The correct answer is AUG. Key Points • Prokaryotic genetic code: The genetic code in prokaryotes is universal and consists of codons, which are triplets of nucleotides in the mRNA sequence. These codons specify the amino acids during protein synthesis. • In prokaryotes, the initial amino acid in protein synthesis is N-formyl methionine (fMet). This amino acid is specifically used to initiate translation. The formyl group is added to methionine to form N-formyl methionine. • AUG is the start codon that codes for methionine in eukaryotes and N-formyl methionine in prokaryotes. It is the universal start codon for initiating protein synthesis. Hence, AUG is the correct answer. • The ribosome in prokaryotes recognizes the AUG codon as the initiation site for translation. This codon is positioned at the P-site of the ribosome during the initiation of protein synthesis. • Other codons such as TTG, CUG, and GGC are not initiation codons. TTG and CUG can act as rare alternate start codons in some bacterial species, but their usage is not universal. GGC codes for glycine and is not involved in initiation. Hence, these options are incorrect. Additional Information • Role of AUG in Translation: • AUG is the most common start codon in both prokaryotes and eukaryotes. It establishes the reading frame for translation, ensuring that the correct sequence of amino acids is synthesized in the protein. • In prokaryotes, the Shine-Dalgarno sequence upstream of the AUG start codon helps the ribosome identify the correct start site for translation. This sequence is complementary to a region of the 16S rRNA in the ribosome. • N-formyl methionine (fMet) in Prokaryotes: • The addition of the formyl group to methionine in prokaryotes is catalyzed by the enzyme methionyl-tRNA formyltransferase. This modification ensures that the amino acid is specifically recognized for initiation. • fMet plays a significant role in initiating protein synthesis and is later removed from the protein chain by specific enzymes after translation. • Functions of Other Codons: • TTG and CUG: These codons are rare alternative start codons in some bacterial species. Their use depends on specific sequences and translational machinery. • GGC: This codon codes for the amino acid glycine and is not involved in the initiation of protein synthesis. Glycine is a non-polar amino acid commonly found in structural regions of proteins. Important Points • Key Features of Codons: • The genetic code is universal, meaning that codons code for the same amino acids in almost all organisms. • Codons are degenerate, meaning that multiple codons can code for the same amino acid. However, AUG is unique as it serves both as a start codon and a codon for methionine. • Applications of Understanding the Genetic Code: • Knowledge of the genetic code is fundamental to fields such as genetic engineering, where genes are manipulated to produce desired proteins. • This understanding is also crucial in molecular biology research, such as studying mutations, protein synthesis, and developing therapeutic interventions.