nature posts this
In order to understand how Sanger sequencing works, it's first necessary to understand the process of DNA replication as it exists in nature. DNA is a double-stranded, helical molecule composed of nucleotides, each of which contains a phosphate group, a sugar molecule, and a nitrogenous base. Because there are four naturally
occurringnitrogenous bases, there are four different types of DNA nucleotides: adenine (A), thymine (T), guanine (G), and cytosine (C).
In this context, does "occurring" mean "happening", "existing" or some other meaning?