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DNA probes, microarrays and genetic fingerprinting

Locating Genes

Techniques used to identify gene location for a variety of purposes include DNA hybridisation and DNA probes. These involve tagging a DNA strand with a fluorescent agent and binding it to a new strand, the probe, which is complementary to the target gene. The new double-stranded DNA is a hybrid.

The extent of binding to the probe (and hence the complementarity between base pairs) is measured by the intensity of fluorescence seen under UV light. This technique can be used to reveal the presence of heritable conditionsdrug responses or health risks. You can get this done today!

Therefore, differences in nucleotide sequences can be used to compare DNA samples. Specific sequences used in comparisons such as illness predisposition and genetic similarity between relatives include microsatellite repeat sequences (MRSs) and single nucleotide polymorphisms (SNPs). SNPs, as their name suggests, are differences of just one DNA base between specific sequences. Closely related individuals are more likely to have the same SNPs. MRSs on the other hand are non-coding DNA sequences of multiple bases, repeated 5-50 times, that vary accordingly between organisms more or less closely related.

Microarray technology

I got some of my DNA screened for several select markers, including for Alzheimer's disease and Parkinson's, as well as many inherited conditions.

Obtaining this data is possible through microarray technology. This allows the analysis of vast amounts of data, like the data present in a human genome, by focusing on specific, relevant sequences. These DNA sequences are added to a microarray chip, and the extent of hybridisation is digitally recorded. The fluorescent or chemiluminescent signal represents the presence of a sequence in that individual.

Microarrays can also be used to investigate gene expression. Since this can reveal the extent of gene expression, it's not merely a test for the presence of a sequence, but rather a test for the active metabolism of a sequence. This is possible because the mRNA rather than the direct DNA present in a cell is extracted, and then the reverse sequence, the cDNA, is produced from it.

Genetic fingerprinting technique

1. The sample DNA undergoes PCR then cleavage at multiple sites with restriction endonucleases (enzymes that cut DNA at specific sequence locations)

2. The resulting many small fragments are tagged using a radioactive molecule

Visualising DNA with gel electrophoresis

A common method of visualising differences is gel electrophoresis which involves loading small volumes of samples on a gel and running a current across it in order to separate the samples by size.

Since the gel has a microscopic matrix inside that provides resistance against sample movement through it, the larger molecules move more slowly while the smaller fragments can move more quickly.

The positive charge is at the bottom of the tank, while the samples are loaded at the top. This way, they will move downwards towards the bottom of the gel because they have a negative charge as molecules. The current is run across the gel for around 30-60 minutes (ensuring the samples don't run too long and hence run off the gel into the buffer solution! if that happens they are lost) after which the sample's progression on the gel can be visualised.

3. They're viewed using a developed photographic film

The bands exposed then undergo simple visual analysis by matching up the template DNA with other DNA that could be similar more or less, depending on situation. Above, the DNA found at a crime scene is compared with that of 3 suspects. The bands of suspect 2 are perfectly aligned with the crime scene DNA.

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