An exception to the statement that large organic molecules are usually assembled by polymerization of a few kinds of repeating simple subunits is RNA. RNA is a polymer, but it is made up of four different types of nucleotides, each with a different nitrogenous base. These nucleotides are assembled in a specific order to form RNA molecules, such as messenger RNA (mRNA), transfer RNA (tRNA), and ribosomal RNA (rRNA), Each type of RNA has a specific function in the cell