Final answer:
The reading frame for translating mRNA into a functional protein is determined by the AUG start codon. It establishes the pattern by which the mRNA is read in sets of three nucleotides, or codons, during translation, to build the corresponding polypeptide chain.
Step-by-step explanation:
The reading frame for translating an mRNA into functional protein is determined by the start codon (AUG) on the mRNA transcript. This codon not only codes for the amino acid methionine but also sets the correct reading frame for translation.
During translation, tRNA molecules bring the appropriate amino acids in sequence to the growing polypeptide chain by matching their anticodons with the codons on the mRNA strand. Each set of three bases, called a codon, corresponds to a specific amino acid.
The process begins with the AUG start codon, which establishes the frame by which subsequent groups of three nucleotides (or codons) are read to define the amino acids of the protein. This reading frame continues until a stop codon (UAG, UGA, or UAA) is reached, signalling termination of the translation process.
Protein synthesis includes three phases: initiation, during which the reading frame is set; elongation, where tRNAs continue to add amino acids according to the mRNA sequence; and termination, where the polypeptide chain is released upon encountering a stop codon.