Final answer:
An mRNA transcript that is 39 bases long can encode for a maximum of 12 amino acids, given that the last three bases are the stop codon UAA.
Step-by-step explanation:
If an mRNA transcript is 39 bases long and ends with the stop codon UAA, the maximum number of amino acids that could be encoded by this molecule would be determined by the length of the mRNA divided by 3 (since three nucleotide bases make up a codon which encodes for a single amino acid). Subtracting the 3 bases for the UAA stop codon from the total 39 bases, we have 36 bases that can be used for coding amino acids. Therefore, 36 divided by 3 is 12, meaning a maximum of 12 amino acids can be encoded by this mRNA molecule before reaching the stop codon.