Medical Diagnosis
Biology: AQA A Level, topic 8.4.6
DNA Probes
DNA probes are used to identify which alleles are present in a sample of DNA. This can be used in medical diagnosis by identifying if an individual has specific disease-causing alleles.
Probes
- A DNA probe is a section of single-stranded DNA.
- The probe is complementary to the DNA of the target allele (e.g. the allele that causes cystic fibrosis).
Hybridisation
- The complementary base sequence in a probe means that if the target allele is present in the DNA that is being sampled, it will bind to the DNA probe.
- When a base sequence binds to a complementary base sequence, this is called hybridisation.
Diagnosis
- DNA probes are labelled with a radioactive phosphate or fluorescent tag.
- If the disease-causing allele hybridises with the probe, this label will be detected.
- DNA probes are used in this way to diagnose diseases.
Diagnosis
Multiple labelled DNA probes can be used to diagnose the presence or absence of disease-causing alleles in a DNA sample. This can be done using two different techniques.
Electrophoresis
- Electrophoresis is a technique that separates fragments of DNA according to size. The sections of DNA are then transferred onto a nylon sheet.
- DNA probes labelled with fluorescent tags are 'washed over' the DNA fragments.
- The DNA probes hybridise with any fragments that are complementary to the probes.
Microarrays
- Microarrays use many DNA probes at once.
- A microarray is a slide with many indents. Each indent contains the DNA probes for a specific gene.
- The fluorescently labelled DNA sample is 'washed over' the microarray.
- Any DNA fragments that are complementary to the probes will hybridise.
Identifying DNA probes
- DNA probes that have hybridised to the DNA sample in either electrophoresis or a microarray can be identified.
- If the probes are labelled with fluorescent tags, they will be revealed by UV light.
- If the probes are labelled with radioactive phosphate, they will be revealed by X-ray.
- The DNA probes that are identified indicate which alleles are present in a sample of DNA.
Medical Screening
DNA probes in electrophoresis and microarrays can be used to screen an individual for a number of genetic diseases.
Information
- Genetic screening can be used to identify -
- If an individual is a carrier of a genetic disease.
- If an individual is at greater risk of developing a disease.
- How likely an individual is to respond to a particular drug.
Genetic counselling
- The information gained from genetic screening can be used in genetic counselling.
- Genetic counselling is used to advise patients about genetic screening.
- The advice considers -
- If screening is advisable.
- What the results of screening mean.
- How to prevent or treat a condition identified through screening.
E.g. BRCA1
- BRCA1 is a mutation that increases the likelihood of developing breast cancer.
- The mutation is more likely to be present in a woman who has a family history of breast cancer.
- Genetic counselling will help a woman decide whether to take a genetic screen. If the BRCA1 mutation is identified, genetic counselling can also help advise what she can do to prevent breast cancer.
Personalised medicine
- The information gained from genetic screening can also be used in personalised medicine.
- The genes in an individual influence how likely an individual is to respond to a particular drug.
- Genetic screening can help doctors prescribe the most effective drugs for their patient.
Test yourself on Medical Diagnosis
Which of the following statements about DNA probes are true?
What is the name of the process when two complementary base sequences bind together?
If DNA probes are labelled with fluorescent tags, how can they be identified?
What are the two techniques used to diagnose disease using DNA probes?
Which of the following statements about microarrays are true?