There are four naturally occurring nitrogenous bases, there are four different types of DNA nucleotides: adenine (A), thymine (T), guanine (G), and cytosine (C). This sequence is broken into a series of three-nucleotide units known as codons. Each DNA sequence that contains instructions to make a protein is known as a gene.
Sequencing DNA means determining the order of the four chemical building blocks - called "bases" - that make up the DNA molecule. The sequence tells scientists the kind of genetic information that is carried in a particular DNA segment.
DNA sequencing was built upon earlier knowledge of DNA polymerases and cell-free systems for replicating DNA. The chain-termination method, which makes clever use of a "defective" DNA nucleotide, now dominates DNA sequencing technology.
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