GCAU refers to the four nucleotides found in RNA: guanine (G), cytosine (C), adenine (A), and uracil (U). These nucleotides are the building blocks of RNA molecules, playing crucial roles in coding, decoding, regulation, and expression of genes. In contrast to DNA, which contains thymine instead of uracil, RNA's unique structure allows it to perform various functions in the cell, including serving as a messenger and a catalyst.