Three Wise Men

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Recently I daily enjoyed watching CSI series on our local TV station. I am not into forensic science but I work in a lab for molecular biology and I know a thing or two about DNA sequencing. I would like to show you my perspective of the sequencing process that is simplified by the CSI series.

In school we learn about genetic material. In brief: chromosomes are composed of genes, genes are constituted of DNA and DNA is arranged in a long sequence of four different nucleotides: adenosine (A), thymine (T), guanine (G) and cytosine (C). The sequence of millions of this four nucleotide letters represents our genome. And here the real work begins. Read further if you are interested in how to obtain a sequence of particular gene, for example to determine the relationship between two persons.

First of all, forensics gather material and bring it to the lab: a hair with a root, some cells under nails, a piece of skin from scratch or a sample of blood. Such material contains cells with genetic material. Lab technician extracts genetic material from cells: more cells mean more genetic material and this leads to more accurate results. After acquiring genetic material appropriate gene or genes are amplified (augmented in quantity). After amplification, a so called PCR product is cleaned from salts and other unused reagents. The next step is amplifying the product in the way that four different dyes are incorporated: each nucleotide letter is represented with different color. Incorporated dye stops the amplification and various lengths of fragments are obtained. In a machine called sequencer fragments move through a special medium with the help of electric current. A laser detects colored fragments and camera records the sequence of detected dyes a.k.a. nucleotides. The result is a chromatogram with four colored curves representing a nucleotide sequence of specific gene.

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A lot of work is implicated to get printed result from a fresh specimen. Such labs occupy specially trained personnel and special lab equipment distribution: a separate room for extracting genetic material, room for amplification, for detection and the last one for sequencing. Trained personnel and separate rooms are needed to avoid any possible contamination of starting specimen or any of the subsequent steps.

However, apart from CSI series lab technician in real-life forensic science works only in a lab. He does not operate with criminals and does not wear a gun. Criminal acts are quickly solved only in TV series. In reality investigation proceeds  much slower.

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