Correct Answer:
D. AUG
The correct answer is AUG. In the genetic code, the codon AUG serves as the primary start signal for protein synthesis. It is the first codon of a messenger RNA (mRNA) transcript that is translated by a ribosome.
Understanding Start and Stop Codons
- Role of AUG: AUG has a dual function. It codes for the amino acid methionine in eukaryotes (or formylmethionine in prokaryotes) and simultaneously signals the beginning of the protein-coding sequence. The ribosome scans the mRNA until it finds this initiation signal.
- Stop Codons – The Opposite Signal: The other three options — UAA, UAG, and UGA — are stop codons. They do not code for any amino acid. Instead, they signal the termination of translation, telling the ribosome to release the newly made polypeptide chain.
- Importance of the Start Codon: Without a proper start codon, the ribosome cannot correctly assemble the translation machinery, and protein synthesis will not initiate. Mutations affecting the AUG sequence can lead to severe genetic disorders.
- Exceptions and Variations: While AUG is the universal start codon, a few organisms use alternative start codons like GUG or UUG on rare occasions. However, in standard examinations and general biology, AUG is the unambiguous primary start codon.
Thus, the primary start codon in protein synthesis is AUG.