DNA sequencer

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A DNA sequencer is a scientific instrument used to automate the DNA sequencing process. It can be also considered an optical instrument as it generally analyses light signals originating from fluorochromes attached to nucleotides.

DNA sequencers have become more important due to large genomics projects and the need to increase productivity.

Modern automated DNA sequencing instruments (called DNA sequencers) are able to sequence multiple samples in a batch (run) and perform as many as 24 runs a day. These perform only the size separation and peak reading; the actual sequencing reaction(s), cleanup and resuspension in a suitable buffer must be performed separately.

The magnitude of the fluorescent signal is related to the number of strands of DNA that are in the reaction. If the initial amount of DNA is small, the signals will be weak. However, the properties of PCR allow one to increase the signal by increasing the number of cycles in the PCR program.

A simple DNA sequencer will have one or more lasers that emit at a wavelength that is absorbed by the fluorescent dye that has been attached to the DNA strand of interest. It will then have one or more optical detectors that can detect at the wavelength that the dye fluoresces at. The presence or absence of a strand of DNA is then detected by monitoring the output of the detector. Since shorter strands of DNA move through the gel matrix faster they are detected sooner and there is then a direct correlation between length of DNA strand and time at the detector. This relationship is then used to determine the actual DNA sequence.

The output of these machines is not perfect as it may contain reading errors and needs to be processed (see Sequence assembly). Until a few years ago, this task was done manually by an operator. The assembly process for a contig made of 10 samples took about 5–15 minutes, depending on the quality of the samples. However, today, modern software can automatically process the output in seconds.

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