Locating genes, genetic screening and counselling Flashcards
How do we determine where a particular gene is located
By using DNA probes and DNA hybridisation.
What is a DNA probe
A short, single-stranded length of DNA that has some sort of length attached that makes it easily identifiable.
What are the two most commonly used DNA probes
1) Radioactively labelled probes which are made up of nucleotides with the isotope 32P. The probe is identified using an X-ray film that is exposed by radioactivity.
2) Fluorescently labelled probes which emit light under certain conditions, for instance when the probe has bound to the target DNA sequence.
Describe how DNA probes are used to identify particular alleles of genes
- A DNA probe is made that has base sequences that are complementary to part of the base sequence of DNA that makes up the allele of the gene that we want to find.
- The double-stranded DNA that is being tested is treated to separate its two strands.
- The separated DNA strands are mixed with the probe, which binds to the complementary base sequence on one of the strands.
- This is known as DNA hybridisation.
- The site at which the probe binds can be identified by the radioactivity or fluorescence that the probe emits.
What is DNA hybridisation
DNA hybridisation takes place when a section of DNA or RNA is combined with a single-stranded section of DNA which has complementary bases.
What must happen before DNA hybridisation can take place and how is this undertaken
- The two strands of the DNA molecule must be separated.
- This is achieved by heating DNA until its double strand separates into its two complementary single strands.
- This is denaturation.
What happens during DNA hybridisation after the two strands of DNA have been separated by heating
- When cooled the complementary bases on each strand recombine (anneal) with each other to form the original strand.
- If other complementary sections of DNA are present (DNA probes), these are just as likely to anneal with the complementary strand.
Describe the steps in determining whether someone possesses a mutant allele that causes a particular genetic disorder
- First the sequence of nucleotide bases of the allele you are trying to locate must be determined.
- This can be achieved using DNA sequencing techniques, however, we now have extensive genetic libraries that store the base sequences of most genetic diseases so you can refer to these to obtain the sequence.
- A fragment of DNA is produced that has a sequence of bases that are complementary to the mutant allele you are trying to locate.
- Multiple copies of the DNA probe are formed using the polymerase chain reaction.
- A DNA probe is made by attaching a marker (fluorescent dye/isotope) to the DNA fragment.
- DNA from the person suspected of having the mutant allele we want to locate is heated to separate its two strands.
- The separated strands are cooled in a mixture containing many of our DNA probes.
- If the DNA contains the mutant allele, one of our probes is likely to bind to it because the probe has base sequences that are exactly complementary to those on the mutant allele.
- The DNA is then washed clean of any unattached probes.
- The remaining hybridised DNA will now be fluorescently labelled with the dye attached to the probe.
- The dye is detected by shining light onto the fragments causing the dye to fluoresce which can be seen using a special microscope.
What is one of the most common uses of genetic screening
- To determine if parents may be the carrier of a genetic disease.
- This allows them to get advice from a genetic counsellor about the implications of having children and work towards a safe solution.
Why is it possible to screen for many genetic disorders at once
- It is possible to fix hundreds of different DNA probes in an array on a glass slide.
- By adding a sample of DNA to the array, any complementary DNA sequences in the DNA will bind to one or more probes.
- In this way it is possible to test simultaneously for many genetic disorders by detecting fluorescence where binding has taken place.
What is another area of genetic screening (not testing for carriers of genetic disease) that is very useful
- Cancer screening.
- Genetic screening can detect oncogenes which are responsible for cancer.
- It can also detect mutations to tumour suppressor genes that may cause cancer by stopping this genes inhibiting cell division.
- mutations of both alleles must be present to inactivate the tumour suppressor genes and to initiate the development of a tumour.
- Genetic screening can help to detect these changes- it can also help people who inherit one mutated tumour suppressor gene and so are more likely to develop cancer.
Why is genetic screening important for personalised medicine
- genetic screening allows doctors to provide advice and care based on a persons genotype.
- For example, some genes may determine how effective as drug is at treating a patient.
Describe how genetic screening and personalised medicine can be useful in painkiller prescription
- To function properly, many pain medications need a specific enzyme to activate them.
- About half of the population have genes that alter the function of this enzyme.
- Screening for the presence of these genes allows the dosage to be adjusted to compensate for the ways in which the genes affect an individuals metabolism of the painkiller.
- This ensures that their use is both safe and effective.
What is an example involving vitamin E that related to genetic screening and personalised medicine
- It has been shown that among people with diabetes, vitamin E reduces the risk of cardiovascular disease for those with certain genotypes, but it can increase the risk for those with a different genotype.
- it is advantageous to screen a person with diabetes before advising on wether or not they should take vitamin E supplements.
What is genetic counselling
Genetic counselling is where advice and information are given that enable people to make personal decisions about themselves or their offspring.
