What is the start codon for protein synthesis in eukaryotic cells?

The start codon for protein synthesis in eukaryotic cells is AUG. This codon is significant because it is the first codon read during the initiation of translation, and it codes for the amino acid methionine. In eukaryotic cells, the presence of the AUG codon indicates where the ribosomes should begin assembling and translating mRNA into a polypeptide chain. This is a critical step in gene expression, as the proper initiation ensures that the entire protein is synthesized correctly.

The other codons listed do not function as start codons. UAC codes for the amino acid tyrosine, while GCC and CAG correspond to glycine and glutamine, respectively. These codons come into play during the elongation phase of protein synthesis but do not initiate the process. Thus, the importance of AUG lies in its exclusive role as the start signal in translation.