The Classes of RNARNA molecules fall into one of three classes: Ribosomal RNA (rRNA), transfer RNA (tRNA) and messenger RNA (mRNA). rRNA and tRNA are involved in the process of translation, and we will discuss them in more detail later. In transcription, mRNA is made and carries the instructions for building proteins.
Within the DNA are discrete regions, called genes, which instruct the cell how to make proteins. It is the expression of these regions, which begins with the process of transcription, that determines the functions of cells. When genes are turned on or off they regulate the function of the cell, and, ultimately, the organism.