Final answer:
The AUG start codon signals the beginning of translation and sets the reading frame for protein synthesis.
Step-by-step explanation:
The group of three nucleotides that signals the beginning of translation is called the start codon. Specifically, the AUG codon plays a critical role in initiating translation. In the process of translation, the AUG start codon sets the reading frame, which is the sequence by which nucleotides are grouped into codons. The AUG codon not only specifies the amino acid methionine but also serves as the signal for the ribosome to begin synthesizing the polypeptide chain. Translation continues with the ribosome moving along the mRNA and bringing together amino acids in accordance with the genetic code until it reaches one of the three stop codons, which signals the end of protein synthesis.