While "bases" is a common term to refer to the nitrogenous bases within a nucleotide, it's technically incorrect to call them "bases" when referring to the entire structure on the mRNA strand. The full structure is called a nucleotide and consists of three components:

* Nitrogenous base: Adenine (A), guanine (G), cytosine (C), or uracil (U)

* Sugar: Ribose

* Phosphate group

So, the nucleotides on an mRNA strand are made up of these three components.