DNA probes

Both gene probes and genetic fingerprinting require gel electrophoresis. Gel electrophoresis is a form of chromatography used to separate different pieces of DNA on the basis of their size.  It uses a slab of agarose gel that is made up of many large, intertwined carbohydrate molecules. 

Gel electrophoresis equipment. (Diagram courtesy of Khan Academy)

1. The DNA samples are placed into wells at one end of the agarose gel and covered in a buffer solution. Fragments of DNA of known length (a ladder) are also placed into a separate well. 

2. An electric current is passed through the gel. Each nucleotide in a molecule of DNA contains a negatively-charged phosphate group, so DNA is attracted to the positive electrode and the DNA molecules move through the gel. Shorter lengths of DNA are more able to move through the agarose since they're less likely to be caught up in the 'web' of carbohydrate. Shorter lengths therefore move further over a given time. 

3. The DNA is then viewed by staining with either a coloured chemical, fluorescent molecule, radioactive isotope, or a DNA probe. The separated fragments are compared to the ladder of known sizes. 

Diagram courtesy of Khan Academy

DNA probes

Genetic screening using gene probes is based on DNA hybridisation (the binding of complementary, single-stranded DNA molecules) and is used to identify specific alleles. Short lengths of DNA, 100-1000 nucleotides long, are synthesised with base sequences found only in the mutant allele under study. These short lengths of DNA are called gene probes. These are amplified by PCR and then labelled either by radioactivity or by attaching a fluorescent tag. 

The genetic screen procedure is:

1. A few cells are taken from a patient by biopsy. 

2. DNA is extracted and digested using restriction endonucleases. If only a limited amount of DNA is available then the DNA may first be amplified by PCR prior to, or instead of, digestion.

3. The DNA is separated using gel electrophoresis and the DNA is transferred to a nylon membrane. 

4. The membrane is incubated with the probe. If the allele of interest is present then the probe hybridises with the DNA for the allele.

5. The membrane is then washed clean of any unbound probe to avoid false positive results.

6. The probe is detected by the radioactivity or by fluorescence.

This technique can be used to screen patients for inherited conditions or health risks. For example, genetic screening can help to identify defective alleles (such as those that cause cystic fibrosis) in parents looking to have children.

The results of screening can also be used in personalised medicine such as possible drug responses. Since different people have different combinations of alleles, even if they suffer from the same disease they may not have exactly the same genetic cause so people respond in different ways to different medicines. Personalised medicines are tailored to an individual’s genotype by using only those that are relevant to their particular combination of alleles.

Genetic counselling