A DNA sequencer is a laboratory instrument that reads the letters of DNA in a sample, millions of fragments at once. DNA is the molecule that carries genes. It is a long chain of four chemical letters, written A, C, G and T. The order of those letters is called the sequence. Reading it is called sequencing, and the study of whole sets of genes is called genomics.
The most common kind of sequencer today reads short fragments. Here is how it works. The laboratory first breaks the DNA into fragments a few hundred letters long. It attaches short tags to each fragment, so that the machine can hold it and so that samples can be told apart. This step is called library preparation. The fragments are spread on a glass slide called a flow cell. Each fragment is copied many times on the spot where it lands, so that it gives a signal strong enough to see. Then the machine adds one letter at a time to every fragment. Each letter carries a coloured dye. A camera photographs the whole slide after each step. The colour on each spot tells the machine which letter was added there. After 150 steps, every spot has given a 150-letter read.
Consider a cancer hospital laboratory in Mumbai that is testing a lung tumour sample. The doctor wants to know if the tumour has a change in the EGFR gene, because some medicines work only on such tumours. The laboratory's laboratory information system (LIS) logs the order. The sequencer reads the tumour's DNA overnight. Software lines up the millions of reads against a standard human sequence, called the reference genome. In the EGFR gene, 18% of the reads show the same changed letter. The result is a known mutation. The report goes back to the doctor, who chooses a matching medicine. This kind of test is described in tumour genomic profiling.
Some sequencers read much longer fragments, thousands of letters each, using other methods. Long reads make it easier to see large changes in a gene, but each read has more errors.
A sequencer produces huge files. One run can produce hundreds of gigabytes of data. So the reads are usually cleaned and stored on a bioinformatics data platform. India's own genome projects, such as GenomeIndia, have used these machines to read the genes of thousands of Indians, so that doctors can learn which gene changes are common in India.