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Gene Probe: Definition, Use & Example

Instructor: Amanda Robb

Amanda has taught high school Biology & Physics for 8 years. She received her M.Ed. from Simmon's College and M.S. from Tufts in Cellular and Molecular Physiology.

In this lesson we'll learn the basics about gene probes. We'll discuss what these molecular biology tools are and how they work to identify microorganisms in microbiology.

What Are Gene Probes?

How do you find a needle in a haystack? With millions of pieces of hay, all the same size and shape, what's the solution to this age-old riddle? What if you had a way to bring the needle to you instead of searching through the hay piece by piece? A very strong magnet would immediately draw out the needle, leaving all the hay alone.

Although we can't see DNA like we can see a stack of hay, identifying a specific piece in a mixed sample is just as hard as finding a needle in a haystack. All DNA has roughly the same size, chemical composition, and shape, so it can be incredibly hard to find the one tiny piece that you're looking for.

Luckily, scientists have applied the biochemical properties of DNA to create a molecular technique called a gene probe. Gene probes are small, single-stranded fragments of DNA that are complementary to a gene of interest. This means they are a perfect match, and when applied to a sample, a gene probe can help researchers identify if their gene of interest is there or not. Let's look at this process in detail.

How Do Gene Probes Work?

All DNA is made of two complementary strands that are a perfect match for each other, twisted together in a double helix. At high enough temperatures, DNA splits apart and the two strands separate. When the DNA cools, the complementary bases match back up and come back together. But in between this heating and cooling, researchers apply a gene probe to the mixture. The probe is a perfect match for the gene they are interested in. It also contains some sort of a tag, such as a fluorescent or colored marker so they can detect a match.

If the gene probe is a match for DNA in the sample, it will hybridize, or attach to the matching sequence when the DNA sample cools. Researchers wash away any unhybridized DNA and detect the tagged probe only if it has successfully hybridized in their sample.

Gene probes can be used to detect a target DNA sequence in a mixture
gene probe

Applications

Gene probes are one of the best ways to identify a particular DNA sequence in a complex mixture. But why would researchers want to know what genes are present? Genes are the instructions for all living things; they're what makes us a human versus any other type of living thing. Some living things are so small that it is impossible to identify them with the naked eye, and DNA is one of the best ways to figure out what organisms are present.

Microorganisms are microscopic living things, some of which can cause disease in humans called pathogens. Gene probes can be designed to detect the presence of specific pathogens in a particular sample. Typically, to identify the presence of a pathogen in a sample, the sample needs to be cultured, or grown in a lab. For some slow-growing pathogens this can take weeks, which is not an ideal situation if the sample comes from a sick patient that needs treatment.

DNA probes offer a rapid alternative to culturing samples in a lab. Additionally, they are highly specific. DNA contains the instructions for all living things and each is unique. Thus, probes are used to specifically identify microorganisms compared to the traditional methods of identification through biochemical and morphological tests in culture.

Examples

Next, let's look at a few examples of DNA probes identify microorganisms.

Mycobacterium tuberculosis

Josh is a patient living with human immunodeficiency virus (HIV). Lately, he has developed a cough and is running a high fever. Since his immune system is dampened by HIV, he is worried the infection might be serious, so he makes an appointment with his doctor. The doctor suspects he might have an infection with the bacteria Mycobacterium tuberculosis, which causes tuberculosis, or TB.

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