The Genetic Code

Free Response

Imagine if there were 200 commonly occurring amino acids instead of 20. Given what you know about the genetic code, what would be the shortest possible codon length? Explain.


For 200 commonly occurring amino acids, codons consisting of four types of nucleotides would have to be at least four nucleotides long, because 44 = 256. There would be much less degeneracy in this case.

Discuss how degeneracy of the genetic code makes cells more robust to mutations.


Codons that specify the same amino acid typically only differ by one nucleotide. In addition, amino acids with chemically similar side chains are encoded by similar codons. This nuance of the genetic code ensures that a single-nucleotide substitution mutation might either specify the same amino acid and have no effect, or may specify a similar amino acid, preventing the protein from being rendered completely nonfunctional.

A scientist sequencing mRNA identifies the following strand: CUAUGUGUCGUAACAGCCGAUGACCCG

What is the sequence of the amino acid chain this mRNA makes when it is translated?


Met Cys Arg Asn Ser Arg

The first step to writing the amino acid sequence is to find the start codon AUG. Then, the nucleotide sequence is separated into triplets: CU AUG UGU CGU AAC AGC CGA UGA. We stop the translation at UGA because that triplet encodes a stop codon. When we convert these codons to amino acids, the sequence becomes Met Cys Arg Asn Ser Arg.