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DNA sequencers are used to automate the DNA sequencing process. Deoxyribonucleic acid (DNA) is a long, linear chain found in the nucleus of cells. Shaped like a double helix and formed from nucleotides, DNA contains genetic instructions. This nucleic acid is also associated with the transmission of genetic information in all organisms and some viruses. DNA sequencers are used mainly in education and biomedical research, as well as for disease treatment and disease prevention. As scientific instruments, they are used in genomics projects and with genetic research. Some products are also considered to be optical instruments. Manual DNA sequencers incorporate radioactive isotopes of phosphorous and sulfur into adenine (A), thymine (T), guanine (G), and cytosine (C) nucleotides. Autoradiography is used to determine the sequence of these basic structural units. When the gel matrix that contains the DNA strands is exposed to X-ray film, causes dark spots to emerge. When the film is developed, the spots indicate patterns. Because this DNA sequencing process cannot distinguish the different types of nucleotides, however, each labeled nucleotide requires a separate lane. Consequently, manual DNA sequencing requires a total of four lanes. Single lane DNA sequencers and unidirectional sequencing are characteristic of manual DNA sequencing processes. Automated DNA sequencers label DNA nucleotides with fluorescent dye instead of radioactive isotopes. Whereas manual sequencing requires X-ray film, automatic DNA sequencing uses a laser to induce fluorescence. Subsequently, fluorescent emissions are collected on a charge coupled device (CCD), which transforms the light pattern into an electric charge pattern. In this way, the presence or absence of a DNA strand is detected by monitoring the detector’s output. Because shorter strands move through the gel matrix more quickly, they are detected earlier. Thus, there is a direct correlation between the length of a DNA strand and its time at the detector. Automatic DNA sequencers rely upon the relationship between these two variables to determine the actual DNA sequence. DNA sequencers may use either the Maxam-Gilbert method or the Sanger di-deoxy method. Maxam-Gilbert sequencing, which is known also as "the chemical method" or "chemical sequencing", relies upon the chemical modification of DNA and subsequent cleavage at specific bases. The Sanger di-deoxy method, as initially defined, required the cloning of the beginning of each reading for single-stranded DNA. Although the Maxam-Gilbert method is newer than the Sanger method, the technique developed by geneticists Allan Maxam and Walter Gilbert is used less commonly because of its complexity, cost, and reliance upon hazardous chemicals.