Topic 8—B: Genome Projects and Gene Technologies- 6. Gene probes and Medical Diagnosis

Question 1

To produce a DNA probe, what do you first need to do?

Answer 1

• Sequence the allele that you want to screen for
• You then use PCR to produce multiple complimentary copies of part of the allele- these are the probes

Question 2

How can DNA probes be used?

Answer 2

• To locate specific alleles of genes (e.g. On chromosomes) or to see if a persons DNA contains a mutated allele that causes a genetic disorder

Question 3

What are DNA probes?

Answer 3

• They are short strands of DNA
• They have a specific base sequence thats complimentary to the base sequence of part of a target allele (the allele your looking for e.g. a allele that causes a genetic disorder)
• this means a DNA probe will bind (hybridise) to the target allele if its present in a sample of DNA

Question 4

Why does a DNA proble also has a label attached?

Answer 4

• so that it can be detected

Question 5

What are the 2 most common labels?

Answer 5

• radioactive label (detected using x-ray film)
• fluorescent label (detected using UV light)

Question 6

How are fluorescently labelled probes used?

Answer 6

1. A sample of DNA is digested into fragments using restriction enzymes and separated using electrophoresis
2. The separated DNA fragments are then transferred to a nylon membrane and incubated with a fluoresently labelled DNA probe.
   - if the allele is present, the DNA probe will bind (hybridise) to it
3. The membrane is then exposed to UV light and if the gene is present there will be a fluorescent band.

Question 7

What can the probe be used as part of ?

Answer 7

• A DNA microarray which can screen for lots of different genes at the same time

Question 8

What is DNA microarray?

Answer 8

• a glass slide with microscopic spots of different DNA probes attached to it in rows

Question 9

Screening for multiple genes

Answer 9

• A sample of fluorescently labelled human DNA is washed over the array
• If the labelled human DNA contains any DNA sequences that match any of the probes it will stick to the array
• So this means you can screen the DNA for lots of different mutated genes at the same time
• The array is washed to remove any fluorescently labelled DNA that hasn’t stuck to it and then visualised under the UV light
• any labelled DNA attached to a probe will show up (fluoresce)
• Any spot that fluoresces means that the persons DNA contains that specific allele
• e.g. If the probe is for a mutated allele that causes a genetic disorder, this person has the allele.

Question 10

Uses of screening with DNA probes

Answer 10

• It can be used to help identify inherited conditions e.g. Huntington’s disease is an inherited condition that affects the nervous system and does not usually start to display symptoms until a person is aged between 30 and 50. People with a family history of the disease may choose to be screened for the mutated allele to find out if they have inherited it.
• It can be used to help determine how a patient will respond to specific drugs e.g. breast cancer can be caused by a mutation in the.HER2 Proto-oncogene and treated with the drug heceptin. Herceptin is only affective against this type of breast cancer because it targets a specific receptor. Screening for this particular mutation helps determine whether herceptin will be a useful treatment or not.
• It can also be used to help identify health risks e.g. Inheriting particular mutated alleles increases your risk of developing certain types of cancer. If a person knows they have these alleles it might help them make choices that could reduce the risk of the disease developing.

Question 11

What are people concerned that genetic screening may lead to?

Answer 11

• Discrimination by insurance companies and employers if people are known to have a high risk of developing a condition

Question 12

What can the results of screening be used for?

Answer 12

Genetic counselling

Question 13

What is genetic counselling?

Answer 13

• It is advising patients and their relatives about the risks of genetic disorders
• It involves advising people about screening (e.g. Looking for mutated alleles if there’s a family history of cancer.) and explaining the results of screening.

Question 14

What can screening help to identify?

Answer 14

• If someone is the carrier of a mutated allele
• the type of mutated allele their carrying (indicating the type of genetic disorder or cancer)
• and the most effective treatment

Question 15

if the results of a screening are positive. (an individual has the mutation.) what is the genetic counselling used to advise?

Answer 15

• The patient on the options of prevention or treatment available

Question 16

Personalised medicine

Answer 16

• The results of screening can also be used in personalised medicine
• Your genes determine how your body responds to certain drugs
• Different people respond to the same drug in different ways- which makes certain drugs more effective for some people than others
• This is where personalised medicines come in
• Personalised medicines are medicines that are tailored to an individuals DNA
• The theory is that if doctors have your genetic information they can use it to predict how you will respond to different drugs and only prescribe the ones that will be most effective for you