Final answer:
The reading frame for translating mRNA into a functional protein is determined by the location of an AUG start codon. The eukaryotic initiation complex uses the 5' cap to find the start site, and Kozak's rules help identify the correct AUG for efficient translation.
Step-by-step explanation:
The reading frame to use for translating an mRNA into a functional protein is determined by the location of an AUG start codon. This AUG codon, which specifies the amino acid methionine, is critical because it sets the reading frame for translation near the 5' end of the mRNA. Translation then proceeds by reading the mRNA in groups of three nucleotides (triplets) until a stop codon is encountered.
In eukaryotes, the initiation complex recognizes the 7-methylguanosine cap at the 5' end of the mRNA, and with the help of a cap-binding protein (CBP) it searches for the correct AUG start codon. Kozak's rules refine this search by defining a consensus sequence around the AUG to determine the initiation site. This consensus sequence, 5'-gccRccAUGG-3', includes a purine (A or G) at the -3 position (R) and a guanine (G) at the +4 position relative to the AUG. The closer an AUG is to matching this sequence, the higher the likelihood it will serve as the start codon for translation.