Final answer:
The fixed reading frame in mRNA translation is a sequence of nucleotides read as codons that establish which amino acids are incorporated into a protein, beginning with the start codon AUG and ending with a stop codon.
Step-by-step explanation:
When the mRNA is translated, the fixed reading frame refers to the sequential set of three nucleotides, called codons, that dictate which amino acids are added during the synthesis of a protein. The translation process initiates at the start codon (AUG), which not only codes for the amino acid methionine but also establishes the reading frame for the entire mRNA sequence. As translation proceeds, every set of three nucleotides is read in order, and tRNA molecules match their anticodons with these codons to contribute the correct amino acid to the growing polypeptide chain. This process continues until a stop codon (UAG, UGA, or UAA) is encountered, signaling the end of protein synthesis. It's critical that the reading frame remains unchanged; if a nucleotide is inserted or deleted (known as frameshift mutations), the subsequent codons would be misread, resulting in a completely different sequence of amino acids which often leads to a nonfunctional protein.