giving of advice and information about the risks of genetic disease and its outcomes. Counselling is a very challenging task. Counsellors must have adequate knowledge and understanding about the topic and need to be well trained in sympathetic counselling techniques. They must help clients come to terms of making their own decisions rather than imposing their own views on them. They should be made aware that the features of a certain genetic disease varies widely. It should also be made very clear of the sorts of support that are available to the child with the genetic disorder and to the parents/guardians as well as family.