Learning Genetics (1)

Fast sequencing techniques enabled the complete decoding of the human genome, and many more model organisms. Scientists are using DNA microarrays to investigate untersuchen everything from cancer to pest control .

Why using a DNA Microarray Analysis? Microarrays are used for genome analysis, to aid diagnosis and for research on differential gene expression. DNA microarrays rapidly and very reliably determine the amount of mRNA from several thousand genes within specific healthy or diseased tissues. Scientists use DNA microarrays to simultaneously measure the expression levels of large numbers of genes or to genotype multiple regions of a genome.

What is a DNA Microarray? DNA microarray technology is based on the hybridization of nucleic acids. Single stranded DNA sequences bind to other sequences via hydrogen bonds between the purine and pyrimidin bases of complementary nucleotide bases. Thus, two single-stranded DNA chains form one double-stranded DNA chain. A higher number of complementary base pairs between two given nucleotide sequences will form tighter non-covalent bonding between the two strands.

During microarray experiments, scientists isolate short DNA sequences, oligonucleotides or cDNA. They represent small sections of genes or other DNA elements (the probes ) with known identities. These are immobilized on a solid support such as a coated glass slide, a silicon chip or a nylon membrane. This is called a DNA chip or a gene array . When the chip is coated with a complete genome, it is called a genome chip . Microarrays on silicon-based matrices are most commonly used. The array is comprised of spots: each spot contains just one DNA sequence. One chip may comprise several thousand spots (so each array consists of many thousands of probes, even up to 80,000). Each sample spot diameter is typically less than 200 microns.

The nucleic acid of interest can be obtained from various tissues. DNA may be used directly or RNA might be reversely transcribed into DNA using a PCR. The DNA of interest needs to be fluorescence labelled before application onto the DNA chip. The complementary sequences then hybridize and bind to each other. After rinsing away non-specific bonding sequences, only the strongly paired strands remain hybridized and bound to the chip. A fluorescence laser scanner finally determines the fluorescence intensity of each spot on the array slide. Since the strength of the signal depends upon the amount of DNA that binds to the probe, microarrays are used to quantify the amount of DNA bound to the probe. Thus, the amount of gene expression within a given tissue can be determined or pathological alterations in gene expression patterns can be diagnosed. The identity of the DNA is given by its position on the chip: thus, one can gain knowledge about the expression pattern and expression levels of thousands of genes with one single microarray experiment.

Which Types of Microarrays are used in Diagnosis? Microarrays can be used to rapidly and accurately detect many human diseases. With the help of diagnostic DNA microarray technology, altered gene expression, mutations or chromosomal dysfunctions can be identified. It helps to diagnose genetic disorders , to identify the expression of disease-relevant genes (biomarkers) or to diagnose bacterial, viral or fungal infections.

Diagnostic microarray experiments can be categorized into three types:

1. Microarray Gene Expression Profiling

Here you can detect the expression of certain genes that may indi-cate certain diseases. For this, the cDNA is derived from the mRNA of known genes and immobilized on the DNA chip. The sample contains genes from both normal and diseased tissues. Spots with higher intensities are obtained for a diseased tissue gene if the gene is over expressed in the diseased condition. This expression pattern from the patient’s tissue is compared to the expression pattern of a gene responsible for a disease. As a result, the deviating expression pattern may point to certain pathological conditions.

2. Microarray for Mutation Analysis

With this important application of microarray technology, one can perform gene mutation analyses to detect genomic alterations and, thereby, detect disease-causing or disease- predisposing mutations in the human genome. Genomic alterations (mutations) may be sequence or single nucleotide polymorphisms. For this analysis, the researchers use genomic DNA (gDNA). The genes might differ from each other by as little as a single nucleotide base. A single base difference between two sequences is known as Single Nucleotide Polymorphism (SNP) and detecting them is known as SNP detection.

3. Comparative Genomic Hybridization (CGH)

This is used for the identification of unbalanced chromosomal abnormalities, in which there is gain or loss of genetic (chromosomal) material. In other words: CGH screens the whole genome for copy number variations – relative to ploidy level in the DNA of a test sample compared to a reference sample. Many human disorders are caused from the increase or decrease of chromosomes or parts of chromosomes, and the microarray experiment is a versatile tool to detect these chromosomal aberrations .

EXERCISE: 1. What is the purpose of a DNA microarray? a) shows genes that are active in a cell b) to detect a virus infection c) to determine the vitality of a given cell

2. What is a DNA microarray? a) cloning b) short pieces of DNA are linked to a slide to bind target DNA fragments c) Western Blotting

3. DNA microarrays contain: a) double-stranded DNA b) single-stranded DNA c) triple-stranded DNA

You’ll find the answers on: http://tinyurl.com/weiterbildung-englisch

BIBLIOGRAPHY 1. http://dict.leo.org. 2. Knudsen S: Cancer Diagnostics with DNA Microarrays, Wiley Verlag, 2006. 3. www.cdc.gov. 4. www.dict.cc. 5. www.webster-dictionary.org.

Entnommen aus MTA Dialog 02/2017

Die Autorin:

Carmen Lobitz mail@lobitz-seminare.de www.lobitz-seminare.de Carmen Lobitz ist freie Trainerin, Lehrerin und MTA. Sie gründete 1998 in Berlin lobitz seminare, die u.a. Englischseminare für Health Professionals anbieten.

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Lösung für Exercise:

1: a, b

2: b

3: b

Learning Genetics (1)

Why using a DNA Microarray Analysis?

What is a DNA Microarray?

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Learning Genetics (1)

Why using a DNA Microarray Analysis?

What is a DNA Microarray?

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