Nucleotides are named for the nitrogenous (nitrogen-containing) base incorporated into the nucleotide, and there are only five types of bases used: adenine, guanine, cytosine (all found in both DNA and RNA), and thymine (DNA only) or uracil (RNA only). The corresponding nucleosides are called deoxyadenosine (DNA)/adenosine (RNA), deoxyguanosine (DNA)/guanosine (RNA), deoxycytidine (DNA)/cytidine (RNA), deoxythymidine (DNA), and uridine (RNA). All of the genetic information in our body is stored in our DNA using sequences of only these four nucleotides. Imagine trying to describe how to make and operate an entire machine using only four letters—that’s just what our genetic information does.
DNA looks like a ladder that has been twisted. The rungs of the ladder consist of pairs of nucleotides, and the way they are arranged creates a genetic code of instructions that tell our body how to grow and function.