Predict if a protein molecule have a sequence of the following amino acids. What may be the sequence of nucleotide in genetic code of mRNA?
Met Phe - Ser

Question

Predict if a protein molecule have a sequence of the following amino acids. What may be the sequence of nucleotide in genetic code of mRNA? 
Met Phe - Ser

Accepted Answer

The correct option is 2: AUG-UUU – UCU

The question asks for a possible mRNA sequence that codes for the amino acid chain Methionine (Met) - Phenylalanine (Phe) - Serine (Ser). The core concept here is understanding the genetic code, where a sequence of three nucleotides on the mRNA (a codon) specifies a particular amino acid. Translation always begins with a start codon, which is universally AUG and codes for Methionine. For the first amino acid, Met, the codon must be AUG. Phenylalanine (Phe) is encoded by two possible codons: UUU or UUC. Serine (Ser) is encoded by six possible codons: UCU, UCC, UCA, UCG, AGU, or AGC. The sequence in Option 2, AUG-UUU-UCU, correctly assigns AUG to Met, UUU to Phe, and UCU to Ser. This specific combination is a valid and complete coding sequence for the required tripeptide. While other valid sequences exist (e.g., AUG-UUC-UCG), Option 2 is the only correct and complete option provided among the choices.

The incorrect option is 1: AUG-UUA – UAG

This option correctly starts with the AUG codon for Methionine. However, the second codon, UUA, codes for the amino acid Leucine (Leu), not Phenylalanine (Phe), which immediately makes the sequence incorrect for the required protein chain. Furthermore, the final codon, UAG, is a universal stop codon. A stop codon signals the termination of protein synthesis and does not code for a Serine residue, which means this mRNA would produce a dipeptide of Met-Leu, then stop. This fundamentally fails to translate into the sequence Met-Phe-Ser.

The incorrect option is 3: AUG-UUC - UAA

This sequence correctly begins with AUG for Methionine and the second codon, UUC, is a valid codon for Phenylalanine. However, the last codon, UAA, is one of the three universal stop codons. Like UAG, a stop codon signals the release of the polypeptide chain and does not code for Serine. Therefore, this mRNA sequence would only produce a dipeptide, Met-Phe, before the translation process is terminated. It is important to remember that stop codons are non-sense codons and are only found at the end of a functional gene's coding sequence to mark the completion of the protein.

The incorrect option is 4: UUU - UUA – UGG

This option is fundamentally flawed because it does not begin with the universal start codon, AUG. Translation always initiates at an AUG codon, which sets the correct reading frame for the ribosome. By starting with UUU, this sequence would code for Phenylalanine (Phe). The subsequent codons, UUA (Leucine) and UGG (Tryptophan), code for the complete but incorrect amino acid sequence Phe-Leu-Trp. This entire sequence fails to code for the required Methionine as the first amino acid and includes two incorrect subsequent amino acids.