DNA sequencing is the process by which the precise order of nucleotides within DNA is determined. Nucleotide bases pair according to specific rules: adenine (A) always pairs with thymine (T) or uracil (U), and cytosine (C) always pairs with guanine (G); this is called complementary base pairing. By using the complementary base pairing rule of nucleic acids, scientists have sequenced an organism's entire set of DNA, or genome.
To learn how to sequence an organism's genome, scientists began with a virus. They first determined the exact nucleotide sequence of the virus by cutting up its DNA into pieces, and then finding the sequence of each piece. They used the fact that adenine will always pair with thymine and guanine with cytosine to determine nucleotide sequences. Techniques often involve the use of modified nucleotides that can be detected during or after binding. By recording the order of the nucleotides that are bound to the unknown DNA strand, researchers can deduce the nucleotide sequence of the original piece of DNA. Shortly after sequencing the first virus in 1977, a yeast became the first eukaryotic (an organism with a nucleus and whose DNA is bound together by proteins) genome to be sequenced. Initially, the process of sequencing was labor-intensive because much of the work of scientists had to be done by hand and thus was time consuming. However, many advances in sequencing techniques have emerged, including the use of machines to sequence thousands of pieces of DNA at the same time. As the process becomes faster and cheaper, large quantities of genetic information are being collected and made available to researchers. Keeping track and making sense of vast quantities of sequence data is one practical application of the field of bioinformatics, which uses computers and mathematics to store and analyze biological data. These data have countless uses, including finding genetic markers for diseases, determining patterns that might not otherwise be visible or clear, and testing for genetic diseases to improve and individualize medical treatments.