3.8.4.2 Differences in DNA between individuals of the same species can be exploited for identification and diagnosis of heritable conditions

Often human diseases are the result of a gene mutation. Recombinant DNA technology has enabled us to diagnose and treat many of these genetic disorders. We need to know where a particular DNA sequence is located. To achieve this we use DNA probes and DNA hybridisation.

DNA probes
A DNA probe is a short single stranded length of DA that has a label attached to make it more identifiable. The most common probes are:

• radioactively labelled probes
• made up of nucleotides with the isotope 32p
• identified using an X-ray film that is exposed by radioactivity
• fluorescent labelled probes
• emit light under certain conditions (e.g when the probe has bound to the target DNA sequence)

DNA probes are used to identify particular alleles of genes in the following ways:

• a DNA probe is made that has base sequences that are complementary to part of the base sequence of the DNA that makes up the allele of the gene that we want to find
• the double stranded DNA that is being tested is treated to separate its two strands
• the separated DNA strands are mixed with the probe which binds to the complementary base sequence on one of the strands. This is known as DNA hybridisation.
• The site at which the probe binds can be identified by the radioactivity/fluorescence that the probe emits

DNA hybridisation

This takes pace when a section of DNA or RNA is combined with a single-stranded section of DNA which has complementary bases. Before this can take place the two DNA strands must be separated. This is achieved by heating the DNA until its double strands separate (denaturation). When cooled the complementary bases on each strand recombine (anneal) with each other to reform the original double strand.


Locating specific alleles
It is possible to locate a specific allele of a gene using DNA probes and DNA hybridisation. E.g to establish whether an individual possesses a mutant allele that causes a particular genetic disorder:

• we must first determine the sequence of nucleotide bases of the mutant allele we are trying to locate. This can be achieved using sequencing techniques/referring to the genetic library
• a fragment of DNA is produced that has a sequence of bases that are complementary to the mutant allele we are trying to locate
• multiple copies of our DNA probe are formed using PCR
• A DNA probe is made by attaching a marker (e.g a fluorescent dye) to the DNA fragment
• DNA from the person suspected of having the mutant allele we want to locate is heated to separate its two strands
• the separated strands are cooled in a mixture containing many of our DNA probes
• if the DNA contains the mutant allele one of our probes is likely to bind to it because the probe has base sequences that are exactly complementary to those on the mutant allele
• the DNA is washed clean of any unattached probes
• the remaining hybridised DNA will now be fluorescently labelled with the dye attached to the probe
• the dye is detected by shining light onto the fragments causing the dye to fluoresce. This can be seen using a special microscope

Genetic screening

If a mutation arises in a dominant allele all individuals will have the genetic disorder. it is important to screen individuals who may be carriers (heterozygous) of a mutant allele. Screening can determine the probabilities of couples having offspring with a genetic disorder. Genetic screening can also be valuable in the detection of oncogenes. Cancers may develop as a result of mutations that prevent the tumour suppressor genes inhibiting cell division.

Another advantage of genetic screening is personalised medicine. This allows doctors to provide health advice based on an individuals genotype. Furthermore, this can improve genetic counselling.