What is a codon and why are they important?

A nonsense mutation in the third codon does what?

• A nonsense mutation in the third codon of the lac repressor will: a prevent transcription of the lac operon in the presence of lactose b prevent binding of lac repressor to lac operator c prevent translation of b-galactosidase d prevent transcription of the lac operon in the absence of lactose e none of the above Answer is B, but why?

• Answer:

  A nonsense mutation is a genetic mutation in a DNA sequence that results in a shorter, unfinished protein product. DNA is a chain of many smaller molecules called nucleotides. During protein formation, DNA (or RNA) nucleotide sequences are read three nucleotides at a time in units called codons, and each codon corresponds to a specific amino acid or stop signal (stop codon). Stop codons are also called nonsense codons because they do not code for an amino acid and instead signal the end of protein synthesis. Thus, nonsense mutations occur when a premature nonsense or stop codon is introduced in the DNA sequence. When the mutated sequence is translated into a protein, the resulting protein is incomplete and shorter than normal. Consequently, most nonsense mutations result in nonfunctional proteins.