Final answer:
The start codon in protein synthesis is typically AUG, but GUG can also be used as an alternative start codon in some organisms.
Step-by-step explanation:
The start codon is the codon that signals the beginning of protein synthesis. In most organisms, the start codon is AUG, which codes for the amino acid methionine. However, there are rare instances where GUG is also used as a start codon. For example, in some bacteria and archaea, GUG can initiate translation instead of AUG. These alternative start codons still result in the incorporation of methionine into the growing polypeptide chain.