MCAT Content / Genetic Code / Missense Nonsense Codons

Missense, nonsense codons

Topic: Genetic Code

Mutations in a DNA sequence can produce errors during protein synthesis by leading to missense codons, which encode an incorrect amino acid, and nonsense codons, that encode a premature stop codon.

Missense codons or missense mutations result from a single nucleotide change (known as a point mutation) in a DNA sequence that leads to an incorrect amino acid in the translated polypeptide sequence. For example, while the codon AAA encodes lysine, a point mutation to AGA will lead to an arginine being added to the growing polypeptide chain instead. Because the genetic code is degenerate, meaning multiple different codons encode the same amino acid, some point mutations do not change the the amino acid that is ultimately added to a polypeptide chain. These are known as silent mutations. For example, a point mutation from UUU to UUC will result in a silent mutation, as both of these codons encode the amino acid phenylalanine.

Nonsense codons or nonsense mutations result from a point mutation that produces a premature stop codon, which signals the end of translation. For example, while the codon UCA encodes the amino acid serine, a point mutation to UAA will result in a stop codon and will erroneously end translation of the polypeptide. These are sometimes called Amber codons.


Practice Questions

Khan Academy


MCAT Official Prep (AAMC)

Practice Exam 4 B/B Section Passage 10 Question 54


Key Points

• Missense codons result from point mutations in the genetic code that lead to incorrect amino acids being added to a polypeptide chain during translation.

• Nonsense codons result from point mutations that result in stop codons, which prematurely terminate the translation process, these are sometimes called Amber codons.

Key Terms

Mutation: A change in the nucleotide sequence of DNA.

Missense codons (missense mutations): A change in a single amino acid that results from a point mutation in the DNA sequence.

Nonsense codons (nonsense mutations): The conversion of a codon from one that encodes an amino acid to one that acts as a stop codon due to a point mutation in the DNA sequence.

Point mutation: A single nucleotide change in the sequence of DNA.

Silent mutation: A point mutation in the DNA sequence that does not change the amino acid that is encoded by a codon.

Amber codon: A stop codon that is a mutation of the genetic code during translation causing premature stop of the reading of the mRNA

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