Final answer:
When using a codon chart, one should always read the mRNA sequence to determine which amino acids the codons correspond to during protein synthesis, starting with the AUG start codon until a stop codon is reached.
Step-by-step explanation:
When using a codon chart, the molecule you should always read is mRNA.
The chart is designed to translate the nucleotide triplets or 'codons' present in the mRNA sequence to determine which amino acid they correspond to in the process of protein synthesis.
For instance, the codon AUG in the mRNA both codes for the amino acid methionine and serves as the start codon, which establishes the reading frame for translation.
Each subsequent set of three nucleotides, read from the 5' to the 3' direction, forms a new codon that specifies a particular amino acid until a stop codon (UAG, UGA, or UAA) is reached, which signals the end of the protein synthesis process.