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 occurring nitrogenous 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?


Naturally occurring means they exist naturally, although it may be possible to create other nitrogenous bases in a laboratory (I'm not a chemist) we would expect to only find the four mentioned nucleotides outside of an artificial environment.

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