Differences in DNA between individuals of the same species can be exploited for identification and diagnosis of heritable conditions (A-level only)

Question 1

DNA probes

Answer 1

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).

Question 2

Hybridisation

Answer 2

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.

Question 3

Diagnosis

Answer 3

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.

Question 4

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:

Answer 4

Electrophoresis
Microarrays
Identifying DNA probes

Question 5

Electrophoresis

Answer 5

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.

Question 6

Microarrays

Answer 6

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.

Question 7

Identifying DNA probes

Answer 7

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.

Question 8

Uses of genetic screening

Answer 8

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.

Question 9

Genetic councelling

Answer 9

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.

Question 10

E.g. BRCA1

Answer 10

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.

Question 11

Personalised medicine

Answer 11

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.