Refer to Figure 13.7 to respond to the following:

A base-substitution mutation that altered the sequence shown in part (a) eliminated the synthesis of all but one polypeptide. The altered sequence is shown here:

 5'-AUGCAUACCUAUGUGACCCUUGGA-3'

Determine why.

Key Concepts

Base-Substitution Mutation

A base-substitution mutation occurs when one nucleotide in the DNA sequence is replaced by another. This type of mutation can lead to changes in the amino acid sequence of a polypeptide, potentially altering its function. Depending on the nature of the substitution, it can be classified as silent, missense, or nonsense, each having different implications for protein synthesis.

Polypeptide Synthesis

Polypeptide synthesis is the process by which ribosomes translate mRNA sequences into polypeptides, which are chains of amino acids that fold into functional proteins. The sequence of nucleotides in mRNA determines the order of amino acids in the polypeptide. If a mutation alters the mRNA sequence, it can disrupt this process, leading to incomplete or nonfunctional proteins.

Start and Stop Codons

Start and stop codons are specific sequences in mRNA that signal the beginning and end of translation. The start codon (AUG) initiates protein synthesis, while stop codons (UAA, UAG, UGA) terminate it. If a base-substitution mutation affects these codons, it can prevent the synthesis of polypeptides or result in premature termination, explaining why only one polypeptide may be synthesized in the altered sequence.