A gene probe is a short, single-stranded piece of DNA or RNA with a base sequence complementary to a particular gene or allele. It is labelled with a radioactive isotope or a fluorescent tag so that it can be detected. When a probe meets its target, it binds (hybridises) by complementary base pairing.Detecting alleles
Probes can be designed to bind only to one allele, such as a mutant allele that causes a genetic disorder. A person's DNA is denatured to single strands and the probe is added. If the probe binds, and is detected after washing off unbound probe, the person has that allele. This is used in genetic screening, for example to identify carriers of cystic fibrosis.Finding which genes are expressed
A gene is being expressed if its mRNA is present in the cell. Probes can therefore show which genes are switched on:
• mRNA is extracted from the cells and copied into single-stranded DNA called complementary DNA (cDNA) using the enzyme reverse transcriptase. The cDNA is labelled with a fluorescent tag.
• In a DNA microarray, thousands of different probes are fixed in spots on a small glass slide. The labelled cDNA is washed over the slide and binds to the probes it is complementary to.
• Spots that glow under a scanner show which genes were being expressed, and the brightness shows how much mRNA was present.
Microarrays can compare gene expression in healthy and diseased cells, such as normal and cancer cells, or show how gene expression changes during development.
Probes can be designed to bind only to one allele, such as a mutant allele that causes a genetic disorder. A person's DNA is denatured to single strands and the probe is added. If the probe binds, and is detected after washing off unbound probe, the person has that allele. This is used in genetic screening, for example to identify carriers of cystic fibrosis.Finding which genes are expressed
A gene is being expressed if its mRNA is present in the cell. Probes can therefore show which genes are switched on:
• mRNA is extracted from the cells and copied into single-stranded DNA called complementary DNA (cDNA) using the enzyme reverse transcriptase. The cDNA is labelled with a fluorescent tag.
• In a DNA microarray, thousands of different probes are fixed in spots on a small glass slide. The labelled cDNA is washed over the slide and binds to the probes it is complementary to.
• Spots that glow under a scanner show which genes were being expressed, and the brightness shows how much mRNA was present.
Microarrays can compare gene expression in healthy and diseased cells, such as normal and cancer cells, or show how gene expression changes during development.