What does the start codon for eukaryotes specify?

The start codon for eukaryotes is specifically the codon AUG, which codes for the amino acid methionine. This is a universal feature in the molecular biology of eukaryotic organisms. The presence of methionine as the first amino acid in protein synthesis is critical for several reasons. Firstly, methionine plays an essential role in the initiation of translation, as it is recognized by the initiator tRNA, which binds to the ribosome at the start codon to begin synthesizing the protein.

Moreover, the fact that methionine is incorporated at the N-terminus of nascent polypeptides forms a basis for eukaryotic protein synthesis and contributes to protein targeting and functionality. This remarkable consistency in the use of methionine across different life forms emphasizes its fundamental role in the biology of all eukaryotic organisms.

Other options correspond to different amino acids that are not coded by the start codon, providing non-initial amino acids that can occur later in the protein sequence after elongation has begun. Understanding this start codon and the significance of methionine is essential for grasping the basic principles of molecular genetics and protein biosynthesis in eukaryotes.