Final answer:
Protein synthesis in cells starts with the start codon AUG, which sets the reading frame and signifies methionine, and ends with one of three stop codons that terminate the process.
Step-by-step explanation:
The process of protein synthesis begins with the initiation stage during which the start codon, typically AUG, is recognized by the ribosome as the starting point for translation. This codon not only specifies the amino acid methionine but also establishes the reading frame for the translation process. As the ribosome traverses the mRNA strand, it reads the nucleotide triplets, or codons, and facilitates the addition of the corresponding amino acids to the elongating polypeptide chain.
This continues until a stop codon (UAG, UAA, or UGA) is reached, signaling the end of translation and resulting in the release of the newly synthesized polypeptide.
These stop codons are also known as nonsense codons, and they do not code for any amino acids. The region following the stop codon up to the end of the mRNA is termed the 3' untranslated region, or 3’UTR. The universality of the genetic code, with a few exceptions, implies a shared common origin for all life on Earth.