Which of the following is NOT a stop codon?

The correct answer is C, AUG, which is not a stop codon. Instead, AUG serves a critical role in the process of protein synthesis as it is recognized as the start codon. In the genetic code, AUG not only specifies the amino acid methionine but also signals the beginning of translation, where the ribosome assembles around the mRNA strand to start synthesizing a protein.

In contrast, the other options—UAA, UAG, and UGA—are all recognized as stop codons. Stop codons play a crucial role in the termination of protein synthesis. When any of these codons are encountered during translation, they signal the ribosome to stop adding amino acids, leading to the release of the complete polypeptide chain.

Understanding the function of start and stop codons is essential in molecular biology, particularly in the context of gene expression and protein synthesis.