AQA A-Level Biology: DNA Probes and Genetic Screening

Differences in DNA between people can be used to find out whether someone carries a particular allele, for example one linked to a genetic disease. This guide explains the tools involved, DNA probes and gel electrophoresis, how they are combined to screen for specific alleles, and how the results are used through genetic counselling and personalised medicine. The central idea is using complementary base pairing to find one specific sequence in a whole genome.

DNA probes

A DNA probe is a short, single-stranded piece of DNA with a base sequence complementary to part of a target allele or region. It is usually labelled with a fluorescent or radioactive tag so it can be located.

Probes are longer than just a few bases for a good reason: a sequence of only a few bases would occur by chance at many places across the genome, whereas a longer sequence is only likely to occur in the target allele, so the probe binds specifically.

DNA hybridisation

DNA hybridisation is the binding of a single-stranded DNA probe to a complementary single strand of DNA, forming hydrogen bonds between the base pairs. This is how a probe attaches to its target sequence, and it is the step that makes the target detectable.

Gel electrophoresis

Gel electrophoresis separates fragments of DNA (or RNA), or proteins, according to their length (number of bases or amino acids) and their charge. DNA is negatively charged because of its phosphate groups.

• The DNA samples are loaded into wells in a porous gel, which is covered in a buffer solution that conducts electricity.
• An electric current is passed through the gel. Because DNA is negatively charged, it moves towards the positive electrode.
• Shorter fragments move through the gel faster, so they travel further.

To work out the sizes of unknown fragments, a standard of fragments of known length is run alongside them under the same conditions, and the positions are compared.

Locating a specific allele

DNA probes, electrophoresis and hybridisation are combined to screen for a specific allele, such as one that causes a genetic disease.

• The DNA is extracted and amplified by PCR.
• It is cut using restriction enzymes at specific sequences either side of the target region.
• The fragments are separated by gel electrophoresis.
• They are transferred to a nylon membrane and treated to make them single-stranded, exposing the bases.
• Labelled DNA probes are added, which hybridise with any target allele present, and the membrane is washed to remove unbound probe.
• The bound probe is then revealed, using UV light for a fluorescent probe or autoradiography (X-ray film) for a radioactive probe.

The same approach is used to screen for heritable conditions such as cystic fibrosis, to check how a patient will respond to a drug (some alleles code for enzymes that metabolise drugs), and to identify health risks such as a predisposition to high blood cholesterol.

Genetic counselling and personalised medicine

The results of screening have to be explained and acted on, which is the role of a genetic counsellor. A counsellor explains what the results mean and the consequences of a condition, discusses available treatments and lifestyle choices that might reduce the risk of it developing (such as regular screening for tumours), and explains the probability of passing an allele on to children, so that patients can make informed decisions.

Screening also enables personalised medicine, treatment tailored to an individual's genotype. For example, identifying the particular mutation causing a cancer allows it to be treated with drugs targeted at that mutation, increasing the effectiveness of treatment.

Screening is not without drawbacks, and these are weighed on both sides. In favour: it identifies carriers, lets people make lifestyle choices and informed decisions about having children, and enables more effective personalised medicine. Against: screening for incurable or late-onset diseases may cause depression or undue stress, results could lead to discrimination by insurers or employers, and testing for many rare or multi-gene conditions would need so many probes that it becomes expensive.

How this fits together

DNA probes and electrophoresis let us find one specific allele among millions of bases, which is the basis of medical screening. The same tools, applied to the repeating sequences scattered through non-coding DNA, can also identify an individual and their relationships, which is the subject of the genetic fingerprinting guide.