Finding the gene you want
Every gene contains a unique sequence of the four bases adenine (A), cytosine (C), guanine (G) and thymine (T). We can test to see if a specific gene is present in a person’s genetic make-up by searching for its unique base sequence.
The search uses a single-stranded piece of DNA called a gene probe. The design of a probe uses the fact that when DNA strands pair up, A only pairs with T and C only pairs with G. The base sequence of the probe matches the unique sequence in the gene that the probe is designed for.
To test a DNA sample using a gene probe, the DNA is first treated so that each of the double-stranded DNA molecules unzips into single strands. The probe is then added to the solution. Because of the way the bases pair up, the probe will attach itself only to the section of DNA that contains a base sequence that matches the probe’s sequence.
Probes are constructed with a radioactive or a fluorescent section, or tag, in them, so that they can be detected after attaching to the DNA.
Detecting the probe gives us information about which chromosome the gene is on, and where the gene is on that chromosome.
We know the base sequences in a number of disease-causing genes, and can find out if they are present using probes specifically designed for them.
At any one time in a cell, some genes are switched 'on', or active, while others are switched 'off', or silent. Active genes express mRNA (messenger RNA copies of the gene that are used by the cell to make proteins).
Microarrays can be used to find out which genes are expressed at any one time. Microarrays are sets of miniaturised chemical reactions arranged on a small glass, filter, or silicon wafer. They can be used to test DNA fragments, antibodies, or proteins. A DNA microarray can record the level of expression for every gene within a particular sample.
Microarrays can assist us by finding:
- drugs that interact with a gene of interest
- individuals with similar biological patterns
- the most appropriate individuals for participating in clinical trials of new drugs
Scientists can use microarray technology to determine the function of genes and get a clearer idea of what is happening inside cells when things go wrong, as happens in disease.